[garyed]

I always treat walls against an unheated garage the same as I would if there was no garage there. I agree that it does block the wind & is usually warmer than the outside air but its very difficult to judge. ManualJ flooring u-factors treat a floor over an unheated garage the same as a floor over a totally open crawl area. I read somewhere that there is an assumption that the garage door will be opened enough to void some of the advantage of the enclosure. I don't think there's any exact way to figure it but I would rather err on the side of caution.

 

[54regcab]

I always treat walls against an unheated garage the same as I would if there was no garage there. I agree that it does block the wind & is usually warmer than the outside air but its very difficult to judge. ManualJ flooring u-factors treat a floor over an unheated garage the same as a floor over a totally open crawl area. I read somewhere that there is an assumption that the garage door will be opened enough to void some of the advantage of the enclosure. I don't think there's any exact way to figure it but I would rather err on the side of caution.

How often in South Carolina do you have to oversize the furnace in order to get the blower capacity needed for cooling? I lived in Columbia SC for 10years, I don't remember it getting that cold (rarely below freezing), but it sure got hot/humid in the summer!!

I've never seen an undersized furnace, but in Oklahoma I wouldn't expect to. I've never seen anybody post about having one in the AOP forums.

 

[garyed]

I see your point. I'm not sure if there is any way to show this with the calculator... I don't remember seeing "partition" under the wall dropdown box.


Yes, I'm fortunate to have that second form of heat. Gives me some wiggle room on my calculations.

I have to apologize because I assumed you were dealing with a standard two story house but I just realized that you are dealing with a conditioned basement & first floor. I hate to tell you this but I haven't added anything yet for basements to the calculator so you are not going to get the proper numbers you really need for that area. There are things like walls below grade, how deep below grade are the floors, moisture content of the soil etc.. to be considered before doing a proper load calculation for basements. I've actually started working on adding those things but for now anything below ground is beyond the scope of the calculator. Sorry

 

[Jaris]

I have to apologize because I assumed you were dealing with a standard two story house but I just realized that you are dealing with a conditioned basement & first floor. I hate to tell you this but I haven't added anything yet for basements to the calculator so you are not going to get the proper numbers you really need for that area. There are things like walls below grade, how deep below grade are the floors, moisture content of the soil etc.. to be considered before doing a proper load calculation for basements. I've actually started working on adding those things but for now anything below ground is beyond the scope of the calculator. Sorry

No problem. So that calculation I came up with was for a floor above a crawl space - you're saying a proper basement calculation would likely ADD some to my BTU requirements?

 

[garyed]

No problem. So that calculation I came up with was for a floor above a crawl space - you're saying a proper basement calculation would likely ADD some to my BTU requirements?

If you didn't count the basement walls already it might but if you did it would probably be less.
The floor load would be a lot less because all the floor above the heated basement has no load at all & the load for an under grade floor is about 1/4 of the load of an unheated closed crawl space. The load from the basement walls would add some btu's if you hadn't already counted them in so it could end up being a wash.

 

[Jaris]

Hmmm.. I'll ponder this.

40k may be enough for most of the year, as long as the basement thing doesn't throw everything off too far.
I'm just really leery of oversizing, thinking that a 60k is going to run on low all the time and rarely step up to stage 2... but I'm likely overplaying that.

 

[beenthere]

beenthere: I never added the garage space as part of the "conditioned" sq ft space in my load calc. The only thing I did was add the two outside walls of the garage to the NE & NW wall load, rather than the two inside walls of the garage touching the conditioned space.

This sound correct?

Outside (exterior) walls are brick like the rest of the house. The garage's inside walls are just drywall. (I think)

NO, the garage's 2 outside walls are not part of the conditioned area. They don't get included.

 

[beenthere]

I see your point. I'm not sure if there is any way to show this with the calculator... I don't remember seeing "partition" under the wall dropdown box.


Yes, I'm fortunate to have that second form of heat. Gives me some wiggle room on my calculations.

Until you try to sell your house, and hve to explain that the furnace can't maintain temp, and have to leave the buyers have money to replace the undersized furnace.

 

[beenthere]

If you were in fact 10,000BTU short with no backup, 2 standard space heaters would make up the difference on those extra cold mornings.

Pretty much defeats teh purpose of a "comfort" system. Sor of becomes a "inconvenient" system then.

 

[54regcab]

Until you try to sell your house, and hve to explain that the furnace can't maintain temp, and have to leave the buyers have money to replace the undersized furnace.

I've just never seen an undersized furnace in Oklahoma (most are 2-3X the size needed), up north may be a different story. Udarrells story of heating a 1937 house @ -13f outdoor temp with 30,000BTU is what amazes me, -13f is just downright frigid !!

 

[beenthere]

I've just never seen an undersized furnace in Oklahoma (most are 2-3X the size needed), up north may be a different story. Udarrells story of heating a 1937 house @ -13f outdoor temp with 30,000BTU is what amazes me, -13f is just downright frigid !!

He doesn't heat his whole house though.

