[JimmyP]

I am trying to caculate my heat load based on electric and propane usage.

From my electric bill I have used 15132 Kwh in the period from March 2012 to March 2013. If I take away 450 kwh /month for non heat usage I get 9732kwh/year.
For the period from April 2011 to April 2012 I used 15182Kwh or 9782 for heat , so it is consistant in usage the 2 time periods. I used April because for March 2011. It was an estimated number. HDD for the periods were March 2011 to March 2012 4397degC 65F = 18.3C and 0F = -17.7C so dT of 36 used in caculations.

using the formula (heatloss/hr / dT) * HDDS for total loss /year in BTU's

Using a loss of 24000btus/hr of heat loss of ((24000btu's hr x24) / 36 ) * 4397= 70.35 Million BTU's You would need 70.35 million BTU's of heat.

From my utility bill i have used 9782kwh's x 3412 btu's/kwh = 33.37million for the year from electric baseboard heaters.
I also use propane and have used 750 liters per the time period. 750 x24603 btu's/l gives me 18.45 Million. a total of 51.8 Million.

Therefore my heat loss should be less than 24000btu's per hour.

Working backward's I have ((51.8million / 4397hdds) * dT 36C)/24 =17600 BTU's/hr. Is this the correct way to do this.

If I am missing something or doing it incorrectly please let me know.

Thanks.

 

[STEVEusaPA]

That's alot of math.....my head hurts. You're not actually calculating heat loss/load, you're just figuring out how many btu's you used for the year. If you used your information combined with some degree day forcasting, you stil wouldnt get to where you're trying to go.
You should do a heat loss based on a room by room calculation. Then you can determine the btu's. needed, and the right size equipment. Combine that with your degree days will give you total btu's for the season, which then can use your data/with electric rates to determine how much it will cost you to heat your home.
Why are you asking? Curiosity, or just for your own information?

 

[JimmyP]

Thanks Steve

I did a load caculation using the HVAC Calc software and I came up with ~ 21000 BTU's/hr. I have been told by two contractors that I need a 3 ton HP or 36000 BTU's. I dont think I do need one that large.

Shouldn't the electric used be able to correrspond to the heat load of the house.

Right now I only have the propane and 5 Kw of baseboard heater and it heats fine.



I re-did my BTU's used for this past winter from Dec 1 when the propane was delivered( we only get it delivered 2 times a year and I am not out of Propane yet.(the other delivery is in July)

I used 6216Kwh of electric for 21M btu's and in the caculation I said all propane was used and my fireplace is 68% efficient. for another 12.9 Million. I used the HDDS for that period and I came up with 21500 btu's /hr.



Jimmy

 

[motoguy128]

Then guess what, you bills and load calculation match (imagine that). Sounds like you only need a 2 or 2.5 ton heat pump depending on climate and equipment models selected. 3 ton would be too large and you'd have dehumidificaiton problems in summer.

 

[JimmyP]

Then guess what, you bills and load calculation match (imagine that). Sounds like you only need a 2 or 2.5 ton heat pump depending on climate and equipment models selected. 3 ton would be too large and you'd have dehumidificaiton problems in summer.

Thanks Motoguy.

That was the point of the exercise, I was thinking I was on the right path. I believed the number I came up with using the HVAC Calc but when the professionals came in and came up with ~ 38000 BTU's/ hr heat load I doubted thier numbers and needed some clarification. That is if I have done it correctly.

Jimmy

 

[jtrammel]

Thanks Motoguy.

That was the point of the exercise, I was thinking I was on the right path. I believed the number I came up with using the HVAC Calc but when the professionals came in and came up with ~ 38000 BTU's/ hr heat load I doubted thier numbers and needed some clarification. That is if I have done it correctly.

Jimmy

It's very common for contractors to fudge their inputs (or not even do a calc) to get ~500sqft per ton, most everyone is so scared to undersize that most grossly oversize.

 

[mark beiser]

Shouldn't the electric used be able to correrspond to the heat load of the house.

No, load calculations for sizing equipment are based on peak design day loads.

Your electric and fuel bills won't show you how much electricity or fuel was used during the few hours the outdoor temperature was at the design temperature on the days it reached it.

Your bills also reflect usage by other items in the home that consume energy/fuel.

 

[JimmyP]

No, load calculations for sizing equipment are based on peak design day loads.

Your electric and fuel bills won't show you how much electricity or fuel was used during the few hours the outdoor temperature was at the design temperature on the days it reached it.

Your bills also reflect usage by other items in the home that consume energy/fuel.

I removed 450kwh's per month for the other items.

Ok so you design for 36000 BTU's /hr heat loss ( which is what thay told me I needed), and you get that output from the heat pump @ 47F. If you are loosing that much heat @ 47F, wouldn't it be groosely oversized If I am only loosing ~ 8000btu's per hr @ that temp. What you reaaly need then is a heat pump that will supply 36000 Btu's at 0 when you will more than likely be loosing 36000 BTU's and not the 18000 or so that the heat pump puts out.

From the load caculation I have a heat gain of 21000 sensible so I need a 2 ton for cooling. which is what they are supposed to be designed for cooling not heating.

 

[motoguy128]

The heat pump doesn't need to supply 100% capacity at design temepratures. Normally you size for AC load, unless you live realy far north. IF you size for heating you have humidity problems in summer when cooling even with a 2 stage system. A well insulated house will have a better balance of heating load to cooling load, since a large part of of heat loss and gain are internal loads. Thsoe loads offset each other, since it gets added to cooling in summer and subtracted in winter. A well done spray foam house with enough thermal mass might not even need a significant heating systems (passively heated) and might have just a cooling system and dehumidifier.

