Sizing thoughts?

Brian_58

I've had a few HVAC people out to look at my current system and prepare estimates. One quality that all these sales people have lacked is conviction in their recommendations and that really bugs me. The entire house isn't more than 1200sf and I need a 90%+ forced air unit . I've had recommendations of everything from 60,000btu to 115,000btu. How can one guy say 60 and the next guy twice that? The 60k guy said "I have double this footage and I have a 60k unit that works fine", i think he's insane. The old furnace was an 80% 70,000btu and I thought it ran a little too much (the motor burned out in 8 years). Since we're increasing efficiency common sense tells me 70k should be about right as a replacement. Any thoughts?

Weezbo

maybe you are right....

the furnace is only one component in the "system".

then again, maybe you are mistaken.

forced air "Systems" are not all created equal. there are variables in pipe sizing and workmanship that significantly reduce the effective delivery of the heated air...

Brian_58

out to look at the "system" I have and replace the furnace only.

Tony Shupenko

You need

to have a heat loss calculation performed. Without knowing insulation levels, glazing, etc., it is not possible to come up with proper sizing.

However, be that as it may, 60,000 to 70,000 btu/hr seems a reasonable range. 115,000 is way oversized.

When you wrote that you thought the unit operated too much, did you mean at the design day (very cold) or at a more moderate temperature? Are you having any comfort problems? Typically, a system should be sized to pretty much run flat out on the design day (thought not may Engineers or Contractors will do this for the dreaded fear for a call-back for lack of heat (read Dan's boiler sizing article in the archives). You want to do this for most effient operation.

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Brian_58

I was wondering how long before performing a heat loss calc. came up! I'l tell you one huge pain in the butt (and then you can lecture me), is having 3" ductwork. Yes, I did not mistype, I have 3" ductwork, and I know its an odd size, and I know it plays into the sizing of a furnace, obviously. One contractor told me he wouldn't even mess with the 3", he'd replace everything with 6", but changing everything over to 6" was at least another $2000 w/o question. I'm not marrying this house and $2000 in ductwork alone in a 1200sf house is nuts in my book. In this case, I'm more worried about functionality of the system than I am about the way things would be done if we had bottomless checkbooks. Everything in this old house is screwed up, why would the heating system be any different?

mitchb

hi Brian

Where are you located? that is a big factor too.

I too have a house 2200 sq ft, with a 2 stage 60 (36 on low fire, 35 year old 4 level side split, all original windows, doors etc, and my unit stays on low fire 90% of the time.

I am in S-W ontario in Canada.

The one guy is not nuts. I yanked a 120,000 unit.

If going hi efficiency you want it to run longer. If you havea 80% @70,000, then 56,000 heats the house. upgrade to a 95% and you only need a 60,000. (56,000 / 0.95 = 58,900

Go 2 stage, and save even more. I do a lot of forced air (95% of the business here) and oversizing is a MAJOR problem everywhere I go. I back up my installs saying if it doesn't heat I replace for free with larger unit. Have yet to do it.

Tony Shupenko

I sense some frustration with the home in general..

and I understand you reasons for not wanting to spend alot (time and/or $). I guess you can refer to some Btu/hr/sq. ft rules of thumb, depending on a qualitatively assessment of of insulation and overall tightness. So say you used 50 btu/hr/sq. ft, you are still coming up with a need for 60,000 + the efficieny, so 65 to 70 again looks about the size.

Even though you got 3" ducts, are you getting the air flow you need? If there are some pockets of cold, maybe you could add a few more branch runs (may also improve fan operation and efficient).

If you are also replacing a cooling system at the same time.... yikes, lets not go there now.

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Brian_58

It couldn't as easy as replacing a cooling system, we're INSTALLING one! And i know, that's where the 3" duct really creates a problem.

mtfallsmikey

Decisions

Yes, the ductwork sounds anemic, $2K to replace, I dunno.But, if you are going to live there for a long time, and not play "flip this house", it is more than worth the investment to properly size,and install the furnace, ductwork, etc. Any mechanical, plumbing, electric work is expensive nowadays..not only labor, but materials are at price levels unheard of since I left the biz almost 10 yrs. ago. You should seek out a contractor in your area that hopefully is listed here on this site..Educate yourself too..

Boilerpro_5

Get a proper heat load...

As to guessing, I have a 1905, 2,800 sq ft two story above grade,1400 sq ft basement,700 sq ft of glass, 9 foot ceiling down and 8 up and only need about 70,000 btu/hr input on the design day here in Northern Illinois. I have hot water heat, so I can go smaller than forced air typically, because I dont' have duct leakage, pressurization in the home and other waste. 60,000 input is quite possibly still too big.

Boilerpro

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