New gas furnace quotes - size/btu differences


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Old 10-02-12, 05:08 PM
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New gas furnace quotes - size/btu differences

Been getting quotes for a new gas furnace and im getting different btu sizes from each installer. Who do I trust?

Heres some info on the house - 1974 split level, attached 2 car garage, full finished basement, original windows, good insulation in the attic. One large picture window in the upstairs living room otherwise the rest are regular windows. upstairs is 846 sq ft, downstairs is the same. house is in central iowa so we get some cold winters.

Furnace right now is an 80%, 81,000 btu furnace crane.

One company is suggesting a 95%, 50k btu(large nationwide company, did a load test)

The other guy is saying a 95% 70k unit. (i checked and the next smaller size in his model is 45000)(one man business, hear great things about the guy, much much cheaper then the others.

This seems like a big difference and I dont want to invest that much and have an issue can anyone shed some light on this?

I had two other installers give quotes but they will only determine the size I need when I set up an apointment to have them install the new one(which seems kind of weird to me)

Let me know you guys think. And thank you very much.
 
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Old 10-02-12, 07:34 PM
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A load calculation is important for proper sizing. Do you know what the load actually came out to? As to the guys who wont do the load until you buy... Running a load is time consuming and costs the company money with no guarantee of return on their investment. Having said that, without a proper load calculation how do they know how to give you a price based on the size of equipment you need? It is very well likely that your current furnace is oversized. Is the 70K unit 2 stage?

PS. Remember you get what you pay for in most cases.
 
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Old 10-02-12, 08:32 PM
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If they really calculated the load, this outfit provided you with a competent recommendation. My bias would be to favor this contractor.


>


It used to be quite common to eyeball an installation, decide on what size furnace might be needed and then size it up to be sure you didn't get a complaint about not getting enough heat on a cold day. Sounds like this guy is that kind of operator.

>


This is an honest approach. They are saying they will invest the time to determine the size of furnace you need after you sign a contract.


What homeowners OUGHT to do is to PAY a good contractor or repairman to do a load calculation and provide advice on what kind of furnace installation might be appropriate. Then you can deal from a position of knowledge and expertise with contractors who can bid on what you actually want and need.

But few people are willing to make that investment. As a result contractors are left trying to make a buck without getting burned by homeowners who want to get valuable information from them without paying for it.
 
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Old 10-02-12, 11:16 PM
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If your furnace is from 1974, anything of equal output capacity will have much higher airflow requirements than your existing furnace.

70k 95% has a very similar output rating to a 80k conventional efficiency unit but needs around 40% more airflow to run at manufacturer's specs.

Odds are that your air ducts are too small for 70k.

That being said, load calcs can only be as accurate as the input data. Significant errors (due to not checking for attic/basement insulation, window type, incorrectly estimating air leakage) can throw off the result off by a few thousand BTUs.
 
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Old 10-03-12, 05:53 AM
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Yes they are both 2 - stage

Load rating calculated came to 42047 btu.

I check on different sizes of the furnaces they priced and it looks like its either 50k or 70k, that seems like a big jump isn't it?

I'm trying to do the right thing here and get multiple quotes, ask lots of questions and get a realiable contractor but really, until it's done and it gets below zero outside there is really know way to know I made the correct decision. So far every contractor(have had four come out) has given different recomendations on others things that should be done as well. I would gladly pay for a correct load calculation but now do I know that it was done correct anyway?
 
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Old 10-03-12, 06:12 AM
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If the load came to 42 then a 50k will be fine. Normal sizes increase by 20k.
 
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Old 10-03-12, 11:16 AM
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My house originally had a 60k BTU furnace and because I wanted a two-stage with variable speed blower the minimum size available was 70k BTU. When it came to being installed the salesman substituted a 90k BTU model. I didn't catch it until the installers were gone and when I called the salesman his response was that he didn't charge me any more for the larger furnace and this way he could guarantee it would never fire at the higher output unless the outside temperature dropped below zero, VERY rare in my area.

I demanded that he install the 70K BTU model and they did. Since then (2006) the furnace has never fired on the second stage UNLESS I manually raised the thermostat setpoint more than the three degree differential for engaging the second stage.

If your heat loss calculation is approximately 42,000 BTUs the 50k BTU two-stage furnace will be a good fit. It will run for a longer period of time on the first stage than did your previous furnace but still burn less fuel and comfort will increase. Be sure to get a two-stage thermostat for this and do NOT let the salesman pressure you into accepting the timed delay in the furnace instead of a true two-stage thermostat.
 
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Old 10-03-12, 01:21 PM
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No.


Think about it. A furnace is usually GROSSLY oversized for the heating load except during the coldest days of the year.

During all but the coldest days the furnace only operates intermittently to maintain the desired temperature.

So a close match between the furnace BTU output and the BTU/hour needed on the coldest day of the year isn't especially important, as long as the furnace is big enough to heat the house on those days.
 
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Old 10-03-12, 02:35 PM
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Oversizing is not smart at all.

The largest unit you should consider getting is 60k.

The lifespan and efficiency of modern furnaces can be greatly reduced if the ductwork is undersized. Units with variable speed blowers attempt to compensate for restrictive ductwork, resulting in excessive noise and premature motor failure while furnaces with conventional motors simply don't move enough air to work within manufacturer's specs. (leading to heat exchanger failure)

Going from a temperature rise of 70-100F (old furnaces) to 40-60F (new mid/high eff models) without reducing capacity means increasing airflow dramatically. A 1600 sq ft house is not likely to have large enough ducts for the 1100-1200+ CFM a 70k unit is designed to move.

The other thing is that oversized units don't run long enough to even heat evenly or even reach rated efficiency. Short cycles also cause significant temperature swings.

If you go for a 2-stage 70k furnace, it will operate like a correctly sized single stage model and never hit high fire. (defeats purpose of getting a 2-stage system to begin with)

Furd's advice is very good.

The bottom line:

The longer the cycles are, the better - provided that sufficient heat is produced to maintain the setting on the coldest night of the year.
 

Last edited by user 10; 10-03-12 at 03:25 PM.
 

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