Central heating downsizing, what to expect?

[PandaBear]

Currently my house (2/F, 1500sqft, living room is what they call a "cathedral ceiling" of 2 story tall) has a 100000 btu 4 burner furnace. It would turn on for a few minutes then pause for 15-30 minutes all the time. The biggest problem however is the noise it generates by the blower pushing exhaust through a vent inside the wall between 2 rooms, waking everyone up in the middle of the night. The next biggest problem is the high natural gas bill whenever we use it.

Currently we have a 2.5 ton AC that we just installed last year and use the same furnace blower fan and it is sized just right (IMO), very quiet (the air mover is not the problem, it is the powered furnace exhaust that is loud). So if I go to a small furnace, how many BTU / burners should I look for? Does it make sense to go to 93% or higher efficiency? (I understand that we have to adjust the water condensation in the exhaust but it is right there with the AC anyways).

Should I stay with PSC fan or ECM fan?

 

[bepperb]

Well I can't speak for your climate but in southeastern Wisconsin that would be overkill for a home your size. Our last 1300sqft had a 60,000 and the current 2000sqft has 76,000 btu/hr. And I would assume our climate is colder.

If I were you I'd shop for a 60,000 btu furnace. Or can you upsize the intake/exhaust to the next size larger PVC pipe and reduce the noise that way (at a lower cost)?

If your current furnace power vents out a PVC pipe (the way your post makes it sound) you will need a high efficiency furnace (80% models vent vertically though chimney unpowered). You would want that anyway, the labor will cost more than the furnace and the price difference isn't that big (probably a sub 5 year payoff, if that).

Your utility/state might have a rebate for a variable speed fan, ours does. And the noise might not bother you, but it would be an improvement.

 

[PandaBear]

Our vent out is through metal vent (zinc plated steel?), not PVC pipe. I would imagine my current unit is put in when the house is build because the builder has a surplus, make a demo home warm without the right insulation, or wants no call back after selling the house.

I would rather not modify any duct/vent work as it may requires tearing out some sheet rocks. The reason I "suspect" it is a power vent is because I hear the fan sound way before the flame is on (probably just funny control on the thermostat instead of a power vent).

So if I have metal exhaust vent I cannot use dual heat exchanger (90%+) furnace without modified vent, correct?

What is the smallest BTU furnace ever made? I would imagine a 36000 BTU would be enough for us (we usually turn the thermostat to 68, and if we do not use the heater the indoor temp is usually only 63-65).

 

[IndyIan]

That explains the 100k btu unit I guess...

Anyways maybe just sound insulate the furnace room would be a simple solution? When we insulated the whole house with roxull, it was unbelievable how quiet it was. It took full on yelling to get someones attention upstairs. Sound simply would not go around corners.
Also just cleaning the power vent fan may help as well. They can get dirty and unbalanced. Maybe look at isolating the power vent ducting itself too, if its resting on a wall.

 

[AandPDan]

Have a heat loss calculation done. It'll tell you how many btu's you need to size the furnace properly. If an HVAC company won't do it then they're guessing at the size.

You can also find a heat loss calculator on-line. Slant Fin, among others, has them.

Here's one for your Ipad: Heat Loss Calculator

 

[MNgopher]

Whatever contractor you choose should do a heat loss calculation. Ask to see it. If they won't provide it, they are taking a total guess or just plopping in stock units they have on hand. Neither is optimal.

Sounds like your unit is likely too big on the heat side for your application. 100,000 btu sounds bigger than what heats my 2 story / 2700 square foot house in Minnesota. Granted, I have no idea what insulation/windows/etc you have, but it sounds like a classic case of using what the contractor had.

 

[rrguy]

Going to a HE furnace with PVC will hopefully allow a simple reroute your exhaust. Should solve your noise issue, get you a rebate to help cover cost? But the exhaust blower on most furnaces I can hear through the cold aire return ducting when they fire up?

 

[JHZR2]

Originally Posted By: AandPDan
Have a heat loss calculation done. It'll tell you how many btu's you need to size the furnace properly. If an HVAC company won't do it then they're guessing at the size.

You can also find a heat loss calculator on-line. Slant Fin, among others, has them.

Here's one for your Ipad: Heat Loss Calculator


+1. This is important.

Is it possible to have a modulating/condensing forced air unit? Not sure if that works because you want as low a temperature as possible, and low delta T doesn't work well for such poor heat transfer coefficients pushing heat to air.

 

[marshall25]

Have a Manual J load calculation done. That is the only way to know what size furnace to install. Also, the lowest efficiency furnace available now is 90% or higher, which must be vented through pvc.

