The following stumper is presented in BPI's October Performance Matters e-Newsletter.
This month's stumper comes from a chump who's actually stumped! Ed Revers of Michell Timperman Ritz Architects in New Albany, Indiana has been scratching his head about this one and is wondering if you can help.
Ed explains that he has been working on a 50 year old house that has a four ton top-of-the-line Florida Heat pump system, with all the energy saving bells and whistles (hot water de-super heater, four-zone stats on Arzel control board, etc). After 2 years of utility bills, the energy savings are nowhere near what the geothermal manufacturers claim they should be. The new energy bills (gas and electric combined) are the same as the past 20 years, during which an old forced-air conventional gas furnace was used. The mechanical contractor has come out several times to check things and reports that it is working fine. The house was super-insulated when the new geothermal system was made operational. It isn't easy to compare old to new systems equally, because an addition of about 25 percent was added to the house when the new system was installed.
Background: The house is in the Louisville, Kentucky area and is now a 2850 sq. ft. ranch with a basement. Mechanical installation costs exceeded $30K, with four grouted wells at 150' depth each. At the recommendation of the mechanical contractor, the house was zoned because the existing house limited duct work configuration options and R-values weren't equal everywhere. The new system performs well (70 in the summer and 68 in the winter), but the energy savings are disappointing. The weather has actually been pretty mild the past few years too. As a newly accredited BPI Building Analyst, Ed considers the house to be tight and well insulated; although no blower door test or duct efficiency test has been performed to verify this. Gas and electric usage is probably below average for a family of four. Any clue as to what would help lower the utility bills and make the system less expensive to operate?
Be really helpful to see the load calc and have a blower door #.
The new system performs well (70 in the summer and 68 in the winter)
A 70 setting for comfort in summer indicates gross oversizing. (poor latent removal means lower sensible is required to achieve comfort.) From there you can suspect cycling losses/short cycling is partially to blame for lack of efficiency.
They didn't expect to save on heating when they have natural gas, did they? Exaggerated expectations by the sales person, and acceptance without research by the homeowner, can also be partially to blame.
Are they tracking entering and leaving water temperatures? Run times? How about External Static Pressure? These things will better tell how much of a design failure we have than "it maintains the temperature we set it at".
looking at the information provided:
1 - is the system a multistage unit to better match load as well as zoning demands.
2 - is the heating capacity properly matched to to home demand - does the backup heat run?
3 - is the cooling capacity too large - so homeowner sets thermostat lower to dehumidify
When you say, "The new energy bills are the same as the past 20 years", my first response would be to show me the data so I can run a billing anlysis. I put together a spreadsheet that analyzes gas and electric billing histories. The analysis simply calculates the average energy usage for each bill and plots it with that month's average monthly temperature. This normalizes the data for different number of read days in each month and adjusts for weather variations.