This is my first winter with my ground loop system. Loop exit temperature now (mid Feb) is down to 33 degrees. Backup strips come on to help when the outside temp is below 20-25 F. Is this normal?
incoming water temp of 33 is great this time of year. 30 is the common design point and units are rated to extract heat even below 30 for incoming water temp so at first glance loop doesn't seem to be an issue. Given this, if your heat strips are coming on, then the unit is not capable of extracting enough heat from the water to maintain the interior temperature set point. You need a larger unit to offset the heat loss of the home in order to prevent the heat strips from coming on. We are in a pretty bad cold spell in most areas of the country right now. Not sure where you are located or how cold it is there now versus your location's ASHRAE design temps. Are the heat strips on constantly, just this month, just during this cold spell? Its often a design/economics consideration that upsizing the unit and ground loop for 100% geo coverage for all heating costs a lot more than using a smaller unit and ground loop with heat strips for brief periods during the coldest weather periods. Pay now or pay later type thing. But that should have been a discussion with your designer/installer rather than them just making that decision for you. You are going to have to describe how much heat strip use is occurring to better understand if this is anticipated for your system/weather conditions or outside of normal bounds.
Not really "normal" to me. The outdoor temp shouldn't be the first reason for aux to kick in. The aux should turn on when the unit can't keep the temp at set point. My unit/tstat is set to turn on aux when inside temp is 3 degrees below set point. Some tstats have an outside temp sensor, i think that set up is more for air source heat pumps.
I wouldn't expect a heat pump to respond directly to the exterior temp but it certainly responds to it indirectly. The ability of any heating system to maintain the interior temperature is dependent on that system to counteract the building's heat loss. The heat loss is a function of the difference between the interior and exterior temperature. So the colder the exterior temp gets, the greater the heat loss (interior to exterior temp increases), the more the unit needs to work to maintain the interior set point. Presumably if the unit can't keep up with the heat loss via the refrigeration cycle, it would turn on the aux heat.
Thanks, gsmith, for your reply. It was just the explanation I needed. I did not expect loop temps to vary so much; I did not expect my backup strips to come on at all except in emergencies. I live in Southern Indiana. The backup strips come on when we have a deep cold snap, with a caveat that I will explain at the end. Meanwhile, here are some numbers: December: Heating degree days (HDD): 837. Backup strips consumed 32 kWh, amounting to 3.6% of that month's HVAC electric consumption. January: HDD of 898. Barely any backup strip use. February through the 19th: 510 HDD, with backup strips consuming 14 kWh amounting to 2% of the HVAC electricity Electricity here costs about $0.10/kWh. Now the caveat. After the backup strips came on in December, I shut off one of the four zones in the house, and I burned wood in the fireplace during cold periods. So, December is probably the best representation of the heat pump limitations. Is this "normal" or an issue for concern.
for a closed loop system, there is wide variation in ground loop temperature because the pipe to ground connection isn't that great of a heat exchanger (why you need so much pipe in the ground). It wouldn't be uncommon in a heating dominated climate to have a ground loop vary from say 30 degrees in winter to 70 degrees (or more) in summer while the deep ground temp is in the 50s. Open loops are different and they stay mostly the same temperature as the source water temperature. alot of people have their system data online at welserver (google it) and you can see the wide variation in incoming water temp across seasons for closed loop systems. I don' t know where in Southern Indiana you are, but using Evansville as an example, the design heating 99% temp per ACCA Manual J is 12 deg (design cooling 1% temp is 92 degrees). I looked up some historical records for Evansville (google is great isn't it!) and got monthly average HDD of 843 for Dec, 959 for Jan, and 760 for Feb. So I would say you had a pretty average December and possibly slightly warmer than average Jan/Feb. I don't know how to incorporate the zone changes and fireplace heating but December is likely your system "normal". So my gut says your system isn't quite designed to cover all heating with 100% geo and likely does 95% or more with geo and the rest augmented with heat strip use during really cold periods. Its not bad but certainly if someone was expecting 100% geo coverage it would be disappointing. Without going through the entire design its hard to say how much bigger the system would have needed to be to cover heating with 100% geo - maybe 1/2 ton up on heat pump size and maybe another 200 or 300 feet of ground loop (totally guessing here but probably not off by an order of magnitude). So I suspect this was done to save upfront cost and keep ongoing electric usage cost minimal - that heat strip use cost less than $5 so really not bad at all. You can go back and look and see how many times the temp got near the 12 degree mark as that is the sort of historical floor for temps in your area and could give you a sense between both the HDD and the min temp how close to normal this winter was and what to expect going forward. I'm not sure how your fireplace is set up, but assuming a standard wood burning/open hearth fireplace they can often cause air infiltration issues because the combustion air has to come from somewhere. So infiltration goes up in the rooms away from the fireplace cooling them off while it gets hot in the room with the fireplace from all of the radiant heat. If my electric was out and had no heat otherwise (ala Texas), you can bet I would have a fire going in the fireplace attempting to heat anyway possible. But for a normal power on/heat pump working situation, the fire place may not be helping and could be hurting. YMMV
Here's my http://www.welserver.com/WEL0899/ chart of annual closed loop temp swing. I'm in southern NY. 4 ton heat pump with 4X 200' deep boreholes. I used very little aux this winter, just short shots during the coldest (below 20F) days.
Tstat only. It engages when >1F off from set temp, but does react to fractional degrees. Because of this, and that I prefer constant temps, I don't program any setbacks.
Thanks again, gsmith, for another great explanation. Evansville was a good guess - it is the nearest airport. I also do not use setbacks; I let the system run against constant thermostat settings. As for the fireplace, it is a NorthStar wood burner: gasketed doors and outside combustion air. This thread was very helpful. Thank you.
