Ground Source Heat Pump Question

EMAIL QUESTION ABOUT HEAT PUMPS
Wed, 6 Mar 2013

Hi Nathan,  I am writing about our house in Lexington which my son and family are living in. He has contacted a firm who is planning to install heat pumps to heat/cool 2 floors of our split 3-level house. It is estimated to cost $20,000 with an interest free loan. I just am somewhat skeptical that heat can be supplied reasonably cheaply with electricity and heat pumps in this climate. I wonder if you are able to send me to appropriate literature or home owners who are using this equipment or perhaps give me your opinion. Sam


RESPONSE

Sam,

I usually think of GSHPs in terms of their coefficient of performance (COP). A good one can get around a COP of 4, meaning for every one unit of energy (electricity) you put in you get 4 units out as heat. According to the 2009 Residential Energy Consumption Survey (http://www.eia.gov/consumption/residential/data/2009/index.cfm?view=consumption#end-use) the average house in Massachusetts uses 65,200,000 Btu of energy on heating per year (652 therms about $652 cost). Since most homes use a regular boiler or furnace of around 80%, I'd estimate they only need 52,192,000 Btu of heat (the other 20% is inefficiencies from the boiler). This is the same energy as 15,296 kWh. To get this from a GSHP with a COP of 4, you'd need 3,824 kWh of electricity (about $382 cost). Conversely, if you provided the same 52 MMBtu of heat with a 95% condensing boiler, you'd use 54,939,000 Btu of natural gas (549 therms and $549 cost). GSHPs will save some energy, but the payback is usually pretty long.

As to whether or not the GSHP will work in Boston, it definitely can. We used a lot at Harvard. I know a guy in Somerville who has one for his house (case study link below). Average ground temperature in Boston is about 50. At shallow depths it ranges between about 30 to 70 (see attached from Logan). The ground temperature usually lags behind the air temperature by a month or two (the coldest months for ground temp are a couple of months behind the coldest air months). Still more efficient than trying to use an air source heat pump where the delta T is greater and less in your favor.

The GSHP will also be more efficient than a typical air conditioner, but the annual air conditioning cost is less than the heating cost. Assume you could save 25% or so on your AC bill (say going from a COP of 3 to 4).

I hope this helps.

Nathan

Resources:

General Info - http://www.greenbuilding.com/knowledge-base/what-are-ground-source-heat-pumps

Somerville Case Study - http://www.greenbuilding.com/zero-energy-homes/case-study-retrofit-somerville-mass