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Jim W 05-10-2009 01:30 PM

Sizing a furnace - can't find one small enough
 
New to this forum but not new to doing everything myself - I am working on sizing a new natural gas furnace for my home.
The original house was only 560 sq ft and was heated with a wall furnace. I have added another 1000 ft and am sizing the new furnace based on results from the HVAC Calc - Residential program.
My results show the total (added 20% to calculated results) heat loss to be 29,198 BTUH. This may seem low but the house is well insulated and 500 sq ft of the addition is ICF walls (R54). Either that, or the program results are incorrect.

The problem is, I can't find a forced air gas furnace that is small enough!

All I can find are furnaces that start at 45,000 BTUH - this seems like overkill. Everything I have read warns against getting a unit that is too large because it will cycle on/off too ofen and wear out.

Anyone have some direction on this?

beenthere 05-10-2009 01:39 PM

40,000 is the smallest you'll find.

Jim W 05-10-2009 02:04 PM

Quote:

Originally Posted by beenthere (Post 272005)
40,000 is the smallest you'll find.

What do you reccomend I shoud do? I'm wondering if several ductless wall mounts would be a better solution. I just don't like the thought of having these things hanging on the walls and would much rather have one central unit.

The mini split systems are fairly nice and would also give me AC (not that I really need it here in San Diego) Problem is, they heat with electricity - not natural gas and could be expensive to use for heating - my primary reason for a system.

Another thing is that the smaller forced air units I have found don't seem to offer a variable speed blower which would be best in this situation. This would "optimize" the system for best performance by allowing it to run almost continuiously (as reccomended) but at very low speeds.

Your opinion is appreciated.

yuri 05-10-2009 02:18 PM

A variable speed air handler with 8-10 kwatts of electric strip heaters is about all you will find. No demand for furnaces that small. Lennox makes a CBX32MV with 8/9/10 kw but they don't sell to the public. Need a Lennox dealer.

beenthere 05-10-2009 02:33 PM

Use a 40,000 BTu.
Just size the duct work for it to be quiet.

Home Air Direct 05-10-2009 02:35 PM

Our line of Whirlpool Furnaces offer a 40,000 btu 95% Gas Furnace, 2-Stage, Variable Speed. The first stage of heat operates at an output of 26,600 btu., with blower definitions from 564 - 1180 cfm. This could be an option. And as Yuri said, a VS air handler and Heat pump could be an option as well.

I am sure other brands offer the same options within their lines. Keep looking.

Good Luck
Jay

Jim W 05-10-2009 05:36 PM

Thanks Yuri and Benthere
 
I'm disapointed that no one makes a smaller forced air unit - especially in this "green" era. I guess a 40,000 BTU unit is my only option.

I did find a 30,000 BTU but it is for big motor homes and probably only runs on LP. Besides, I doubt if they are built to last more than a few seasons.

Your feedback is very much appreciated!

Jim W

Home Air Direct 05-10-2009 06:14 PM

Quote:

Originally Posted by Jim W (Post 272088)
I'm disapointed that no one makes a smaller forced air unit - especially in this "green" era. I guess a 40,000 BTU unit is my only option.

I did find a 30,000 BTU but it is for big motor homes and probably only runs on LP. Besides, I doubt if they are built to last more than a few seasons.

Your feedback is very much appreciated!

Jim W


The reality is that until now, the need for furnaces smaller than 40,000 btu has probably been less the 1% of the market. And, no matter how precise you figure your heat load, HVAC is not yet a micro technology. At this stage of the design game of HVAC, we still have to work with ranges of acceptability.

But I do not quite understand your disappointment, as there are several models of furnaces that are 95%+ in efficiency, that have a first stage firing capability of under 30,000 btu with variable speed drives? That is about as "Green" as you are going to get in the fossil fuel world.

Good luck to you in your search. Bring your unanswered question back here. There are some excellant field pros here that can give real world answers that go beyond the manufacturers hype.

