Over sized furnace


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Old 08-21-06, 01:39 PM
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Over sized furnace

I have an older home that is new to me. I have been looking at the HVAC systems with the thoughts of some upgrading and also trying to figure out how to cut back on my costs to heat and cool as the energy usage has been much higher than I anticipated. All the HVAC equipment was installed in 1997.

Currently there is a three ton heat pump for the second floor and an oil furnace with a 2.5 ton air conditioner on the first floor. I have used the Warmair.com site to calculate the heat loss/gain in order to verify the sizing of my existing systems. In the case of the second floor heat pump (both heating and cooling) and the first floor air conditioner the calculations came pretty close to the size of my equipment. On the other hand, the heat loss for the first floor was calculated at about 40,000 BTU while the existing oil furnace is rated at 96,000 BTU output (based on about 81% efficiency).

I am well aware of the issues of over sizing an air conditioner, but I donít know what the issues are in the case of an over sized oil furnace. So what are the implications of having a heating system that is so much larger than necessary? I have noticed that other than the coldest periods it will often have short cycles, but is that a problem for oil furnaces? Could I realistically save any money going with a system that is properly sized? I have propane so I could switch to a propane furnace if there are cost savings to be realized.

Many thanks,

Mark
 
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Old 08-21-06, 01:57 PM
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I have a feeling that your furnace was originally sized to heat the entire house.

Yes, there certainly are ramifications to short cycling an oil-fired furnace. Wear and tear on the equipment is the first thing that I would be concerned about although the first noticable thing would probably be uneven heating in the various rooms of the house.

As far as the equipment angle, there is the heating and cooling of the combustion chamber and heat exchanger. Most likely you are not running the furnace blower continuously so that would also be subject to the stresses of stop-start operation.

You are probably running somewhere between a 0.8 and a 1.0 GPM nozzle on your burner. You could try lowering that by about 20% and see how that increases your cycle time. One thing to be aware of is that a lower fuel rate will translate into a lower exhaust stack temperature and if allowed to go too cool it will lead to increased corrosion rates in the final stages of the heat exchanger and in the stack piping. It is best with a non-condensing furnace to keep the stack temperature no lower than 350 degrees F.
 
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Old 08-21-06, 02:21 PM
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furd,

Thanks for your thoughts. You seem to always have an answer for me.

I concluded that the size of the oil furnace was due to the fact that smaller oil furnaces are hard to come by. Maybe due to the stack temp issue you mention.

I currently have a .85 nozzle and have thought about going smaller. But, I wasnít really sure what the impact would be and whether I could save any on reduced fuel oil consumption. My stack temp runs about 500+ now so I should be able to reduce some amount.

I do keep my blower fan running continuously and that must be the reason that I donít have a more noticeable temp swing. When I realized how big the furnace was, the first thing I thought about was temp swing, but it was not bad this past winter (first in this house).

Do you have any idea if reducing the nozzle size will help with reducing fuel oil consumption? Or, if getting a properly sized propane unit will cut my heating costs?

Thanks again,

Mark
 
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Old 08-21-06, 02:45 PM
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With that high of a stack temperature I would probably try a 0.65 or 0.70 GPH nozzle. Nozzles are cheap but unfortunately combustion testing and adjustment is usually not a DIY project because of the cost of the equipment.

Anything that will lower your stack temperature will effect a fuel savings. You may want to check out the fuel cost comparator at

http://www.warmair.com/html/fuel_cost_comparisons.htm

to see if a change to propane might be cost-effective. You must, of course, work into the cost/benefit analysis the cost of a new furnace (with installation), the length of the payback term and the cost of borrowing the money (if necessary) to finance this endeavor.
 
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Old 08-21-06, 05:25 PM
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Nozzle size

Be careful in reducing the nozzle size. I don't think I would drop lower than .75 to start. If you can provide furnace & burner details I might be able to provide some better information. Of particular intrest is the furnace make & model as well as the make of the burner. Proper combustion testing is of utmost importance.
 
