Drawbacks with highest efficiency heat pumps

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Old 09-04-06, 07:56 AM
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Drawbacks with highest efficiency heat pumps

I am looking to upgrade some or all of my HVAC equipment. Currently, I have an oil furnace on the first floor with an 11 SEER 2.5 ton air conditioner and a three ton heat pump (also about 11 SEER) with electric back up on the second floor. The A/C and heat pump are builder grade York units.

My current thinking is to replace the first floor with a heat pump and a propane gas furnace as the back up, and on the second floor simply replace the heat pump with new. In both cases, the air handlers will have the efficient ECM variable speed blowers and the gas furnace will be multi-staged on the heat side.

In order for this effort to have any reasonable payback period, I have been looking at the higher or highest SEER units from the top manufacturers. This puts me in the multi-speed compressor categories (or in the case of Trane two compressors) and, unfortunately, not using R22 refrigerant.

So, what are the issues with two staged compressors? Are the compressors actually two speeds or do they simply have a bypass valve that reduces the amount of refrigerant that is being pumped when operating in the first stage? Does Trane have a better mouse trap with their dual compressor set up?

Lastly, when calculating the cost of running the heat pump with a gas backup, what would the average COP be? It seems like it would be much higher than the traditional setup with electric backup, which effectively reduces the COP.

Many thanks for your thoughts and comments.

Mark
 
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Old 09-04-06, 04:04 PM
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Will you stay in the home for the next ten years?? Have you gone to http://warmair.net
and compare fuel cost for where you are. I like the two compressor best not the 2 stage ones. But if you drop down some .The Trane XL15i is about the beat buy. Seer of 15.10 and a HSPF 8.05 and still R22. With the V/S blower it has the humidistat control on it.

when calculating the cost of running the heat pump with a gas backup, what would the average COP be? It seems like it would be much higher than the traditional setup with electric backup, which effectively reduces the COP.
The cop will stay the same. You cant count the cop when your over on gas. The cop of the heat pump is to what the outside temp is at that time it is running.

ED
 
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Old 09-04-06, 05:31 PM
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Ed,

Thanks for your reply and comments.

Yes, I plan on being in this home for the rest of my life and hopefully the Lord is planning on that being more than ten years.

When I started researching all of this, the system you mentioned is what I thought I wanted. But, it is only about 25% more efficient than my current equipment, and thus, the pay back is just too long for it to make sense to me.

Yes, I have gone to warmair.com and done the fuel comparison analysis. I understand what you are saying about the COP not being different when coupled with a gas furance. But, I wasn't sure what COP to input in the analysis when considering some of the highest efficiency systems. What do you think would a reasonable COP to use?

So what do you like about the two compressor approach and what don't you like about the two stage approach? Lastly, I thought I had heard that these newest systems aren't providing as much as might be expected. What do you know about that?

Many thanks,

Mark
 
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Old 09-05-06, 05:56 AM
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Depending on area of country one lives, climate, and electric rate, I think propane backup is a poor choice considering its cost. I suggest studying these comparison numbers very carefully.

I think a good COP should be minimum 2.4 up to a high range of 3 for the size system you are considering.

A 2 stg heat pump will help with better dehumidification and operating costs. You have to determine if the extra cost is justified.

I have to admit that I prefer Trane/AmStd Heat pumps but Carrier/Bryant have the most sophisticated controls in the marketplace with their Infinity/Evolution models.

IMO
 
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Old 09-05-06, 05:56 AM
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mchild

Depending on area of country one lives, climate, and electric rate, I think propane backup is a poor choice considering its cost. I suggest studying these comparison numbers very carefully.

I think a good COP should be minimum 2.4 up to a high range of 3 for the size system you are considering.

A 2 stg heat pump will help with better dehumidification and operating costs. You have to determine if the extra cost is justified.

I have to admit that I prefer Trane/AmStd Heat pumps but Carrier/Bryant have the most sophisticated controls in the marketplace with their Infinity/Evolution models.

IMO
 
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Old 09-05-06, 07:00 AM
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TigerDunes,

Thanks for your thoughts on the COP. I have used 2.5 so it sounds like I am in the ballpark.

Using the warmair.com site, I have input a cost of .115 cents for electricity with a COP or 2.5 and $2.31 per gallon with an efficiency of 94% on the propane furnace (two stage). It yields a cost of $1.41 for a heat pump and $2.69 for the propane furnace. It also calculates the cost of electric baseboard heat at $3.16, which I have assumed (please tell me if my assumption is flawed) is similar in cost to operate to the electric backup in the typical heat pump. Therefore, the cost of propane as a backup to the heat pump would be cheaper than electric backup.

