HVAC Heat/Cool Load calculation example ?

[mixer440]

I'm posting a Heat/Cooling load calculation that I did using a fantastic program posted here on this website (posted by permission).

Just about everyone here has stated the importance of doing a heat/cooling load calculation by experts who know how to do them. Just to understand how they might work I tried the program and came up with the following results.





To me (with the untrained eye) my results below look low. Especially when two of the possible HVAC companies that I contacted that did do a load calculation came up with much higher values:

Company One: 2 stage 78K/50K BTU with a 4 ton H.P.
Company Two: 2 stage 37K/58K BTU with a 3 ton H.P.
Currently Have: mid 90's Single stage 100K/80% with a 3ish H.P.

2800 SqFt
My Results: Heating: 35,826 Cooling: 40632 (sorry about the small size of the graphics below)








I realize that these load calculations are only one "tool" and require some artistic application, but again----just for my curiously---why would there be such a large difference ?


Thanks for the comments

[udarrell]

I live in SW WI & have a 57,500-Btuh propane furnace; with the temp at around 12-F & wind chill at -3-F it only takes 16,292-Btu during with one houir & 7 minutes off.

I use a SWING temp room- thermostat set at a 7-setting for over a 3 close to 4-F swing.

Runtime was 17-minutes at 958.333-Btu/per minute, or 16,292-Btu.

It would take high winds at deep sub-zero temps to get to the 57,500-Btuh capacity of the furnace; if it ever does!

Of course, it's a 95% with outdoor combustion air intake; the much colder weather combustion air temp drops the fire chamber's temps as it dilutes it; similar to pouring real cold cream into a boiling hot cup of coffee.

Winter design at La Crosse WI at 99% is -13-F; at 97.3% it's -9.
Madison WI is the same distance north; 99% is -11-F; 97.5% is -7-F.

It will be cruising at those temps because, I've checked it below zero, - but can't find those results to publish.

For those rare periods when it's extremely cold you could have a portable electric heater to supplement the furnace for those short periods of time; ... if ever needed.

I couldn't see the square footage of your home? Mine is a 2-story with a deep basement & heating around only 1000-sf.

[mixer440]

That would be 2800 sqft single level, medium tight, W:R21/C:R42/F:R25, Ducts in vented crawl:R6

.....

I couldn't see the square footage of your home? Mine is a 2-story with a deep basement & heating around only 1000-sf.

[ADillon]

your cooling load falls right between 3 and 4 tons so it looks like you're in the ball park there. The heat load looks low but it could be correct. usually the smallest resi furnace is 60k or 80k

[mixer440]

Thanks ADillon.

So if the smallest furnace is around 60K then you'd want that for the 1st stage--or--whatever stage is used least. Right ?

Off Topic: I like your picture, but have you seen this version: http://www.kulfoto.com/funny-picture...urger-and-head

your cooling load falls right between 3 and 4 tons so it looks like you're in the ball park there. The heat load looks low but it could be correct. usually the smallest resi furnace is 60k or 80k

[SkyHeating]

I think that heat loss seems possible, so you might want a 3.5 ton air conditioner, but not all 60K BTU furnaces have a blower motor that can do 3.5 tons of airflow. You have to match the BTU's on the furnace, airflow on the furnace and coil and condenser so sometimes the correct combination does not meet the ideal heat load. If the 3 ton heat pump has been working for you I probably would not get a larger 3.5 ton, if your 3 ton has not been keeping up then I would look at the 3.5 ton heat pump. If you choose the 3 ton heat pump pretty much any two stage 60K BTU furnace will match up with that heat pump size.

[beenthere]

60,000 would be second stage.

[mixer440]

SkyHeating: I'm unsure of the exact size of the Heat Pump and who knows what it is after 20 years. Here's the number:



I'm curious, what does a dealer do if a company doesn't have 1/2 sizes----go up or down ?

[SkyHeating]

SkyHeating: I'm unsure of the exact size of the Heat Pump and who knows what it is after 20 years. Here's the number:



I'm curious, what does a dealer do if a company doesn't have 1/2 sizes----go up or down ?

Based on that model number it is a 3.5 Ton. If you look at the model look for a number with a 0 before it, then take that number, divide by 12 and that is how many tons the unit is for about 98% of equipment.

All companies have half sizes until you get to two stage units.

[mixer440]

Thanks for the info.

Q: Let's say for example then that you have/need 3.5 tons. If you get a 2 stage HP, then how do you size that ?
Q: What's your opinion on 2 stage units from Carrier or Trane ?

[stvc]

I live in SW WI & have a 57,500-Btuh propane furnace; with the temp at around 12-F & wind chill at -3-F it only takes 16,292-Btu during with one houir & 7 minutes off.

I use a SWING temp room- thermostat set at a 7-setting for over a 3 close to 4-F swing.

Runtime was 17-minutes at 958.333-Btu/per minute, or 16,292-Btu.

It would take high winds at deep sub-zero temps to get to the 57,500-Btuh capacity of the furnace; if it ever does!

Of course, it's a 95% with outdoor combustion air intake; the much colder weather combustion air temp drops the fire chamber's temps as it dilutes it; similar to pouring real cold cream into a boiling hot cup of coffee.

Winter design at La Crosse WI at 99% is -13-F; at 97.3% it's -9.
Madison WI is the same distance north; 99% is -11-F; 97.5% is -7-F.

It will be cruising at those temps because, I've checked it below zero, - but can't find those results to publish.

For those rare periods when it's extremely cold you could have a portable electric heater to supplement the furnace for those short periods of time; ... if ever needed.

I couldn't see the square footage of your home? Mine is a 2-story with a deep basement & heating around only 1000-sf.

Hmmm, I never thought about how the outside combustion air temp may effect efficiency. I'm curious, I might have to dig into that. I would guess that the rating is based on a standard outdoor temp but have no idea how big the change might be.

[stvc]

Just found this.

The temperature of incoming combustion air affects the measured efficiency: measured
SSE will be slightly lower in cold weather and slightly higher in warm weather. AFUE
ratings for sealed-combustion furnaces that use outdoor air are based on a national
average outdoor temperature of 42oF. A furnace will test about 1 to 1.5 percentage points
below AFUE on a cold winter day based solely on the difference in intake air temperature
2
(and a furnace tested on a hot summer day will similarly test about 1 to 1.5 percentage
points above AFUE).

Copied the info from here http://homeenergyplus.wi.gov/docview.asp?docid=13851

[mixer440]

So, I guess that a 97% furnace is really a 95% furnace, since you'll never be using it in the summer and almost never above 40 degrees if you have a heat pump. Oh well, what's approximately 2 degrees over 10 years life ? (Hmm, let's see: 60,000 BTU/2%Xheating days below 40x10years=)

Just found this.

A furnace will test about 1 to 1.5 percentage points
below AFUE on a cold winter day based solely on the difference in intake air temperature
(and a furnace tested on a hot summer day will similarly test about 1 to 1.5 percentage
points above AFUE).

Copied the info from here http://homeenergyplus.wi.gov/docview.asp?docid=13851

[vstech]

to answer your question, the difference may be in the numbers you vs the contractors used for the software. wall construction, duct leakage, window size/awnings etc all affect the load. shade, roof design, also the web numbers may not precisely relate to your design condition.

[ADillon]

As far as efficiency loss from outside air goes, it is pretty minimal. If you view it as a concer, then you should have the nit installed single pipe so that you are pulling combustion air from the conditioned space. (you must have adequate free area to pull from)