How To Tell If Furnace is Correct Size?


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Old 02-09-08, 05:00 PM
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How To Tell If Furnace is Correct Size?

I'll start by trying to answer the questions the sticky post said you need before you can help:

1. I live in Albuquerque New Mexico. In the winter, temps can be in the teens. in the summer 100+ is not unheard of.
2. House style and construction details. Pueblo Ranch style made of Concrete Stucco (wood frame) with a "flat roof". ~2400 sq. ft.
3. Make, model and age of equipment related to the problem. Rheem RGPH15EARJR - almost 7 years old.
4. Fuel type. Gas - blown air
5. Water temperature and pressures of boiler systems. N/A
6. What type of zoning do you have with your boiler system. N/A
7. Thermostat type. (Huh?) Honeywell programmable?
8. Anything else that would be useful. Floor plan of our home: http://i36.photobucket.com/albums/e4.../FloorPlan.jpg

We had this home built in 2001. The house is basically divided into three sections (see floor plan): The master suite section, the living/dining/kitchen section, and the smaller bedrooms and office section. We have always thought it was cold in the master suite suite (on the west side of the house)during the winter. It is furthest from the furnace and has only one door connecting it to the rest of the house. Just stepping thru the door, there is a noticeable temperature difference. During the day I measured 67 degrees in the master while the living area is at 73 degrees.

My theory is that this is the result of a poor floor plan design: Master closed off from the rest of the house and far away from the furnace. But, I'd like to know if my furnace might be undersized.

How can I tell if my furnace is sized properly for my square footage?

Thanks in advance.
Phil
 
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Old 02-09-08, 06:23 PM
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Only way to no if it is sized right is to do a heat load cal on it. Un even temps are almost always do to poor duct design.
 
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Old 02-09-08, 06:43 PM
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Airman pretty much told you all. I will add that poor duct design is far more common than you might think.

Can you modify your floor plan to show where the supply and return ducts have their registers? If you could add the dimensions of the ducts it might help.
 
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Old 02-09-08, 07:41 PM
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Gas and Oil Heating Furnaces > How To Tell If Furnace is Correct Size?

You may want to consider looking at your supply AND returns.

The far end of your house is probably getting less warm air supply and probably is not picking up the cold air from the floor. This prevents enough warm are from heating the room and fools the thermostat that is is a different room.

With one level, just getting the cold air back to the furnace in the winter should help a great deal since more warm air can heat the room.

Dick
 
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Old 02-09-08, 09:13 PM
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As other has said, may be a poor duct design. I am taking it you have flex duct in the attic, and all your vents are in the ceilings.

What makes you think it's undersized? if it was undersized, on the coldest day, it runs non stop, and not able to bring up temp.

With the model you shown, you have a 150,000 BTU furance, that's a LOT of heat... It could be it's oversized.. Oversized furance is not going to run long enough to allow the far rooms warm up.
 
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Old 02-10-08, 06:43 AM
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Thermostat location also plays a big part. If you modify that floor plan picture to show the ducts please also show the thermostat location.
 
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Old 02-10-08, 07:36 AM
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MRPB,

I have just begun to learn the basics of HVAC recently, so I would not presume to give any one advice (in fact I am still behind the 'eight ball' with my furnace at the present--as my avatar indicates); but, your situation seems so similar to mine that I would like to share a few things.

First, as Jay said your problem might be too much Btu's. In the front of my ranch (kitchen & living room it is always noticeably warmer than in the back bedrooms. My thermostat is in the hall between the front and back. Part of this is due to the fact that I have a garage on the one end, and the two back bedrooms have two exterior walls. But, my furnace is oversized at the present and the burner (I have oil heat) only runs for about 5-6 mins tops. I think this is called "short-cycling" and this explains part of the poor comfort level in the back of the house.

But, otherwise something interesting happened recently with this situation. I have always had higher humidity in the back of the house as well . . . but when I put in a pipe to bring in some outside air for combustion air and make up air the humidity leveled out and was the same in the front of the house as the back, and as well the temps got closer, I was almost 4 degrees off, now I am just 2 degrees off. I'm working in another thread with Grady to change nozzles to bring down my Btu's. If I understand this correctly, when I reduce the output and the energy consumed, I will cause the furnace to run longer, about 15 mins for the burner cycle (as opposed to 5-6 mins). I'm thinking this has got to even out the temps quit a bit, if not all the way after bringing in outside air.

For what it's worth.

Rick
 
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Old 02-12-08, 05:33 AM
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Originally Posted by Jay11J View Post
As other has said, may be a poor duct design. I am taking it you have flex duct in the attic, and all your vents are in the ceilings.