I forget, are you using a dehumidifer for latent loads? If so, then yes, you could oversize a little for heating and go with a 3 ton 2 stage heat pump and use the whle house dehumidifer to manage latent loads. That's a very good setup.

 

[JimmyP]

Motoguy

I don't have a dehumidifer. The house is 3 years old and doesn't have any mechanacial ventilation ( only the kitchen and bathroom fan exauhsts no intake).

We are in the country and we open windows in the summer for cooling. It is in eastern canada so my main concern is heating. The house faces south east and has a lot of windows facing that direction > 200 sq feet on south east and south west sides. I was planning on a 2.5 ton but I don't think they come in 2 stages. I would like to have a infinity greenspeed and may go with that in a 2 ton if the price is not too large. I am waiting to hear from a carrier rep for an estimate. The problem is he said he didn't need do do a load caculation and he will send out a quote based on information I supplied in an e-mail. I suspect he will come back with a 3 ton or higher as well.

I think the 2 ton infinity suits my requirements fine. I will only need 900 cfm max so smaller duckwork( there are none installed) and I plan on putting in an HRV as well.

 

[motoguy128]

If your going 2.5 ton, you'd go with a 3 ton if 2 stage. Humidity shouldn't be an issue that far north so oersizing a little for heating isn't a bad idea. Carrier recommend oversizing their Greenspeed 1 size for most applications, so you might go with a 3 ton. However, I have been told that the 2 ton is quite a bit less expensive. Which is odd, since it appears to be the same compressor and size, just restricted in maximum compressor speed. Maybe the inverter is 1 size smaller.

 

[JimmyP]

2.5 ton was oversizing

If your going 2.5 ton, you'd go with a 3 ton if 2 stage. Humidity shouldn't be an issue that far north so oersizing a little for heating isn't a bad idea. Carrier recommend oversizing their Greenspeed 1 size for most applications, so you might go with a 3 ton. However, I have been told that the 2 ton is quite a bit less expensive. Which is odd, since it appears to be the same compressor and size, just restricted in maximum compressor speed. Maybe the inverter is 1 size smaller.

The 2.5 was oversizing for heating needs. and a lower balance point. I think the 3 ton would be unnecessary. The Greenspeed Infinity 2 ton would be sufficient according to thier heating performance values.

Jimmy

 

[motoguy128]

You might price the Greenspeed. Huge benefits in terms of comfort as well as low temp capacity. 90% of the time the the system would be at part load and airflow around 400-500 cfm. Really quiet. The unit is so efficient in cooling at part load and low temps that it would cost almost nothing to cool your home if you did leave the windows closed. I think at minimum capacity at 75F outdoor temp it uses something like 500 Watts to deliver about 13000BTU's. Great stuff.

 

[JimmyP]

Yes, I agree and from all I have read about it and from what most are saying in here is would be a great system to have.

Thanks

 

[dan sw fl]

What size equipment do you need when the daily Low is 20'F and the high 8 hours later is 50'F?
IN- Out T
70-20 = 50'F dT 6 am
70-50 = 20'F dT 2 pm
So, heat loss for dT 6 am will be about 2.5 X heat loss for dT 2 pm

Is your Design Temperature which is used for sizing equipment going to be 35'F?
Is an inside temperature swing of nearly 20'F acceptable?

 

[JimmyP]

Desine temperature used

What size equipment do you need when the daily Low is 20'F and the high 8 hours later is 50'F?
IN- Out T
70-20 = 50'F dT 6 am
70-50 = 20'F dT 2 pm
So, heat loss for dT 6 am will be about 2.5 X heat loss for dT 2 pm

Is your Design Temperature which is used for sizing equipment going to be 35'F?
Is an inside temperature swing of nearly 20'F acceptable?

I used dT of 65F or 36C for the caculation. This is the worse case scenerio. The guy who said i needed a 3 ton used dT of 65F. I should not need this amount ever. We do not have 30 degree temperature swings here?

Jimmy

 

[JimmyP]

Dan

For dT of 50F I need 16382 BTU's/hr and for dT of 20F I need 6553 BTU's /hr

 

[JimmyP]

Dan

HDD's used in caculation from Dec2012-March2013 was 2395C or 4311F.
coldest average low temperature seen here was 15F or -9.3C dT of 55F and for that I need 18020 BTU's /hr based on usage over that time period.

Jimmy

 

[motoguy128]

Just realize that if if the delta T goes to 65F for more than a couple hours your hosue may start to cool off. It's possible that in a cold snap, it could sustain a 50F delta T for more than 24 hours, so you home may cool down to 60-65F. If that's acceptable or you plan to have a supplemental heat source like a small wood stove or direct vent fireplace, then you'll be fine.

 

[JimmyP]

Just realize that if if the delta T goes to 65F for more than a couple hours your hosue may start to cool off. It's possible that in a cold snap, it could sustain a 50F delta T for more than 24 hours, so you home may cool down to 60-65F. If that's acceptable or you plan to have a supplemental heat source like a small wood stove or direct vent fireplace, then you'll be fine.

Thanks for that information.

I do have 2 directvent fireplaces installed. One in the basement and one on main floor in the living room. So I would be ok. If it does get down to 5F it would only happen for one or 2 days a year. the average low in the winter is 15F while the ave daily temp is 25F.