Jim W 05-10-2009 06:30 PM

I guess "dissapointed" is an overstatement - and you are right about the high efficiency furnaces currently availiable.
Thanks again.
Jim W

sktn77a 05-10-2009 08:32 PM

Have you considered a regular split system heatpump and air handler - these are available in smaller sizes. I don't care for the ductless mini-splits either.

Jim W 05-11-2009 12:54 AM

Quote:

Originally Posted by sktn77a (Post 272153)
Have you considered a regular split system heatpump and air handler - these are available in smaller sizes. I don't care for the ductless mini-splits either.

I really haven't looked into a heat pump because AC is not really needed here (unless you ask my wife in a few months). As far as I know, heat pumps use electric to heat with which would be expensive when natural gas is availiable. I could be wrong about that though (heat pumps only using electric for heating).

I also wouldn't feel comfortable installing a heat pump myself but a gas furnace seems do-able in my estimation. The ducting will be the biggest challenge but I'm commited to doing it right so it's gonna take some time.

yuri 05-11-2009 09:00 AM

A heat pump gives you more heat per dollar than straight electric heat strips down to 32deg F or so. Co-efficient of performance theory. I would check with your insurance company and local gas utility. Where I live NOBODY other than a licensed gas fitter can install a gas furnace as a permit and inspection have to be done. Failure to do so and you have no insurance and need a good lawyer.

Jim W 05-11-2009 08:48 PM

Quote:

Originally Posted by yuri (Post 272317)
A heat pump gives you more heat per dollar than straight electric heat strips down to 32deg F or so. Co-efficient of performance theory. I would check with your insurance company and local gas utility. Where I live NOBODY other than a licensed gas fitter can install a gas furnace as a permit and inspection have to be done. Failure to do so and you have no insurance and need a good lawyer.

Thanks Yuri, I have and will continue to work closely with my local inspector (the current addition is under permit). If he wants a licenced HVAC person to install the furnace, I will definatly comply. The requirement is understandable for obvious reasons but, whomever I hire will only do what is absolutly required to comply, and I will do the rest. Don't mean to offend the licenced folk out there but I've had my fill of poor workmanship and high prices. Sorry, but a licence means you are capable of passing a test. It doesn't necessarily mean you are capable of performing the task.

I had two HVAC contractors give me bids several months ago and they both insisted that I get AC as well (didn't listen), never did a complete load calc but used a "rule of thumb" based on square footage and quoted 12K installed. I can't figure out where they get these huge numbers but the brand new SUVs they're driving give me a clue.

BTW I hold a valid California Class A Contractors licence but I'd never say I'm capable of reading grade stakes tomorrow morning - I am "licenced" though - want me to build a road for you?

I'll get off my soapbox now - go ahead and flame me if you choose

I realize I may have a tough time finding someone to only install the furnace and let me do the rest.

I will look into the heat pump as well and appreciate your imput.

Regards,
Jim W

yuri 05-11-2009 09:19 PM

As long as the unit is installed properly and a fitter hooks up the gas line without doing some crazy bending of a flexible gas connector etc I would recommend you go for it. What scares the hell out of me is the thought of my neighbor buying a furnace at HDepot and hacking it in. Pay careful attention to the venting tables, slope/grade. Some things that look easy tend to be more complicated than they look. As Beenthere said you need to have proper sized ductwork or it may be noisy.

Good Luck

Jim W 05-11-2009 10:03 PM

Yuri, I agree and am extremly aware of the doing it right, or not doing it at all approach. Sometimes I wish I didn't have this do-it-myself attitude. I has left me with no time for the fun stuff in life. It's always "as soon as I'm done with this project I'll take that vacation.

The projects never end and the vacation never begins.

Thanks so very much for all your advice, it is truely appreciated.

BTW, I'm ready for my plumbing inspection and I'll talk to the inspector about the furnace while he's here.

Jim W


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