What makes you think it's undersized? if it was undersized, on the coldest day, it runs non stop, and not able to bring up temp.

With the model you shown, you have a 150,000 BTU furnace, that's a LOT of heat... It could be it's over sized.. Oversized furnace is not going to run long enough to allow the far rooms warm up.

OK, based on what you've all said, the furnace is probably not undersized. Here's an updated floor plan with register and thermostat location. http://i36.photobucket.com/albums/e4.../FloorPlan.jpg

You are correct there are flex ducts in the "attic" with registers in the ceiling, but there is no access hole, so I can't tell you what size the ducts are. From memory seven years ago, I think they were about 12" - 18" in diameter.

Thanks to you all for your help so far, what now?
Phil
 
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Old 02-12-08, 05:44 AM
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To start off, I would close down the three inner vents in the living room/kitchen and maybe the one in the dinning room.

Also with your vents in the ceiling, not going to mix with the cool air in the floor.

Also it is common for furnace in the south to be oversized in heat due to they need larger blower for cooling.
 
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Old 02-12-08, 10:41 AM
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In my opinion you have a poorly designed duct system.

Generally speaking the supply vents should be located to "wash" the exterior walls. This attacks the heat loss (or heat gain in summer with A/C) where it first hits the house.

You show only the single return air grille. In your spread out house you really need several return air ducts and I strongly suspect that the duct from your existing return air grille to the furnace is undersized.

At the very least you need a return air duct from your master bedroom.

Re-locating the thermostat might also be advisable after you get that new RA duct in place and you would need a complete balancing job done on the supply ducts.
 
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Old 02-12-08, 11:12 AM
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i agree with Furd, more return is needed. but it not a common thing down south to have return in every room like we do up here in the north.. Guess they figured it's too much work in the attic and being cheap.
 
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Old 02-12-08, 11:58 AM
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Wink

First off here. Do you close that bed room door all the time?? With no return in that room . Do you have about 2" to 3" that the door clears the floor or rug???? To let return air get out when the door is closed?
On the 12"X5" registers Just what kind are they ??Do they throw to the out sidewalls or not?? Like said we take the runs to the outside walls all the time.You might get registers ,ones that will help you blow the air where you need it. Back to the pipe size that would help to know . If the furnace run is just short runs. You might be able to deorifice it or cut out a burner??? So it and the blower will run longer. How does the AC do ???????

Rick's Furnace. Most of the time you can drop the nozzle size down one size. You have to check and see what size your chamber is there and what size you can use in it.

just my .02 cents worth
 
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Old 02-13-08, 01:45 PM
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OK, the consensus seems to be poor duct design: Is it worse when I tell you that I forgot to mention the return air duct is also in the ceiling?

Yes, my wife keeps the bedroom door closed a lot, but it seems colder even when the door is left open.

Closing inner vents in the kitchen/living room and possibly the dining room seems easy enough.


Adding another air return with ducts and all in the ceiling for the master suite sounds expensive (or out of my skill set ). What if I vent the wall between the bathroom and vestibule down near floor level?

I could probably move the thermostat, if needed.

The air registers have internal louvers that move, but are not easily aimed at the external walls. Maybe they don't do that here because of the moisture in the air: we don't have refrigerated air - we use a swamp cooler in the warm months.

Talk of reorficing and dropping nozzle size went over my head. Please explain?

Thanks again,

Phil
 
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Old 02-19-08, 04:04 PM
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The furnace is over sized. google this what are the problems with an oversized furnace. http://www.oec.ca/images/FurnaceOversizing.pdf
 
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Old 02-20-08, 03:31 PM
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Energywise, that is an interesting link but it is not the last word. A properly sized furnace MAY exhibit the same characteristics as an oversized furnace if the ductwork is inferior.

Unfortunately, it seems that inferior ductwork is more the norm these days,

Mrpb, forget anything concerning "dropping nozzle size" as it pertains to oil-fired furnaces. Re-orificing would mean changing the rate at which your gas furnace burns fuel and may be an option but your duct system is really the first problem that needs correction.
 
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Old 02-20-08, 03:40 PM
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Seems the uneven heating is the real problem

"During the day I measured 67 degrees in the master while the living area is at 73 degrees."

Closing registers is an iffy solution at best.

I would say (as others have) that the ductwork system could be better. Maybe leaving inside doors open a bit rather than totally shut will make a difference (so that air flows better throughout the house). If that solves it, transverse ducts may be the real answer.

One thing that might help is to have the fan setting on "on" so that it circulates air continuously. If you have a variable speed fan, this might do it. If the fan isn't variable speed, maybe not as good an idea.

At any rate it's not an undersized or oversized furnace causing the uneven heating.
 
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Old 02-20-08, 04:29 PM
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