Major furnace/ac job-Help!


  #1  
Old 05-07-03, 12:22 AM
Darkhorse
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Major furnace/ac job-Help!

I'm sorry this is so long and has a lot of questions but we're talking $12K and major tearing up in the house. Please help if you can. Here's some background. I need a new furnace and central air system in my house. I'm in a ranch on a slab. The cold air returns are under the slab and flood with water so the four places I had out for estimates say that I have to seal off those and go with a new return system in the attic. The house measures 90'X 42' on the outside and has a 400 square foot garage. I think the house is around 3000 square feet. It's 50 years old with old windows, no wall insulation and R-19 in the attic. The house has large picture windows and the shops that have come out said to calculate for low insulation value. I live near Lake Erie. I currently have a 5 ton AC from the late 60's and an 18 year old 175,000BTU furnace. Here are my questions:
Shop A says I need 93,000 BTU's of heating and 3.5 tons of AC. They want to put in an Amana two stage variable speed high efficiency furnace(their top of the line) and a Trane XL13i AC. Even though they want to use a 3.5 ton size outside they want to use a 5 ton inside coil with an expansion valve. They said I need 2000 cfm of airflow due to my 70' supply trunk with 18 branches and another 25' trunk with two branches. In total I have 21 supply registers counting the plenum register. They say the bigger coil is necessary for my airflow requirements. They want to go with a 115,000 BTU furnace only due to the fact that the next size down is a 90,000 BTU. I'm afraid of all this considering how much bigger my current system is and the fact that it performs well. They also said I need the 5 ton blower. Do they sound right?

Shop B wants to install a Carrier 58MVP variable speed high efficiency furnace and a Carrier 38TXA AC unit. They want to use a 100,000 BTU furnace as they say the 94% AFUE will give just the right output. I wanted to move up the 120,000 but they said unnecessary. They want to use a 4 Ton system and say the larger 5 Ton coil mixed with a 3.5 Ton outdoor unit suggested by company A is unnecessary. They are also staying with the standard metering device and don't see the need for the Expansion Valve.

As for the cold air return system the labor will be massive. It will run a trunk about 70 feet with eight branches coming off of it. It will be in the attic. These places were going to use duct tape and fiberglass instead of mastic then fiberglass insulation. They both said they want no more than ten feet of flex at the end of each branch. They said any more will be restrictive. My new returns will be in the ceiling because of the slab. My supplys are high on the wall in each room. I would think the two would compete against each other but they said with a variable speed and continuous fan I will have no problem. Will returns in the ceiling with supply high on the wall work or will I be an icicle from the waste down?

Is a thermal expansion valve necessary? One company says yes and the other no. What about the 3.5 outdoor with the 5 ton indoor for the AC? Who is right? Both have given me estimates of $12,000 so price isn't an issue. Would you say Amana/Trane or Carrier? I'm also going with a media air filter and was wondering what the return boot and the main return trunk in the attic should be sized at? Sorry for the length but this is a lot of money and I'll have to live with the results for a long time. Thanks.
 
  #2  
Old 05-07-03, 02:42 AM
firsthvac
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If I read you post right, you have good reason to be concerned.
I'm wondering if the companies bidding on the job actually showed you a load assessment on your home? Judging by what information you've given, it sounds like no one has done a thorough assessment.

1. You have an old home with virtually no insulation.
2. Unless you've done some major sealing to doors, windows, electrical penetrations, foundation, etc., my bet is the house has a high air infiltration factor.
3. Judging from the models you've given, the furnace will be the primary air handler and there is no reason to install an excessively larger coil for a smaller a/c.
4. How come no one has mentioned the possibility of using 2 smaller units to accomplish the same purpose since you have such long air supply runs?
5. Why does company A want to mix brands when matched systems normally give the best efficiencies?
6. How does company B know a 100k furnace will give just the right amount of heat without doing a load assessment?

It is the contractor’s responsibilty to perform a proper, complete sizing calculation for the home. building. Unfortunately, many installer use 'rule of thumb' guides, 'experience', and 'rating plates' without anything to double check their conclusions. Both methods are ok for giving rough estimates but they ARE NOT proper sizing methods. Correct system sizing requires consideration of a many more factors than simply reading the nameplate of the existing unit. Many factors affect the load requirements and a good experienced licensed estimator will measure, measure, measure.

