[garyed]

The program advised to use sq feet of house over a basement, which for me is 1460. The remaining room I have upstairs is over a slab, so I added the linear feet of that room to the 1460.

If you want to be more accurate you can choose both types of floors. On the page where you choose your floor type there's an option for floor1 & floor2. You can choose the crawl space for floor1 & slab for floor2 then you just enter the sq. ft. for type 1 & linear ft. for type 2 on the calculation page. It was designed for a situation exactly like yours.

 

[udarrell]

Well, that CFM blank on that calc is only for either using the recommended fresh air or another amount you put in the blank; if you use no mechanical ventilation then leave the blank space empty.

I need to explain why I thought it was necessary to figure the infiltration rate; I have for years used a work sheet on which you had to figure the sensible & latent infiltration Btuh effects.

You used the cu.ft volume of the conditioned space Times(Xs) whatever ACH you selected; it was guess work; you multiplied that result by 0.0167 to get the infiltration CFM; then you multiplied that infiltration CFM by 22 to get the sensible load & by 31.96 to get the latent load.

The load-calc automatically figures the infiltration by what you select.

I apologize to everyone for trying to use a worksheet or a spreadsheet method in connection to the calc. then the use of that calculator further threw me off.
At my age, Old habits are hard to let go of...
op:

 

[dan sw fl]

5:25am @ 21F outdoor temperature (design for our area 17f)
Current indoor temperature 65F, just turned up thermostat to 70. Lets see how long it takes...

Update :
5:45 system just reached 70f and shut off. Looks like even my 44k furnace is oversized, should have taken longer than 20 minutes to recover.

It should have taken about 3 hours + to recover.

 

[udarrell]

5:25am @ 21F outdoor temperature (design for our area 17f)
Current indoor temperature 65F, just turned up thermostat to 70. Lets see how long it takes...
Update :
5:45 system just reached 70f and shut off. Looks like even my 44k furnace is oversized, should have taken longer than 20 minutes to recover.

Is that 44,000-input * .95% is 41,800-output? 20-mins /60-mins is .3333% * 41,800 is only 13,933-Btu. I use a SWING room stat using #7 swing-setting, at 69°F it runs about 13-minutes to go from 68 to 70°F.

Under all conditions, I can always get the amount of longer runtimes I want, for more efficient cycles.

At 31°F 11/7/31, 6:08 am, a complete on/off on again cycle, was 79-minutes; with 14-mins runtime / 79-mins is .177215% * 57,000-Btuh output propane, results in only 10,101-Btuh at those conditions.

Last 2-years it never went over 30,000-Btuh even in the coldest conditions in SW WI; I use -13°F design...+1250-sf two story with a deep basement, 1937 farm home with a lot of storm-windows. Previously had a 140,000 80% when new, .85 nozzle Oil furnace that was way oversized.

 

[54regcab]

It should have taken about 3 hours + to recover.

If they only made furnaces in sizes smaller than 44k :gah:
Contractors rarely buy the 2 burner furnaces, their low sales numbers gives manufacturers little incentive to make anything smaller.

 

[54regcab]

Is that 44,000-input * .95% is 41,800-output? 20-mins /60-mins is .3333% * 41,800 is only 13,933-Btu. I use a SWING room stat using #7 swing-setting, at 69°F it runs about 13-minutes to go from 68 to 70°F.

Under all conditions, I can always get the amount of longer runtimes I want, for more efficient cycles.

At 31°F 11/7/31, 6:08 am, a complete on/off on again cycle, was 79-minutes; with 14-mins runtime / 79-mins is .177215% * 57,000-Btuh output propane, results in only 10,101-Btuh at those conditions.

Last 2-years it never went over 30,000-Btuh even in the coldest conditions in SW WI; I use -13°F design...+1250-sf two story with a deep basement, 1937 farm home with a lot of storm-windows. Previously had a 140,000 80% when new, .85 nozzle Oil furnace that was way oversized.

WOW, Just WOW. I knew oversizing was a problem down south, I figured those giant furnaces may have really been needed when it got to -15f. It makes me wonder how many houses could be heated with a 44k furnace.

Furnace is a 44K 90%. According to This Thread there isn't much difference between a 90 and 95%. Current thermostat is setup for cycles per hour. I have it set at the lowest setting of 2CPH (Choice is 2,4,6). I am interesting in hearing your ideas on how short cycle times affect furnace efficiency. I've read your blog on AC short cycling, but nothing on furnaces.

