I have a problem with higher-than-expected electric use (about6 75% more). Aux heat uses a switch, which I had on just once for about 24 hours last winter. The system has been providing heating/cooling without problems, except for the electric usage. With the sub-zero temperatures this past winter, I figure the system is not underprovisioned.

The first thing I notice is that the LAT (heating) coming from the attic air handler is about 10 degrees cooler than the LAT coming from the basement air handler. I am not sure if that is normal.

I also recently discovered that the controller has a Stage 1 setpoint of 105' with a differential of 5', and a Stage 2 setpoint of 103' with a differential of 3' (which to my logical mind seems wrong). The controller normally shows a temperature of about 94-99 degrees, and seems to turn off the geothermal unit when it hits 99 degrees (which again to me does not seem right). Whenever I look at the controller, it has S1 and S2 lit (which I assume means that the heat pump is almost always on Stage 2).

Any thoughts would be appreciated.

Further details:

1. Location is central Massachusetts
2. Heat/cool load calculations: 54,195 Btuh (heating) 37,661 Btuh (cooling)
3. Heat pump is a 5-ton dual stage water-to-water GeoComfort GWT060, installed in late 2012. 2 MPH036A air handlers.
4. Closed loop, vertical, 2 300ft bore holes.
5. Average cost/kWh is $.19. Estimated annual usage over past year: 17,640kWh (above normal electric usage)
6. EAT/LAT: 1st floor: 72.5' 103.2', 2nd/3rd: 70.3' 90.0' (using an IR thermometer, after running about 10 minutes)
7. EWT/LWT: 33.5' 40.5' (using an IR thermometer, after running about 10 minutes)
8. 100% load by geo, balance point: -3' ("Outdoor Design Temp")
9. n/a
10. Projected kWh: 10,364/year (5,765 heating, 1,967 cooling, 2,632 hot water). Actual: ~18,000 total (after subtracting estimated base electric usage). Previously we used oil, with a K-factor of 7.3, using about 1,000 gallons of oil a year (for heat and hot water).
                 -Scott

Scott,

How are you heating your DHW?

Chris

Posted By ChrisJ on 15 Dec 2014 04:46 PM
Scott,

How are you heating your DHW?

Chris

Hi Chris,

The heat pump has a desuperheater, so it has heated water going to a tank, which then connects to an electric water heater. When I checked the temps earlier, the pipe bringing water from the heat pump to the tank was 99.4 degrees, so from what I can tell that appears to be working OK.
                       -Scott

What is the LLT (leaving load temperature) for the supply line going to your buffer tank? I assume you have a buffer tank.

To get 103 leaving air temp, you probably have at least 113F water going to the air handlers, which means you have at least 123F leaving load temp from the Heat pump, which also means your system is running not very efficient. Do you have an outdoor reset controller? You cannot trust he temp of your sensor in your buffer tank (again, assuming you have one).

To get 103 leaving air What is the LLT (leaving load temperature) for the supply line going to your buffer tank? I assume you have a buffer tank.

To get 103 leaving air temp, you probably have at least 113F water going to the air handlers, which means you have at least 123F leaving load temp from the Heat pump, which also means your system is running not very efficient. Do you have an outdoor reset controller? You cannot trust he temp of your sensor in your buffer tank (again, assuming you have one).temp, you probably have at least 113F water going to the air handlers, which means you have at least 123F leaving load temp from the Heat pump, which also means your system is running not very efficient. Do you have an outdoor reset controller? You cannot trust he temp of your sensor in your buffer tank (again, assuming you have one).

What is the LLT (leaving load temperature) for the supply line going to your buffer tank? I assume you have a buffer tank.

To get 103 leaving air temp, you probably have at least 113F water going to the air handlers, which means you have at least 123F leaving load temp from the Heat pump, which also means your system is running not very efficient. Do you have an outdoor reset controller? You cannot trust he temp of your sensor in your buffer tank (again, assuming you have one).temp, you probably have at least 113F water going to the air handlers, which means you have at least 123F leaving load temp from the Heat pump, which also means your system is running not very efficient. Do you have an outdoor reset controller? You cannot trust he temp of your sensor in your buffer tank (again, assuming you have one).

Posted By docjenser on 15 Dec 2014 08:01 PM
What is the LLT (leaving load temperature) for the supply line going to your buffer tank? I assume you have a buffer tank.

To get 103 leaving air temp, you probably have at least 113F water going to the air handlers, which means you have at least 123F leaving load temp from the Heat pump, which also means your system is running not very efficient. Do you have an outdoor reset controller? You cannot trust he temp of your sensor in your buffer tank (again, assuming you have one).

Hi,

Yes, I do have a buffer tank, but no outdoor reset controller.

The pipes to/from the buffer tank and the heat pump are nearly fully insulated, so I cannot get a temperature reading. The output of the buffer tank has a small amount of exposed pipe, which was reading right around 100 degrees (but likely higher, given that there was not much exposed pipe). When I was getting the 100 degree temperature there, the controller was displaying 92 degrees.
                                 -Scott

Posted By rscott13 on 15 Dec 2014 08:52 PM

Posted By docjenser on 15 Dec 2014 08:01 PM
What is the LLT (leaving load temperature) for the supply line going to your buffer tank? I assume you have a buffer tank.

To get 103 leaving air temp, you probably have at least 113F water going to the air handlers, which means you have at least 123F leaving load temp from the Heat pump, which also means your system is running not very efficient. Do you have an outdoor reset controller? You cannot trust he temp of your sensor in your buffer tank (again, assuming you have one).