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Old 08-21-06, 05:46 PM
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furd,

Does lowering the stack temp tend to increase or decrease fuel consumption?

I have done the fuel comparison analysis and oil is a little cheaper. Therefore, my question is what kind of savings might I expect with going to a properly sized furnace - whether it be oil or propane? Or, asked another way, how much excessive energy am I using with equipment that is about 140% to large?

Thanks again for your thoughts,

Mark
 
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Old 08-21-06, 05:54 PM
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Grady,

Can't find all that you asked for. The label says that it was manufactured for:

Heat Contoller, Inc.
Jackson, Michigan

The model number is OBUA95-D3.5S. i couldn't find anytihng of the burner.

Thanks,

Mark
 
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Old 08-21-06, 06:01 PM
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Furnace

Following a hunch, I checked R.W. Beckett's OEM Spec Guide & found the burner specs for your furnace. It has a Beckett AFG burner with an F3 burner head. The factory nozzle is spec'd at .85 gph x 70º with a solid cone. Having an F3 head, You should not drop below .75 gph. Stick with the 70º solid cone nozzle. You will have to reduce the air by one or two numbers on the air shutter. I strongly urge having a pro come in & do a proper combustion analysis. Basic parts changing on an oil burner is pretty simple but adjusting the firing rate really should be left to someone with the proper test equipment.
 

Last edited by Grady; 08-21-06 at 06:21 PM.
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Old 08-22-06, 06:39 AM
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Grady,

Thanks for looking that up. I do plan on having my service company do the work if I decide to go forward with it.

Dropping the nozzle to .75 is a 12% reduction in the amount of fuel that the furnace can burn in a given period of time. Is it reasonable to assume that the functional capacity of the furnace is also reduced 12% to 84,000 BTU? If so, then that makes the furnace only about 100% over sized - which is better.

But, will I actually see a savings in heating the home in the amount of fuel oil consumed? Seems to me that I will have some reduction in the number of short cycles and thus it will be better for the longevity of the equipment, but not much if any reduction in overall consumption of fuel oil.

Thanks again,

Mark
 
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Old 08-22-06, 06:59 PM
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Run time vs. fuel consumption

On each start up, it takes time for the heat exchanger to warm & during that period, you are getting virtually no heat into the home. It is, for all practical purposes, going right up the stack. This being the case, you certainly should see a reduction in total consumption, with everything else staying the same.

If you want to drop the input below .75 gallon per hour, you would need to change the end cone to a "F-0", not a big deal at all. The two things you need to watch out for are maintaining the minimum temperature rise across the heat exchanger (as specified by the manufacturer) and keeping the stack temperature high enough to prevent condensation (generally about 350ļ).
 
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Old 08-22-06, 07:44 PM
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Grady,

That is really good news. How low would you guess I could go if I changed the cone? If I remember correctly my stack temp was in the 500 - 550 range with my current .85 nozzle. I don't know what the temp across the exchanger was. I would love to get the output down as close to the needed BTU as reasonably posssible considering the temp limitations you mentioned.

One other thing that I failed to mention is the system is vented through the wall with a draw fan. Does that have an impact on this the lower limit of the stack temp?

Thanks again for your help.

Mark
 
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Old 08-22-06, 07:54 PM
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Thumbs down Sidewall Vent

Man, do I ever hate sidewall venting an oil fired appliance. Talk about an energy waster, they are probably number one. Draft control thru the heat exchanger with these things is nearly impossible. They just cause heat to be ripped right thru the furnace. You want to save money? Put up a chimney.

If you change the end cone, you might be able to drop the firing rate to .60 gph.
 
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Old 08-23-06, 06:38 AM
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Grady,

Well Iím glad I remembered that little tidbit about the side wall installation. Is the issue the amount of air the vent fan pulls through the exchanger and with it lots of heat?

Knowing that, what would you guess I am losing with having this side wall installation? Would it be as high as 30% of my heat is going out the vent?

I can see a scenario that with loosing huge amounts of heat out the vent and then going down to a much smaller nozzle I may not have enough heat for the house.