If the companies your spoke of did give you a good accurate load assessment, it will include at least the following items:

The size, shape, and orientation of your house (how it faces the compass points), inside/outside temperature differences, square footages of windows & doors, insulation, construction type, the number of people in the home, local average humidity and temperatures, as well as load factors for lights, appliances, and people (because all these do give off heat). A really good company will also have done an air infiltration test due to the age of the home.

If you didn't get a copy of the load assessment, insist the bidding contractors show you a detailed load calculation before signing the contract. They should have no problem in sharing the results with you. If they balk...go elsewhere.

Due to the size of your job and your concern for the placement of the return air ducts, be sure any of the bids include the WRITTEN load calculation, name and models of the equipment, warranties specific to equipment, installation materials AND labor. Also have them include where and how they are installing the equipment, how the payment(s) will be made and the date of completion. This will be your only proof they did or didn't do the job right.


Concerning some of your other questions:

You will need the maximum amount of airflow you can get considering the length of your supply trunks and the number of supplys being used, therefore a blower change may be needed.

Use of an expansion valve is a more efficient way of metering the refrigerant to the coil since it more accurately insures the proper amount of refrigerant is in the coil under a wider range of conditions, but is not absolutely necessary.

The total return air duct size and filtration should always attempt to meet or exceed the equipment manufacturer's minimum recommendation. Without more detailed information and actually inspecting the job location, it would be improper to say exactly what size you will need.
 
  #3  
Old 05-07-03, 10:49 AM
Darkhorse
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Originally posted by firsthvac
If I read you post right, you have good reason to be concerned.
I'm wondering if the companies bidding on the job actually showed you a load assessment on your home?

They did a load calculation and showed me the results. They both showed that I needed 39,000 BTUs of cooling and company A(Amana/Trane) showed that I needed 93,000 BTUs of heating while Company B(Carrier) showed that I needed 89,000 BTUs of heating. In fact all four of the companies I had out agreed that I needed 3.5 tons of cooling. Although, Company B(Carrier) says they want to bump me up to a 4 ton due to the fact that the supply trunk is 50 years old, probably leaky and in a hot attic. They are the same ones that say the TXV is not necessary. Company A says I need 3.5 tons of cooling but wants to use a 3.5 ton outdoor unit with a 5 ton indoor coil and a TXV.




1. You have an old home with virtually no insulation.

R-19 in the attic. Two rooms with wall insulation(I forgot those) All windows but two bedroom windows are 50 years old with large picture windows facing the lake and an 8 foot patio door that is very leaky. The slab is not insulated either.


2. Unless you've done some major sealing to doors, windows, electrical penetrations, foundation, etc., my bet is the house has a high air infiltration factor.

Just storm windows and some replacement weatherstripping on the entry doors but it doesn't work too well.


3. Judging from the models you've given, the furnace will be the primary air handler and there is no reason to install an excessively larger coil for a smaller a/c.

So you would say going with the 4 Ton setup and staying with a 4 ton outdoor unit and a 4 ton indoor unit would be safe even considering they say the ductwork is capable of moving 5 tons of air?


4. How come no one has mentioned the possibility of using 2 smaller units to accomplish the same purpose since you have such long air supply runs?

It isn't really feasible considering the layout in this house and we've always had strong airflow even in the farthest reaches of this house.



5. Why does company A want to mix brands when matched systems normally give the best efficiencies?

Their main AC that they use is Trane. They've been a Trane dealer for as long as I can remember. They also used Trane furnaces as well but switched to Amana a couple of years ago. They told me the reason was that the quality of Trane's furnaces had slipped and they were very loud. I wonder if it really had something to do with the fact that Sears will install Trane now at a lower cost than they can handle? Their second line of equipment for furnaces and AC is Heil.



