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Factor in warm-up time to estimate new system’s savings

Editor's note: The Energy Services Bulletin features real answers to real questions posed to our staff at the Energy Services Power Line. We hope you find it useful.

Question:
One of our customers is applying for a grant to upgrade their HVAC system. For the application, they need to compare the old system’s energy use with the new one that would replace it. The old system was a 416,000-kW/hour gas boiler. The proposed system will be a 10-ton air-to-air heat pump with a coefficient of performance of 10.2 and a 35-kW electric heat backup. It will be used to heat a theater intermittently. Can you offer guidance in computing this comparison?

Answer:
Thank you for providing the Power Line with the facility’s monthly gas bills, cost of gas and electricity and the seasonal energy efficiency rating of the heat pump. This information made it possible to calculate anticipated energy costs.

We calculated the savings by dividing the gas bill for each month by the cost of gas to obtain the number of Therms used. This number was then converted to thousands of BTU per month. Dividing mBTU/mo by the number of BTU per kWh (3,413) yielded total kWh/mo. Dividing kWh/mo by the SEER of the proposed heat pump gave an estimate for the monthly kWh using the proposed heat pump, expressed in the table below as HP kWh. Finally, the HP kWh was multiplied by 7.32 cents per kWh, your customer’s electricity cost, to get the new heat pump’s equivalent monthly heating cost, or Elect $:

Date	Gas Cost	mBTU	HP kWh	Elect $
1/30/2004	$821.33	84,673	2,432.3	$178.04
3/1/2004	$1,007.39	103,855	2,983.2	$218.37
3/30/2004	$809.33	83,436	2,396.7	$175.44
4/30/2004	$470.25	48,479	1,392.6	$101.94
6/1/2004	$14.56	1,501	43.1	$3.16
6/29/2004	$13.65	1,407	40.4	$2.96
7/30/2004	$13.39	1,380	39.7	$2.90
8/30/2004	$15.47	1,595	45.8	$3.35
11/1/2004	$14.97	1,543	44.3	$3.25
11/29/2004	$74.94	7,726	221.9	$16.24
12/29/2004	$604.63	62,333	1,790.5	$131.07
TOTALS	$3,859.91	397,929	11,430.6	$836.72
Annual Savings =		$3,023.19

This simple analysis ignores a very important difference between the systems: the heat pump’s lower capacity will extend warm-up times, especially during cold weather. Where the boiler system required three hours to bring the space up to temperature, the heat pump system will take at least three times as long. Heat pumps are normally equipped with electric resistance elements to add extra heat for warm-up and when the heat pump cannot meet the load using the compressor.

Use of electric heat reduces the seasonal efficiency of the heat pump to a value approaching 1 under worse case conditions. It may be advisable to provide extra capacity. An additional 10 to 20 tons may be needed to accommodate the warm-up requirements and keep the SEER at high levels.

If you need direct technical assistance or further research on this topic, contact the Power Line at 1-800-769-3756, or submit your request on-line.