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Edward Brassington Posted: Jun 08, 2010 10:22 PM	Heat loss calculations I have a customer that is building a new 2000 sq ft ranch style bungalow. He is considering geothermal heat, and after our meeting today is now considering spray foam (closed cell). However, he wants to know if there is a different heat loss calculation to use with spray foam when compared to batt insulation. I assume it is based on air changes per hour, but what numbers would be used to make sure he is not buying too big of a geo-thermal unit. His current calculations indicate that he will need an 80,000 btu system.
mason Posted: Jun 09, 2010 12:10 PM	Yes, you would want to reduce the air exchanges per hour when using SPF (assuming it is installed correctly). The exact amount can vary considerably depending on the type of windows used, climate, type of construction, building's use etc.) As a ball park reference, you could use the energy performance testing performed by Architecutural Testing Inc. sponsored by SPFA. It showed fiberglass insulated wall assemblies having 20 to 40% less efficiency than SPF insulated walls at low and high temperatures when air infiltration was induced into the wall assembly. The test results are referenced on some of my articles at the website, masonknowles.com.
Robert Jordan Posted: Jun 10, 2010 10:34 AM	Find a Passive House consultant. Having a very tight and well insulated house obviates the need for geothermal. The money going into geothermal could be much better spent on the building envelope. Everyone wants the wizz bang.
John Shockney Posted: Jun 10, 2010 05:02 PM	From an HVAC contactors point of view let us do a little math, your HVAC guy says that the house needs 80,000 btu’s of heat divide that by 12,000 btu;s/ton of refrigeration = 6.67 tons for your geo system and that system would be too oversized to properly air condition the house. So what the contractor will do to make the system work will be sizing the geo heat pump for the cooling needs and install backup elect heat to make up the difference. This is where I have trouble with some of the payback claims made by the big geo guys on smaller homes because when you need the most heat (below 30) the heat pump will not keep up with the heat loss and you are running elect resistance heat and if that is the case why not use an air over heat pump for less than ¼ the cost and put the money into better insulation. The return on investment is then les than 5 years instead of 15-20 with the geo system!!! Personally I would recommend a 2 stage 3ton air over heat pump with 15kw of backup heat that gives you the a wider range of performance and the comfort without the problems of an oversized system. Also don’t forget proper ventilation. Hope this helps Airpro

Author

Comments

Edward Brassington

Posted: Jun 08, 2010 10:22 PM

Heat loss calculations

I have a customer that is building a new 2000 sq ft ranch style bungalow. He is considering geothermal heat, and after our meeting today is now considering spray foam (closed cell). However, he wants to know if there is a different heat loss calculation to use with spray foam when compared to batt insulation.

I assume it is based on air changes per hour, but what numbers would be used to make sure he is not buying too big of a geo-thermal unit. His current calculations indicate that he will need an 80,000 btu system.

mason

Posted: Jun 09, 2010 12:10 PM

Yes, you would want to reduce the air exchanges per hour when using SPF (assuming it is installed correctly). The exact amount can vary considerably depending on the type of windows used, climate, type of construction, building's use etc.) As a ball park reference, you could use the energy performance testing performed by Architecutural Testing Inc. sponsored by SPFA. It showed fiberglass insulated wall assemblies having 20 to 40% less efficiency than SPF insulated walls at low and high temperatures when air infiltration was induced into the wall assembly. The test results are referenced on some of my articles at the website, masonknowles.com.

Robert Jordan

Posted: Jun 10, 2010 10:34 AM

Find a Passive House consultant. Having a very tight and well insulated house obviates the need for geothermal. The money going into geothermal could be much better spent on the building envelope. Everyone wants the wizz bang.

John Shockney

Posted: Jun 10, 2010 05:02 PM

From an HVAC contactors point of view let us do a little math, your HVAC guy says that the house needs 80,000 btu’s of heat divide that by 12,000 btu;s/ton of refrigeration = 6.67 tons for your geo system and that system would be too oversized to properly air condition the house.

So what the contractor will do to make the system work will be sizing the geo heat pump for the cooling needs and install backup elect heat to make up the difference. This is where I have trouble with some of the payback claims made by the big geo guys on smaller homes because when you need the most heat (below 30) the heat pump will not keep up with the heat loss and you are running elect resistance heat and if that is the case why not use an air over heat pump for less than ¼ the cost and put the money into better insulation.

The return on investment is then les than 5 years instead of 15-20 with the geo system!!!

Personally I would recommend a 2 stage 3ton air over heat pump with 15kw of backup heat that gives you the a wider range of performance and the comfort without the problems of an oversized system. Also don’t forget proper ventilation.

Hope this helps

Airpro

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