What is your opinion on the fact that my return grilles will be in the ceiling? This really concerns me as I would think it would tend to steal supply air from the registers which are mounted on the wall less than a foot from the ceiling. Can this setup work? All of the contractors say that with a variable speed furnace that I should have no problem and suggested running the blower fan continuously. I would think the house would be colder, especially from the waist down. They said a single stage or two stage furnace wouldn't do the job as well.

Another question that I forgot. I'm going with a media air filter. An EZFlex by Carrier. It supposedly has less restriction, less pressure drop and a higher Merv rating of 10 compared to the Spaceguard at 8. I can't go with an electronic due to the fact that I have asthma and it gives off ozone. I found out at my old house that I'm sensitive to ozone. We had an electronic air cleaner there. Company A says that i would need two media filters since I will have more than 3.5 tons of airflow and Company B says that one will be fine with the 4 Ton unit they say that i need even though my home's ductwork is capable of moving 5 tons. What do you think?

Again, I'm sorry for the length and all of the questions. Thanks for your first post and all of your help. I've called out the four most well-known contractors in my area so I thought it would be easy but they all said this job was "unique." I'm afraid to pull the trigger on this with all of the differing opinions they're giving me and the drastic size difference versus my old equipment. Like I said I have an 18 year old furnace at 175,000 BTUs and they want to cut it down to a 100,000.
 
  #4  
Old 05-07-03, 11:29 AM
T
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Your 18 year old furnace will probably have an efficiency rating of at most 70%. That will give it a real heating value of 0.7X175,000 or 122,500 BTU. If they use 94% efficiency units (and I believe all variable speed units are), then you'll only need a 130,000 BTU furnace to give you the same "real" heating value. Perhaps the original unit was oversized.
Then again, I'm not a heating contractor..
 
  #5  
Old 05-08-03, 05:28 AM
firsthvac
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Glad to hear you did get a load calculation. I find it interesting that Company B wants to up the size of the A/C because of the possible supply line loss and gain. The same should have been considered in the Furnace sizing. But if the companies have that concern, why aren't they recommending sealing and insulation of the existing duct?

As far as the returns going into the ceiling. If they are located in a manner which promotes the maximum possible loop of airflow, you should have no problem. For example, place the return opposite of the supply as far away as possible and set the supply register air downward. Keep in mind that a/c-heating is a closed loop airflow system and any "stealing" which is done is still retained within the system. It is the thermostat set at the average room height which determines the comfort level of the home.

While running a fan constantly has been shown to best maintain an even air temperature, the drawback is a cooler feeling to the body from the air movement across the skin. A majority of folks don't have issues with it as they will adapt to the environment. If you feel cooler, shut the fan off and raise your thermostat a degree or two to compensate for the difference. With the increased efficency of your new system, you shouldn't see an increase in your utility bills.

On another note...it's true that Trane is no longer what they once were, but moving to Amana isn't much of an improvement and mixing components does not promote the maximum efficiency of a system. Eventhough Carrier's quality and design in packaged units has severely declined, the split systems still retain good reliability with simple maintenance and use of common off-the-truck components. I would take the Carrier system for two major reasons...first, the system components are matched which helps to limit the number of problems that can be encountered with installation, maintenance and operation as well as providing for a better airflow throughout the home. Two, the larger a/c will insure you have sufficient btu's availible for cooling in the extreme temperature situations which is becoming a more common occurance in the today's world climate. I would however ask for the next size up in furnaces since your insulation and infiltration factors are poor at best and a 94% 100,000btu furnace will only provided 94,000btu's under ideal circumstances, which your home is obviously not.

About the air filter...if it provides better filtration with less restriction, use it
 
  #6  
Old 05-08-03, 11:28 AM
Darkhorse
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Thanks a lot for the reply. It definitely helps. Do you think the 4 Ton AC with the 4 ton outdoor unit and the 4 ton indoor coil would be the best choice? If I understood you correctly you do. That's what the Carrier people want to do. What did you think of the Amana people's idea of using a 3.5 ton outdoor unit with a 5 ton coil? Is a 5 ton coil necessary due to what my ductwork can handle? They say my exisiting supply, due to the size of the main trunk and the fact that it has 18 branches and registers along with a smaller trunk that feeds two other registers, makes it capable of moving 2000 cfm's. The Amana people think this is necessary but the Carrier people think that a 4 ton unit,inside and out is fine. The Carrier people say that as long as the blower is capable of moving 5 tons that the 4 ton AC will be fine because that is what, in their opinion, what my application calls for. What do you think? I'm not familiar with mismatching coil and condenser sizes.