Can the Trane heat pump be operated properly with Carrier controls? And how do the gas furnaces compare between the two manufactures?

Thanks again,

Mark
 
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Old 09-05-06, 08:50 AM
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mchild

I am assuming your figures are correct but your logic is somewhat flawed. A heat pump with auxilliary heat (electric heat strips) in heating mode provides heat both from the heat pump and the aux heat strips. So you would be receiving heat from both sources-the cheaper heat from your heat pump plus the expensive heat from your heat strips to supplement your heating needs on those colder days. Using strictly emergency heat provides the expensive electric heat without any help from the heat pump. I can't tell you what percentage as this depends upon your heating needs and the size of heat strips required to meet your heating needs on those cold days. However, I just believe that heat strips may be a better choice than a propane backup. Plus you should be aware that your SEER and HSPF ratings will be slightly higher with a var speed air handler as opposed to a var speed propane fueled furnace. You can also have your heat strips staged. For example, the load calcs state you need 10KW heat strips. Then get 2 5KW modules installed and have them staged where you use just one 5KW module until you require the extra 5KW module.

I have never heard of using Carrier Controls on a Trane system and I suspect there would be warranty and compatibility issues. I would not turn around for the difference between top of line Carrier furnace vs Trane furnace.

Depending on where you live, I believe electric rates will be more stable in the future than nat gas or propane rates.

IMO
 
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Old 09-05-06, 09:53 AM
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TigerDunes,

I completely agree that my reasoning may be flawed that is why I am asking for help. And I truly appreciate you and everyone else helping.

I fully understand the electric back up heat strip setup in the typical heat pump and how it also can serve as a stand alone emergency heat system.

Here is where I may be flawed in my thinking. I assumed that the gas furnace would come in and “supplement” the heat pump on those coldest periods just as the electric strips do. Does it work differently? Is it one system or the other running at any point in time? The heat pump runs to a point where it can’t provide sufficient heat and then it completely shuts down and the gas furnace comes in?

If the above is the case then I can see your point on the benefits of having an air handler with electric heat strips (staged if possible) over a gas furnace backup. But, if the gas furnace “supplements” then it seems like the comparison to baseboard heat is reasonable in that the back up system, whatever it is – baseboard, heat strips in the air handler, or a gas furnace simply runs along with the heat pump until the set point on the t’stat is reached and they all shut down or step back one stage. So I guess the question really is does the gas furnace supplement or replace the heat pump when the heat pump can’t keep up?

I have been studying the product sheets of several condensing units and have found just what you are speaking of with regards to the highest SEER ratings are often with a certain variable speed air handler and the gas furnaces sometimes are a little less. Just goes to show how different a system might be than the manufactures advertised “up to SEER” rating. I had found that I should be able to match a system up that will get me within .50 SEER and HSPF of the highest rated setup for a given BTU output. But, that assumed the gas heat supplemented the heat pump so I may have to re-think this if that is not the case.

Many thanks,

Mark
 
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Old 09-05-06, 10:00 AM
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Can the Trane heat pump be operated properly with Carrier controls?
No way would I try that

A 2 stg heat pump will help with better dehumidification and operating costs. You have to determine if the extra cost is justified.

That is why I say stay with the single stage compressor and a V/S blower . It has a humidistat control on it that will slow down the blower till it gets the humidity down then goes back to high speed.
On the furnace I cant say what one is best. Its been so long that we have sold or put one in I dont no.
We started to put heat pumps in back in the 70's with electric heat back up. Only have some one call about a high bill if a sequencer get stuck on.
On the COP always had used 3.0 . But forget the COP its all in the HSPF now. That ## is from the average from a -17o to +47o.

Not to long ago here in MO. they said that at 0o you got a $1.08 of heat from a heatpump as a $1.00 of electric heat in anything else.

Now to look at the ### of units you are looking at go to. http://www.aridirectory.org/ari/unitary.html
This will tell you what the HSPF on all are. Also if you are going for a tax credit on this new unit. With the bill of sale on it you also have to have a print out from this www of the unit. Too turn in to the IRS
Dont forget that with the gas furnace you have to go over to it before the heatpump has to go into a defrost mode
ED
 
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Old 09-05-06, 10:28 AM
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Ed,

See my earlier response where I discuss the reason why the 15 SEER system doesn’t seem to make sense for me.

I understand that down there in God’s country you don’t have much need for gas furnaces. I’m in the foothills of the Blue Ridge Mountains of Virginia. Noticeably chillier up here in the winter.

I understand that the gas furnace will have to come on during defrost cycles just as the heat strips come on. I am assuming the dual fuel t’stat should cause the system to function that way.