Hi,

Yes, I do have a buffer tank, but no outdoor reset controller.

The pipes to/from the buffer tank and the heat pump are nearly fully insulated, so I cannot get a temperature reading. The output of the buffer tank has a small amount of exposed pipe, which was reading right around 100 degrees (but likely higher, given that there was not much exposed pipe). When I was getting the 100 degree temperature there, the controller was displaying 92 degrees.
                                 -Scott

Tough to believe the temperatures. When your air leaves at 103F your supply temperature by definition has to be higher. You should get a needle thermometer for the p/t ports.

Posted By docjenser on 16 Dec 2014 01:02 AM


Tough to believe the temperatures. When your air leaves at 103F your supply temperature by definition has to be higher. You should get a needle thermometer for the p/t ports.

It looks like I made a mistake there -- the 100F reading was the ELT, not LLT.

There is something that looks like it *might* be a p/t port on the pipes going from the buffer tank to the air handlers, but they don't look like the pictures of p/t ports I am seeing online. They have what looks like a bottle cap that screws off, and a yellow handle. Does that sound like it could be a p/t port? I do not see p/t ports on any other pipes (loop, heat pump to/from buffer tank, etc.).
                           -Scott

Post a pic

Posted By Bergy on 16 Dec 2014 05:35 PM
Post a pic

Ah, good idea -- here it is:

Make up an adapter-Hose to PT port and insert a digital meat thermometer.

"Previously we used oil, with a K-factor of 7.3, using about 1,000 gallons of oil a year (for heat and hot water)."

Ok so depending on fuel oil price you saved some but not a lot. It certainly could be better. No outdoor reset means you are often making hotter water than you need and that means wasting energy. If you are not using auxiliary at all then you have extra capacity that could be making DHW instead of using an electric water heater which is also not ideal.

We know last winter was unusual. You can look up HDD (heating degree days) for your area and decide whether it would have caused a 10% or 20% (in energy use) or whatever jump depending on previous years.

Question did you have AC before? Did you use it last summer?

If upstairs ducts are passing through unconditioned space then yes air coming out would necessarily be colder than downstairs. If they are excessible you may consider beefing up whatever insulation is on them.

Do you know the efficiency of your old furnace? If so, you can check the accuracy of the estimates (ie, convert 1000 gallons to kwh)

You should find that an outdoor reset thermostat pays for itself quickly.

Posted By joe.ami on 17 Dec 2014 09:24 AM

Question did you have AC before? Did you use it last summer?

If upstairs ducts are passing through unconditioned space then yes air coming out would necessarily be colder than downstairs. If they are excessible you may consider beefing up whatever insulation is on them.

Previously we used window air conditioners, so the change to central air was very nice.

Extra insulation certainly couldn't hurt. I've been thinking about the idea of using spray foam in the attic under the roof, turning the attic into semi-conditioned space -- it has a number of advantaged, but costly. Adding insulation to the ductwork would likely be worthwhile, though.
                                 -Scott

Posted By jonr on 17 Dec 2014 10:39 AM
Do you know the efficiency of your old furnace? If so, you can check the accuracy of the estimates (ie, convert 1000 gallons to kwh)

You should find that an outdoor reset thermostat pays for itself quickly.

Unfortunately I have forgotten the efficiency of the old furnace -- I think the installer took the tags with him when he removed the furnace .

I'm going to look into an outdoor reset thermostat. It sounds like another good way to improve the efficiency.
                                  -Scott

Ok while the geo is a more efficient air conditioner, it likely conditioned more space. If you removed previous cooling kw's from your baseline electric use then that is a problem.

I'm having trouble reconciling your oil/kw use. 18,000 kws is around 62m btu right (baseline times whatever COP as the guys reminded me)? So if your fuel oil burner was only 50% efficient thats still at least 67.5M. If your oil burner was 80% efficient and your fuel oil was worth 145,000 (more like a #7) btu's then the numbers are really 116M. Is my math right?

It's possible that not sucking air from your home for combustion has really brought the infiltration (thus the btus) down, but I'm having a difficult time making sense of the data before us.

Spray foam in the attic may be a very good idea.

Rscott, Be a bit careful with ir thermometers and accurate temp readings. If the pipes are say copper (which is highly reflective in the ir spectrum), you will not get accurate readings. You are seeing part of the reflected background temp of the room as well as the temp of actual pipe. Take a piece of black electrical tape and wrap it around an exposed pipe. Use the IR thermometer on the tape and measure the temperature after a few minutes. Much more accurate temp readings. Cheers, Eric

Joe has you on the right track. Assuming a COP 0f 3, you used 61 Million btu’s last year in a very cold winter. Does that include domestic hot water? As far as oil goes- 1000 gallons is 139 Million source BTU’s However typically hot water alone accounts for several hundred gallons of oil- and that is generated very inefficiently in the summer with the waste heat increasing your window unit cooling load. A crude ballpark would be to assume overall heating and hot water efficiency is around 65%. Which is around 90 Million btus for heating. Anyway you cut it you seem to be using a lot less btu’s for heating and cooling. That part is good. On the other hand at $0.19 kwh *17400 = 3300$ 1000 gallons of oil at Current avg price in CT is 3086$ But when people were paying $4.48 gallon last year it was much more expensive. Where costs are going for oil is a crapshoot. Cheers, Eric

With a COP of 3, 18,000 kwh of electricity = 184 Mbtus. But I'd guess more like COP <= 2.7 in this case (which is why you want outdoor reset). Your usage is above the estimate and above what 1000 gallons of oil suggests.