As for the furnace, the next size up is 120,000 btu compared to the 100,000 that was suggested by 3 of the 4 contractors. Our 50 year old supply lines are insulated. Well, the main trunks are but the branches are not where they disappear under the flooring in the attic but when they reappear in the open joist area they are again covered in insulation. I've looked at them up close and I see no sign of any duct tape, mastic or silicone. That makes them pretty insulated but unsealed. I mentioned this to the contractors and they said their load calculations were based on a worse case scenario with temps in the teens and so on. My current furnace is 18 years old and the input is 175,000 btu's. I'm guessing but if you figure about 65% efficiency that would make its output just under 114,000 btu's. The new 120,000 size would put out 112,800. That's pretty close. We're satisified with the heating of the current furnace with the exception of the room that is 70 feet away. Airflow is a little weak back there. They all say my unit is grossly oversized but the unit I found here in the early 1980's was 180,000 btu's. Would there be any detriment to moving up to the 120,000 unit you suggested versus the 100,000 unit? Isn't it better to be too big versus too small? I realize operating cost but what about comfort? Would the house be stifling hot compared to the 100,000?

What do you think about the two versus one on the media filter issue? I was told if you are going above 3.5 tons that you need two. Is that EZFlex capable of allowing me to run one or do I need two?


A lot of questions and some dumb ones I know but this is important. Thanks for your time.
 
  #7  
Old 05-08-03, 07:46 PM
firsthvac
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Go with the 4 ton Carrier system and ask for the 120,000 btu furnace. There is no logical reason to install a 5 ton a/c coil with a 3.5 ton condensing unit. An indoor coil only determines the amount of btu's availible to the airflow for heat exchanging purposes and is only capable of delivering the maximum btu's of the indoor coil itself or the outdoor unit, whichever is less. In other words...if you have a 5 ton indoor coil capable of delivering 60,000 btu's but the outdoor unit has a maximum capacity of the smaller 3.5 tons (42,000 btu's), then the indoor coil will only be good for 42,000 btu's. Why put in added resistance to the airflow when it's not needed?

Additionally, the Carrier dealer is correct about the blower being the determining factor in the amount of airflow. 99% of furnace blowers that can handle airflow requirements for a 4 ton system are the same blowers used for 5 ton systems. Such is true in your case.

As far as upping the size of the furnace...your load calls for 90,000 to 100,000 btu's of heating. A few additional btu's in your case will not hurt the overall performance and the operational cost will still be less than what you are paying with your 18 year old system. Like I said before, the ratings are based on ideal conditions, not on poor conditions like yours. By the way, the heat load temp should have been based on around 5 degrees outdoor temperature for 1-3% of the heating season in your region of Ohio.

In regards to the filter issue...one will be enough as long as the size is equal to or greater than the total filter inlet square inches recommended by the equipment manufacturer. The type of filter used is dependent on the homeowner needs. Now is also the time to have the supply ducting sealed and cleaned, especially since you have allergies.

One other note to consider...get an extended warranty if it is available. Call it insurance against unexpected breakdowns, routine maintenance replacements, etc. I know it can run a few hundred extra dollars but you can be sure it will be used before it expires (unlike small appliance warranties which seem to expire just before a problem occurs...lol) and it will save you money in the long run. You're a smart person and it would be foolish to not protect an investment of this size.
 
  #8  
Old 05-08-03, 10:21 PM
Darkhorse
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Thanks for the reply, firsthvac. That really helped clarify everything for me. I had been worried that with the 120,000 BTU size supposedly being bigger than what I needed that it would render the two stage variable speed furnace useless. I was concerned that it would short cycle or that even on low fire stage that it would be as powerful as what my worst case scenario load calc. would need. I had also feared that the high stage heating with the variable speed would be stifling. That's why I was so concerned. Thanks.
 