 

[Jaris]

If you want to be more accurate you can choose both types of floors. On the page where you choose your floor type there's an option for floor1 & floor2. You can choose the crawl space for floor1 & slab for floor2 then you just enter the sq. ft. for type 1 & linear ft. for type 2 on the calculation page. It was designed for a situation exactly like yours.

Ok, plugged the numbers in and bumped the load up 6k to about 47,000 BTU's.

60K furnace looks like the way to go!

EDIT: Actually I'm smack dab in between a 40 and 60k furnace.

 

[54regcab]

Ok, plugged the numbers in and bumped the load up 6k to about 47,000 BTU's.

60K furnace looks like the way to go!

Why did you bump the load up?

You read udarrells post about his 30,000BTU heat loss @ -13f in a 1937 farm house.
Previous owners thought house needed 140k, and were WAY OFF.
I'd like to hear his thoughts about only needing a 45k furnace in your house.

 

[Jaris]

Why did you bump the load up?

You read udarrells post about his 30,000BTU heat loss @ -13f in a 1937 farm house.
Previous owners thought house needed 140k, and were WAY OFF.
I'd like to hear his thoughts about only needing a 45k furnace in your house.

What I meant is that I input the new floor numbers into the calculator per Gary's post, and that changed the requirements from about 41k, to 47k. (having part of the floor on slab, the rest on crawl space)

 

[udarrell]

WOW, Just WOW. I knew oversizing was a problem down south, I figured those giant furnaces may have really been needed when it got to -15f. It makes me wonder how many houses could be heated with a 44k furnace.

Furnace is a 44K 90%. According to This Thread there isn't much difference between a 90 and 95%. Current thermostat is setup for cycles per hour. I have it set at the lowest setting of 2CPH (Choice is 2,4,6). I am interesting in hearing your ideas on how short cycle times affect furnace efficiency. I've read your blog on AC short cycling, but nothing on furnaces.

Well, I don't fully heat two upstairs bedrooms; the east bedroom gets some heat but is dampered down at the takeoff.
The other two West bedrooms are on one duct run & a two-way diffuser; when it is real cold I shut the diffuser off on the SW bedroom.

I will check it this winter with all the ducts & registers wide open. I believe it will then be close to 35,000-BTUH, but we'll see... that is still way below what anyone would have thought & co-insides well with the load-calcs.

 

[Jaris]

Alright guys, about ready to lay down the cash for the unit. Would you go with 40k or 60k?
According to my last load calc, I'm right between the two at 47,000.

I have an existing 2.5 ton AC, so I'm not sure the 1,000/1,200 CFM would matter between the two furnaces.

 

[54regcab]

My vote goes to the 40K

The output of the 40k you are considering is within 2,000BTU of the furnace I recently installed. PLENTY of heat as shown by the "study" earlier in the thread, also see udarrell's -13f test on his 1937 farm house.. Getting closer to the actual load will lead to better comfort and lower cycling losses.

 

[Jaris]

Thanks!

 

[jtrammel]

I'd go with the 40k. I love the tm9v furnace. If ever necessary you've always got the electric back up.

 

[garyed]

I'll be the oddball here. I'd go with the 60k. I'd rather oversize a little than undersize. ManualS allows for up to 40% over the heat load calc but I'm not sure what it allows for under sizing.

 

[beenthere]

Alright guys, about ready to lay down the cash for the unit. Would you go with 40k or 60k?
According to my last load calc, I'm right between the two at 47,000.

I have an existing 2.5 ton AC, so I'm not sure the 1,000/1,200 CFM would matter between the two furnaces.

View attachment 437771

Did you try redoing the load calc without including the sq ft of the garage. The interior garage walls should be calc-ed as partitioned walls.

Might knock another couple thousand BTUs off your heating load.

 

[Jaris]

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)

 

[54regcab]

An unheated garage is typically warmer than outdoor temperatures and blocks the partition walls from the wind. Being on the North side of the house it can be a significant difference.

Continuous heat blowing from the vents makes it feel warmer than the thermostat indicates. *If* the 40k is a little short on the coldest nights of the winter, you could turn on the electric backup for a little bit.

 

[Jaris]

An unheated garage is typically warmer than outdoor temperatures and blocks the partition walls from the wind. Being on the North side of the house it can be a significant difference.

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.

Continuous heat blowing from the vents makes it feel warmer than the thermostat indicates. *If* the 40k is a little short on the coldest nights of the winter, you could turn on the electric backup for a little bit.

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

 

[54regcab]

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.

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.