 

[doitmyself]

Originally Posted By: marshall25
Have a Manual J load calculation done. That is the only way to know what size furnace to install. Also, the lowest efficiency furnace available now is 90% or higher, which must be vented through pvc.


There are still 80% efficiency furnaces available. When I asked my HVAC buddy who would purchase these, he stated that in climates where heating is used very little, the payback to install a 90%+ furnace might not be there or that the 80% unit might make sense as a backup/hybrid to other heat (ie. heat pump).

http://www.bryant.com/products/gasfurnaces/legacyline80.shtml

 

[HTSS_TR]

Originally Posted By: doitmyself
Originally Posted By: marshall25
Have a Manual J load calculation done. That is the only way to know what size furnace to install. Also, the lowest efficiency furnace available now is 90% or higher, which must be vented through pvc.


There are still 80% efficiency furnaces available. When I asked my HVAC buddy who would purchase these, he stated that in climates where heating is used very little, the payback to install a 90%+ furnace might not be there or that the 80% unit might make sense as a backup/hybrid to other heat (ie. heat pump).

http://www.bryant.com/products/gasfurnaces/legacyline80.shtml

I agree.
Since I need to run my heater about 15-30 hours a year or less, this year I have heater on about 4-5 hours, my 44 years old furnace is a little noisy but it doesn't make sense to replace it. I don't know the old furnace efficiency, I bet it's less than 60-70%.

 

[MNgopher]

I did check and I have an 88,000 btu furnace heating a two story 2700 SF home in Minnesota. Given that, 100000 btu sounds high for a climate like the OP's and the space he's indicated (again though - not knowing the insulation efficiency).

80% efficiency units are still available. The latest attempt to mandate 90% efficiency units in the northern states, and allow 80% in the southern states due to low payback probability in those climates was recently scrapped. Thus, you can still get 80% units in northern climates.

The main reason had to do with the change in exhaust that occurs with the higher efficiency units. In retrofit applications, there can be significant costs to changing the exhaust type that wipe out the savings from the efficiency gain. We'll see how soon before a new standard gets traction. Its only been 20+ years since we mandated 80% as the bottom end...

 

[PandaBear]

So what is the smallest size available on the market? It seems like 80% is the most reasonable choice as I don't turn on the heater every night, and a smaller unit will come with a smaller forced air fan and less exhaust gas through the vent, and that may be enough to quiet down the noise.

 

[Schmoe]

100000 BTU's???? Dang....that'll warm your buns quick. Also sounds like your system isn't balanced either.

 

[IndyIan]

I think you need to talk to a few HVAC guys and find one that understands what you want to do. They should have a far better idea than us on what's avaiable in your area and what will fit in your current space and system.

 

[sleddriver]

Originally Posted By: PandaBear
...The biggest problem however is the noise it generates by the blower pushing exhaust through a vent inside the wall between 2 rooms, waking everyone up in the middle of the night.


Solving this problem acoustically may incur far less expense than replacing the heater, even though it seems to be WAY over-sized for your climate. Sounds as though an acoustical blanket could be used to wrap this vent and blower or the blower needs to be replaced or the squirrel cage cleaned or replaced if it's making that much noise.

 

[PandaBear]

The noisy part of the system is the what they called "Oval Vent" between the wall. Padding it with insulation blanket would means some sheet rock work on the bedroom wall, not something I'd like to do.

In a nutshell, in Kalifornia, it is easier to just buy a newer furnace instead of doing vent and duct work as it is much cheaper in labor. 80% efficiency smaller unit looks good enough, with quieter fan even better. I think that's likely the direction I'll go.

 

[PandaBear]

Also wondering:

Since I found the model number of the existing furnace, and what components it come with, can I replace the parts like the burner, heat exchanger, draft inducer motor myself (or ask a HVAC guy) instead of replacing the unit?

The thing is still pretty good condition as I imagine it is not really used before we purchase the house and we only started using it recently.

 

[sleddriver]

Sheetrock work is easy. The labor cost is far lower than removing the old/installing the new unit, not to mention its cost and paying tax on top of that.

By lowering the air velocity through the duct, you'll lower the noise as well. If your indoor unit is an upflow type, in a small closet, you can wrap the whole thing in an acoustical blanket, which will make it very quiet.

 

[doitmyself]

Type B/W double wall oval vent is supposed to be installed in walls with open air space around it. INSULATION could create fire or other system problems. This type of DIY cobble repair enters the realm of creating life threatening consequences.

I think the OP should pay a competent HVAC person to evaluate his system and determine what the problem is and if there is an economical and safe fix. It is not unusual for general contractors to botch installations. Over sized furnace with undersized or otherwise improper chimney vent sounds like a possibility here. All we can do is speculate.