HVAC anyone?

Paul1000RR

Paul1000RR

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Furnace went to hell and time to get another one. Trying to figure out how to calculate the necessary btu rating for my home, total square footage is 1920 including basement. From what i read its recommended 50-60 btu per sq in colder climates which would work out to 90,000 - 115,2000.

However had a guy quote me and say i only need a furnace with 40,000 - 60,000.

Anyone work in this field and know the truth?

Old one was 80,000 at 80 percent efficiency
New one is 40/60,000 at 95 percent efficiency

Thanks
 
Don't buy a furnace that is too big. Sometimes less btu is better than more btu because otherwise it doesn't operate in the right power band so to speak. The goal is to maintain a level even temperature. Too little btu means it will be going flat out all the time. Too much btu means it'll be cycling on and off too much and delivering spiky heat. In the middle means it'll be in the lower stage more without cycling on and off so much (especially at max).

Don't include your basement in the calculation either unless it's fully renovated and you live in it.

An estimator will give you their calculation.

The discussion about 2-stage vs modulating is something you have to decide for yourself, but I can tell you that a modulating fan is a good idea in my opinion no matter what type of machine you have.

Installer matters more than the brand I feel. You want to have a good installer, experienced, not a hack. Ok to have a junky machine if it is set up and backed by a real pro.
 
Sunspark said:
Don't buy a furnace that is too big. Sometimes less btu is better than more btu because otherwise it doesn't operate in the right power band so to speak. The goal is to maintain a level even temperature. Too little btu means it will be going flat out all the time. Too much btu means it'll be cycling on and off too much and delivering spiky heat. In the middle means it'll be in the lower stage more without cycling on and off so much (especially at max).

Don't include your basement in the calculation either unless it's fully renovated and you live in it.

An estimator will give you their calculation.

The discussion about 2-stage vs modulating is something you have to decide for yourself, but I can tell you that a modulating fan is a good idea in my opinion no matter what type of machine you have.

Installer matters more than the brand I feel. You want to have a good installer, experienced, not a hack. Ok to have a junky machine if it is set up and backed by a real pro.
Click to expand...

It is fully renovated and i do spend a lot of time in it.
 
Had mine replaced last year. I think I got 45k btu for 1000+ sq ft not including basement. Seems to work evenly, doesn't cycle on and off and roar like the old 20+ year old machine it replaced. It's a small house only 2 bedrooms and 1 bathroom. Basement is unfinished. From what you're saying, I don't think you will ever need more than 60k btu so the estimator was correct. You may very well be fine with 45 if you have a good modulating fan. It's not an exact science. The quality of your insulation and the quality of the load balancing plays a role. It's a forced air system. If you have too many registers open or closed in spots other areas will be affected, likewise the registers for recirculating (this air isn't used by the furnace, it gets the air from outside in high-eff) if you don't have enough or well placed "in" registers you end up with negative or positive pressure in spots. The ideal is when every floor can return air in a balanced manner. My house is not ideal. I only have return air on the main floor not top or basement.

Hi-efficiency will need to exhaust out the side of the house so if you currently exhaust out the roof that shaft won't be used anymore.
 
40000btu is too small for that size house plus basement. I'm not sure house layout, tin work sizing or if its a tight or loose house, but I'd think your looking in the 60/70,000btu range. You just don't want some hugh btu furnace cause then your heat cycles will suck and not provide a consistent comfy heated house.


Once you find the right size, you next get too ponder the wonderful world of brand type, which stage gas valve you want, single, 2 stage, etc etc, and the type of stage you want with the blower fan.


I myself like simple and cheap. Single stage furnace, 2 pipe venting and basic non programable stat. I personally don't think you save that much money buying multi stage fancy option furnaces.. Your on natural gas, it's way cheaper then oil, propane or electric so your already saving money :)


Furnaces are like buying a car, you want basic or you want fancy with features, both do the same, gets you from point A to B.


Others on here will give you some good opinions and suggestions..
Check out hvac-talk.com for good info.
 
I went through this two years ago. The best advice for your own peace of mind is to do your own heat loss calculation. I brought in 8 different contractors and none of them did a heat loss calculation. All of them just asked the square footage and looked at the old furnace (mine was 10 years old but the A/C was toast). I was told different sizes by different contractors. By doing my own calculation I made sure I got the right size. An oversized unit wastes energy and doesn't heat or cool effectively because it doesn't run for a complete cycle. If your old furnace is really old, >15 years then you may be looking at a smaller unit as today's designs are more efficient.

I used this site: http://www.hvaccalc.com/main.asp

For $49 you get a short lived subscription which lasts a couple of months. That's fine because once you've done the calculation you won't need the software again. It's worth IMHO.
 
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