  #9  
Old 05-13-03, 11:33 AM
Darkhorse
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Attention Firsthvac and anyone else interested

I'm sorry to bother you on this again but I met with the contractor I decided on today. It was the Carrier dealer that I mentioned in this thread.

They were the ones that were going to put in the 4 ton ac. The competing contractor had suggested a 3.5 ton outdoor unit and a 5 ton coil. You had said this was unnecessary and he agreed. The other contractor had wanted to do it to increase airflow through my ductwork being that my original supply was sized to move 5 tons. The other contractor had also said running the 5 ton coil would help remove latent heat as well as increase airflow. Do you still agree that I should put in the 4 ton indoor and outdoor vs the 3.5/5 setup even though my ductwork will support 5 tons of airflow? Can you see any scenario where that would be a benefit? My original AC is 5 ton. My house is 90'x42' with a 20' x 20' area making up an attached garage. It's a ranch on an uninsulated slab. My house was calculated as low insulation value(I have R-19 in the attic, wall insulation in only two rooms, 50 year old windows with 4 picture windows facing Lake Erie. Their sizes are:
7'9" X 3'6''=there is a 50/50 chance I will replace this window
8' X 5'3"
16' X 5'3''
7' 4" X 3' 9"=only this last one will see replacement for sure

All the contractors say they have accounted for these windows, low insulation value, facing Lake Erie, etc, but they all feel the 100,000 BTU size is the right one. It turns out that the contractor that would install the 115,000 size was only doing so because Amana's next size down was a 90,000 and we need 93,000 according to him. He said if they made a 100,000 he wouold install it too.

As for the furnace, the contractor I picked but haven't signed with had come up with a heat load calc. of 93,000 BTU's and was going to put in the 100,000 BTU 58 MVP with the 94% and variable speed. The output on high fire is 93,000 and on low fire is 61,000. I suggested stepping up to the 120,000 size which would have an output of 112,000 on high fire and 73,000 on low fire. He objected going to the 120,000 BTU saying that the extra 19,000 BTU's of heat on high and the extra 12,000 on low would cause the furnace to short cycle and being high efficiency wouldn't promote efficient burning of contaminants possibly shortening the life of the furnace. I basically felt going with the larger would let it run on low fire the majority of the time. What do you think? He was adamant about not going with the bigger size. I remember that you agreed with me and suggested the larger furnace. My current furnace is 18 years old with an input of 175,000 and the tag says the output is 140,000. He says that can't be the actual output because in 1985 they didn't make an 80% furnace. He said the output is closer to 50% making it have an output of 87,000 BTU's but another contractor says a Rheem/Ruud(that's what I have) from 1985 would have an AFUE of about 65% making this thing have an output of 113,750. Who is right on the efficiency and what would you do?

As for the air cleaner he says one large EZFlex media filter rated for 2000cfm, as long as the furnace opening matches the surface area of the filter or at least has a transition installed making it match, will be okay with the 4 ton AC saving me from having two filters. He also mentioned putting the EZFlex in the attic right in the return before it drops down to the furnace. Do you agree with his two ideas? Sorry this is so long and repetitive compared to my other posts but spending $12,000 on something I'm uneasy of has left me indecisive for once in my life. Thanks again.
 
  #10  
Old 05-13-03, 09:03 PM
firsthvac
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Dave--

In the mid 80's, 65% would be the correct efficiency for most furnaces of the time including yours. The increase to a 5 ton coil will do nothing significant to increase airflow nor help in the removal of latent (unseen heat).

As I said before...the outdoor unit in your case is the determining factor for the amount of heat that can be removed since the size (capacity) of the outdoor unit will only reject a maximum of 4 tons (48,000 btus) Therefore, the system will only be capable of taking in a maximum of 48,000 btus and only if it is 100% efficient (which it isn't) It is a basic law of refrigeration thermal dynamics and physics. If any one HVAC manufacturer ever discovers a way to remove more heat than there is capacity of a system, then we can all do away with efficiencies since a system of that sort would be more than 100% efficient. Eventhough we have data that says some units can reject a larger amount of btu's than they are rated for, the fact is that those tests which produce the data are very carefully controlled laboratory conditions, none of which are applicable to what goes on in the field. To date...no one has been able to make anything 100% efficient (and I'm not just talking a/c systems here) much less more than 100% efficient. Therefore, common sense says why install something you will never use.