An accurate heat loss calc my reveal that the OP can use the 40,000 BTU furnace and heat his whole house to comfort temp. but if it reveals that a 40,000 can't. Then either another brand that has a 50,000 or a York 60,000 should be selected. Having to use supplemental heat whe teh outdoor temp drops below design is one thing. Sizing that you need supplemental at design is another.

 

[54regcab]

He doesn't heat his whole house though.

An accurate heat loss calc my reveal that the OP can use the 40,000 BTU furnace and heat his whole house to comfort temp. but if it reveals that a 40,000 can't. Then either another brand that has a 50,000 or a York 60,000 should be selected. Having to use supplemental heat whe teh outdoor temp drops below design is one thing. Sizing that you need supplemental at design is another.

Agreed, and living in the south I can only go by what others have experienced for northern climates. Heat loss calculations for Oklahoma typically come in lower than the blower capacity needed for the AC. A 40-50K furnace will heat most Oklahoma homes that require up to 3 tons of cooling.

Seeing as udarrell lives in a northern climate I'd like to hear his thoughts on all of this.

 

[beenthere]

Here in south PA, most homes need slightly more air flow for cooling then heating. But often you can use the same air flow for heating as cooling and still be within allowable temp rise of the heat exchanger.

Move up to northern PA, and heating air flow requirement increases a lot.

 

[udarrell]

Agreed, and living in the south I can only go by what others have experienced for northern climates. Heat loss calculations for Oklahoma typically come in lower than the blower capacity needed for the AC. A 40-50K furnace will heat most Oklahoma homes that require up to 3 tons of cooling.

Seeing as udarrell lives in a northern climate I'd like to hear his thoughts on all of this.

I believe most load-calcs incorporate a 15-mph wind; therefore anything above that wind velocity would increase heat-loss...

WIND is the second greatest source of heat loss during the winter. High winds can occur
on the cold nights and when they do, heat loss can be higher because of air scrubbing
the outside of the space covering. Winds can also force their way through cracks in the
structure, causing infiltration and drafts. In fact, up to one-third of the annual heating
energy goes to heat this moving infiltration air many times each winter day.

Well, now I can't find the source of that quote.

There are formulas to use; however, I would simply get the temperature & wind Velocity from my full-time weather radio & record the actual Btu/hr used to maintain the 69°F set-point. at a 7-Swing setting the temp drops to about 66°F comes ON & shuts OFF at 70°F; I dress warm & feel comfortable within that temp-range.

On 11/7/13; At 31°F & a wind-chill of 3°F it only took 10,100 Btu/hr to maintain indoor temp; it amazes me... I didn't record the wind-mph velocity...

Well, we can get -25°F or below & that could be with a strong wind therefore I don't feel that a 57,000-output Btuh is oversized in this climate. Was up in northern WI many years ago on a coyote hunt when it was around 40 below zero plus some wind blowing...snowmobiles & cars wouldn't start, slowed our big organized hunt down to a crawl...

I don't want to advise you; were it me I'd go for a single-stage 40,000-input & 38,000-output with a programmable Swing room-stat with the electric back-up...I am betting you'd never need the electric back-up, but who knows, there can be short periods of exceptions. You can always do things to lower the heat-loss so 38,000 would handle it...it is totally your decision; most of the time it will be way over capacity at 38,000-output, so you'd need a Swing RM-TH. You can do a search for LUX PRO PSP511LCa & ACE ATX 1500...

 

[Jaris]

NO, the garage's 2 outside walls are not part of the conditioned area. They don't get included.

If you don't include the 2 outside walls, then shouldn't the 2 inside walls be included? If so, what input should be used on the calculator?

 

[beenthere]

If you don't include the 2 outside walls, then shouldn't the 2 inside walls be included? If so, what input should be used on the calculator?

Partitioned walls. You enter the temp difference.

 

[garyed]

If you don't include the 2 outside walls, then shouldn't the 2 inside walls be included? If so, what input should be used on the calculator?

I always use the same walls for the garage that I do for the outside walls so it would not make any difference.

 

[Jaris]

Thanks for the help again guys.

most of the time it will be way over capacity at 38,000-output, so you'd need a Swing RM-TH. You can do a search for LUX PRO PSP511LCa & ACE ATX 1500...

udarrell, can you explain why you would want a single stage furnace with a swing t-stat - I thought this whole 2 stage/modulating furnace movement was all about comfort and longer run cycles?

I was thinking a focus pro/white rogers with 2 stage heat would be good.

 

[beenthere]

I was thinking a focus pro/white rogers with 2 stage heat would be good.

When i use a stat with a swing in my house. I actually end up setting the temp higher, to be as comfortable as with a CPH stat set lower.

 

[54regcab]

Supposed to be BELOW design temperatures with high winds this weekend. Lets see how my "mini furnace" does with this cold snap. Some of the guys at the shop think I smoked my breakfast putting such a small furnace in a 1587sqft house. They thought I was nuts in 2012 when I installed a 2ton AC, but it cooled well even when temperature got in the triple digits.