Many thanks,

Mark
 
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Old 08-23-06, 03:31 PM
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Sidewall vent

No, you are not loosing that much heat just more than you would with a chimney. My experience with sidewall power vents has been anything but good but you could be one of the lucky ones who has little or no trouble.
 
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Old 08-24-06, 12:29 PM
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Grady,

I contacted the tech service advisor at Heat Controller and asked how much I could de-rate this furnace. They said this particular model does not allow for much, if any, de-rating and said that a .75 nozzle would probably be as low as I as could go. Not much of an improvement from my standpoint.

From your comment that I might be able to go lower by changing out the end cone and then use a .60, I was beginning to think I could actually get down to a reasonable BTU with this system. But, I guess not.

Thanks again for all your help,

Mark
 
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Old 08-24-06, 03:49 PM
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Downfiring furnace

The big thing when downfiring to being able to keep the temperature rise across the heat exchanger within the manufacturer's specs. If you asked, did they tell you what the minimum acceptable temperature rise was?
 
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Old 08-24-06, 04:05 PM
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Wink

Grady
You think they could cut the nozzle to a 65 GPH and kick up the pump PSI????

ED
 
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Old 08-24-06, 04:19 PM
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Nozzle size vs. pump pressure

Ed,
By doing so you are virtually defeating the purpose of dropping the nozzle size. For example a .65 nozzle @ 120 psig will yeild about .71 gph & at 140 psig it will give you about .78 gph.
 
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Old 08-24-06, 06:33 PM
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Grady,

They inferred that the temp rise would be the problem with trying to de-rate it. They said the 84,000 BTU model that was sister to mine was possible to de-rate, but not mine. They said the minimum temp rise would be listed on the label. I checked and it states 50o to 80o F.

I mentioned changing the cone and was told it would not help.

Mark
 
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Old 08-24-06, 06:48 PM
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Downfiring

Changing the end cone would allow you to drop the nozzle size & still get a good flame. The temperature rise is a different issue. Depending on the fan size & speed as well as the restriction from the ducts, you may or may not be able to stay within the limits. Do you know how much temperature rise you have now after the furnace has been running steady for 5-10 minutes? To check this you need two thermometers. Put one in the return duct as close to the furnace as possible & the other in the supply just downstream of the A/C evaporator coil if you have A/C, without A/C put it just after the first 90ļ turn the air would need to make as it leaves the furnace. Subtract the return temp from that of the supply & you have the temperature rise.
 
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Old 08-24-06, 07:28 PM
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Grady,

No I don't know my temp rise. I guess I can fire her up and check it. May get a bit warm in here.

Thanks,

Mark
 
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Old 08-25-06, 04:50 AM
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Temp Rise

If you choose to check the temperature rise, you will also need to verify the fan is set to it's lowest speed.
 
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Old 08-26-06, 07:25 AM
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Temp rise reading

Grady, et al,

I have checked the temp rise as suggested. I first rewired the fan motor to operate at the lowest speed when in the heat mode. I ran the furnace for 11 minutes continuously and checked the intake temp (about one foot from the blower) at 80.1 and the exit temp (just beyond the A coil) at 140.5. Thus, there is a total rise of 60.4. Tell me if Iím wrong, but that doesnít seem like much room to de-rate this and keep it at a minimum rise of 50.

Mark
 
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Old 08-26-06, 05:41 PM
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Temp rise

Good work, Mark. You learn well, Grasshopper.
You are absolutely right. There does not seem to be much room for downfiring. If I had to guess, I'd say .75 is going to be about it. Sounds like sombody did a good job on the ductwork. This is a problem when using one appliance to do two jobs (heating & cooling). The ducts have to be sized for the heavier air flow load. I have long been, and continue to be, a big adovcate of heating with a heating system & cooling with a cooling system.
 
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Old 08-26-06, 07:17 PM
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Grady,

Learning, but the Master still has the stones.

Many thanks,

Mark
 
 

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