Mark
 
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Old 09-05-06, 10:44 AM
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With the gas the heatpump coil will be on top of the furnace So it will make the freon gas hot if in defrost. In the electric back up the heater is after the heatpump coil so the heat dont go over the coil.

ED
 
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Old 09-05-06, 11:30 AM
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So how does the defrost cycle of a heat pump work with a gas furance?

I have never seen it done, but is there a reason the coil can't be located before the heat exchanger such as right at the return air connection to the unit (like where an electric air clearner is often connected)? In that case it would be before the blower also. I don't know how that would affect the functioning of the blower or the coil.

Mark
 
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Old 09-05-06, 11:33 AM
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mchild

It's either the heat pump or backup furnace operating-never both. Heat strips with a heat pump paired with an air handler provide aux heat-the amount of supplemental heat to meet the BTUs required to heat your home on those cold days. The backup furnace though provides the total amount of BTUs required to heat youir home on those cold days.

Since you indicate you live in Virginny, check with your electric utility on an all electric rate for heating.

IMO
 
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Old 09-05-06, 11:41 AM
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TigerDune,

Ahhh, so there is the flaw you were mentioning. I do plan on contacting my electric company once I have a clearer view of what I am going to do.

So, just to satisfy my natural curiosity, why couldn’t the gas furnace be down stream from the evaporator coil (see my earlier post) and function as the heat strips do?

Mark
 
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Old 09-05-06, 12:17 PM
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just to satisfy my natural curiosity, why couldn’t the gas furnace be down stream from the evaporator coil (see my earlier post) and function as the heat strips do?

CAUSE The cold air as it passes over and around the hear exchange will sweat and rust it right out. I have had to take two furnace out that some one put in that way.

ED
 
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Old 09-05-06, 12:37 PM
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Okay, I think I understand.

If there is a combo of a heat pump and gas furnace, when the heat pump goes into defrost then the furnaces comes on which further heats the evaporator coil and thaws the condenser with super heated refrigerant. Right?

Mark
 
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Old 09-05-06, 01:16 PM
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Again, I appreciate all that I am learning here.

One other question comes to mind on the two staged heat pumps. Since my heating and cooling needs are different for this system, could the two staged heat pump be more effective in meeting the divergent needs of both?

My cooling needs are about 24,000 BTU’s and my heating are about 44,000 BTU’s. With these needs, if I were to use a single stage compressor then it would have be rated at 24,000 BTU’s since making it larger would create humidity problems in the summer. Then, during the heating season, since it is 20,000 BTU’s short of what is needed, then the heat strips would be called on frequently to make up the difference. Thus, there would be a higher energy cost due to the high usage of the heat strips.

But, with a two staged 4-ton heat pump the first stage of cooling would produce about 24,000 BTU’s which would be sufficient for the cooling and de-humidification needs and the second stage at 48,000 BTU's would be sufficient for the heating - except during the coldest periods when the heat strips would supplement.

Is this practical?

Mark
 
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Old 09-06-06, 05:10 AM
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mchild

While your idea on oversizing with a 2 stg HP is reasonable, you must be very careful if you pursue this course of action. I think using a 4 ton unit is wrong. You must know what percentage BTUs in cooling a particular 2 stg model will provide in low stg. Keep in mind your ductwork must match in size the BTUs output. For instance, you could consider a 3 ton high eff heat pump that produces 24k BTU in low stg(67%) and 36k BTU in high stg. You still would require heat strips but I think this might be a good strategy but of course you should review this carefully with your selected dealer.

IMO
 
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Old 09-06-06, 06:14 AM
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TigerDunes,

I plan on doing just as you suggest. I was really asking in a general sense as to whether this appoach was reasonable - the BTU requirements have not yet been finalized so I need to do that before taking this thought any further.

Before making any equipment seleciton I would make sure that it, in fact, meets the cooling and heating needs. That is, the first stage must properly provide for the cooling and de-humidification needs and the second stage provide a reasonable contribution to the heating needs. I understand that the first stage isn't necessarily 50% of the total.

Plus, as you suggested, I have to make sure the ducting is correct. And, as you also suggest, I think based on duck sizing limtations a three ton would probably work without changing the ductwork (wish these systems came in half ton increments).

The purpose for all of these discussions is so I can be educated enough that I don't get "sold" whatever is sitting in the back of some dealers warehouse. Every home I have owned has had the A/C oversized and my current oil furnace is about 150% larger than what is needed. I think I have about one ton excess cooling too (each of the two systems about one half ton oversized).

Thanks again,

Mark
 
 

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