Now concerning the furnace size...you heat loss should only be an average of 73,000 btu's with a maximum loss on extreme days being around 112,000 btu's. The amount of gas contaminants left after burning is determined by the air/gas mix prior to burning. Fuel gases will only produce so many btu's per gallon of fuel. Efficiency of a unit is the ratio of btu's the unit generates to the btu's the unit transfers to the airflow. So as you can clearly see, burning of contaminants is not related to how efficient a unit is other than if the air/fuel mix is not correct (insufficient intake air, improper flue piping, incorrect gas orifices, etc.)then carbon build up on the inside of the heat exchanger can result in reducing the efficiency of the unit. This will happen with any efficiency of furnace, not just high efficiency types. If the airflow across the heat exchangers is not sufficient (not restricted, undersized ducting, too low a blower speed, etc) then short cycling could be a problem, but that is not the case here.

As far as whose right about the efficiency of your current unit...the tag is. Several manufacturers were beginning to produce 80% fuel efficient furnaces. They were the 'very high' efficiencies of their era (in the same boat as 95+% furnaces in today's time)

Stick with the 120,000 btu furnace. It will have a better airflow output than it's smaller cousin and be producing the heat you will require without burning problems.
 
  #11  
Old 05-13-03, 10:07 PM
Darkhorse
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Thanks a lot firsthvac. That's what I'll do.

I was unclear on part of your post:

Now concerning the furnace size...you heat loss should only be an average of 73,000 btu's with a maximum loss on extreme days being around 112,000 btu's.
You're referring to the output of the 120,000 BTU furnace and not my load calc. numbers here I assume? They said my load calc. was 93,000. I just wanted to make sure I understood you. I've wrote such long posts I don't know how you've managed to not get confused by me. I'm going with the 120 but I just wanted to make sure you knew that they calculated 93,000 BTU's.


You didn't mention, did you agree with the contractor that the one EZFlex filter would work with my system considering I will have 4 tons of AC? Some said one filter while others said I would need two due to the fact that it's over 3.5 tons.

Also, my existing and staying put supply side ducting can move up to 5 tons(2000 cfm) of air in my system but the contractor is sizing the new return system for 1600 cfm(4 tons) and 1700 cfm if I get the 120,000 BTU furnace which I'm going to do. He said normally you match the supply and return in terms of airflow but in this case he said it's not necessary for the new return ducting to be capable of moving 5 tons(2000cfm) like the supply side since I won't have a 5 ton system anymore. Do you agree?

What are your thoughts on mounting the EZFlex in the attic if space will not allow in my furnace room? Is it as effective not being as close to the furnace blower? It's going to be tight with the filter next to the furnace due to the fact that I could have to give up a doorway to the rec room. Thanks again for all your help. Maybe I should send you a consulting fee.
 
  #12  
Old 05-14-03, 05:59 AM
firsthvac
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Dave--

Sorry about the typo concerning the btu's...it should have said 93,000 btu's instead of 73,000

As far as the ducting goes, don't sweat it. The slight oversize of your return duct will actually help enhance the performance of the furnace blower by allowing the intake side of the blower access to air. Think of it this way...it's like your nose flaring slightly to allow more air intake to your lungs when your jogging. It may sound funny, but the same principle is actually true of heating/cooling airflow.

Where the filter is concerned, your heating/cooling will require a filtering surface area of about 600 square inches for the 120,000 btu furnace. So whatever combination of filters achieve this will be adequate. (For example, 1@24x25 or 2@16x20) What matters about the location is whether or not the total air intake is filtered and access to the filter is a convienent as possible for changing purposes. In most cases the closer to the blower air intake the better but not required since other factors may not allow for it (i.e. construction, unit size, floorplan, etc) We, personally, do all we can to avoid putting filters in attics to limit the amount of particulates that can enter the system if a filter access comes off (or is left off) and because the inconvienence of changing them tends to keep homeowners from doing so on a regular basis .

You're welcome...I'll send you a bill...
 
 

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