The number 1 reason people invest in a geothermal heat pump is for the operating cost savings. We have had tremendous success saving people money through this difficult winter and in the face of considerable energy price uncertainty. I routinely talk to customers who have paid $275 to $375 for a recent peak winter month. This is a terrific number especially since our recent winter months have been wicked. This consumption also includes their lights, fridge, TV, etc. When compared to any alternative, geo is unmatched and has been proving it every day. Here’s how to make sure there are no unpleasant surprises.
There are two key factors you must watch. These are high generation charges and undue reliance on back-up heat. Simple measures can be taken to avoid these unnecessary expenses and ensure you enjoy the cost efficiency your geo system happily delivers day after day, no matter what Mother Nature throws at you.
High generation charges
A relatively new variable in the electricity market is called ‘third party electrical supplier”. Northeast Utilities was compelled to exit the generation business some years ago in an effort to bring market competition into the electricity business. You may have noticed your power bill is a bit complex. One of the charges is the generation charge. NU offers a standard service for this component, or you can sign up with a number of third party suppliers. These suppliers are unregulated and the concept is they will compete on price or type of generation (green) to attract customers. This introduces another variable for electricity consumers, and is not specific to people with geo heat pumps, but of importance to everyone.
A terrific article on this is available in the Courant at:
This article is a must read. Also, any homeowner, geothermal or conventional, should visit the state website listed below to inform themselves as to the options available. It is true that electric costs are more important to people with geo systems, but in truth, they are important to all of us.
Excessive auxiliary heat (also called back-up)
Auxiliary heat, almost always electric resistance can spike your electric bill if it is coming on when you don’t want it to. We refer to this component as the toaster. It is a large electric resistance element that draws typically 10 kw. The operating cost of this unit is roughly 4 times higher per btu than the heat pump. Most times when I get a high bill concern, it has to do with the toaster. The toaster is there for good reasons. It provides inexpensive to install back-up heat and is a source of emergency heat in case of a problem. It is really important that the owner understands the auxiliary heat because it’s really handy when you need it,but you don’t want to use it unless you have to.
Heat pumps are controlled using multi-stage thermostats. If the heat pump (first stage) can’t satisfy the temperature, then the second or third stage kicks in, and the heat pump and the electric work together to satisfy the demand. The homeowner would be hard pressed to determine which component is doing what percentage of the work. For this reason, we like to leave the toaster disabled, either by leaving its circuit breaker off, or by installing a kill switch on the thermostat signal. In this way, the owner will be alerted if the heat pump is not keeping up. The house gets cold. Unpleasant, but better than a big electric bill and I will be called right away. One side note, if you are going on a winter vacation, enable the toaster before you leave. It’s good for some insurance against damage if something fails in your absence. Our rule is to leave the toaster disabled unless you need it. The following lists some situations that could affect the electric bill. All of them can be addressed but the first step is to identify the situation. My intent is to indicate we’ve seen it all, and we are well equipped to keep or in some cases, return, a geo system to the outstanding performance that is expected.
Setting back the temperature
Some people like to turn the thermostat up and down, or use a programmable thermostat. Whether for comfort or energy savings, this is fine as long as the toaster is disabled. If the toaster is enabled, and it comes on to bring the house back up to temperature, any savings and more are gone. There is a lot of disagreement about setback with heat pumps. I like a moderate setback as long as the toaster is off.
Mechanical failure of the heat pump
If the heat pump breaks and shuts down the toaster takes over, and the controls are seamless enough that the owner won’t know the heat pump stopped. We had a customer early this winter open up a outsized electric bill. Somehow, the electric heat breaker was left on, and when the unit developed a large refrigerant leak and shut down, he couldn’t tell. Again, leaving the toaster off will give the owner notice of a problem and give us a chance to make repairs promptly. Any machine can break, but the penalty should be as little as possible.
Undersized and over sized equipment is often blamed. Sizing the unit is a bit of an art form and it starts with a detailed and accurate load calculation using ACCA manual J. 50 btu/sq.ft. simply will not do. We run our own loads and try to size the unit to handle 100% of the design heating load and let the toaster remain disabled year over year. A case can be made that the most cost effective sizing is something less than the design heat load. Since design outdoor conditions are very short in duration, spending the extra money for the bigger system can be thought of as not cost efficient. In our area, a heat pump sized at 75% of the design load will provide 95% of the annual heating requirements. Since the toaster has so little run time, spending more money to eliminate it may not pay. The down side here is that the house will reach a balance point. This is the outdoor temperature at which the heat pump is running all out, and if it gets a degree colder outside, that degree is coming of of the indoor temperature. I don’t mind balance points in the positive single digits or low teens, but when you approach +20, you have to be careful. Informing the owner what to do and what to expect is crucial. It can be OK, but the owner needs to be paying attention. Some strategy needs to put in place to manage the back-up because it will be needed when it gets really cold.
Another design tool is ACCA Manual S, which limits the heat pump size to 125% of the calculated cooling load to prevent over sizing in air conditioning. For this we use the cooling capacity of the heat pump on low speed. Frankly, I have never witnessed system problems in cooling due to over sizing (short cycling and poor dehumidification). I have seen problems with over sizing where the unit puts out so much air, in both heat and cool mode, you have trouble getting all of it into the house in a comfortable way.
In the real world, all of the calculations may have been accurately performed, and the system installed accordingly, but if some aspect of the house doesn’t measure up to the assumptions, the system will effectively be undersized. Insulation levels and air sealing can affect heat pump performance. On many occasions we have fielded complaints about cost or comfort to find deficiencies with the insulated shell of the house.
Short loops are a often the whipping boy of outside trouble shooters. I don’t deny the possibility of loop troubles, but we believe in the old ARI standard 330. This standard documented the performance of heat pumps running on 32 degree source water temperature. The coldest loop I ever saw ( we have over 1000 installs) was 29 degrees, and the unit was purring right along. Now there is a penalty in both capacity and efficiency, but it’s not terrible.
Flow rate in the loop can be a problem. Propylene glycol has the consistency of Karo syrup, especially when it gets really cold. It’s hard to pump. The industry standard for flow rate is 3 gpm/ton. We like to design for 5, settle for 4. The enhanced flow rate goes a long way towards supplying the unit with btus to extract
Another loop issue is insufficient antifreeze concentration. We were called into a job for high bills (not our install). The unit would run great all summer and into December, but would then shut down, the electric came on and monthly bills got pretty big until spring came. This went on for three years. The loop solution had a freeze point of 23 degrees. As the loop got colder, there would be flash freezing in the evaporator, and the loop flow would cease. The freeze point was reduced to 17 or so resulting in satisfaction.
Since most loops are parallel circuits manifolded together, there is the potential for one or more circuits to become air bound and stop flowing. For this reason, we insist on our manifolds to be in the building to let us purge each separately and prove flow through each. Inaccessible below grade manifolds preclude this form of trouble shooting.
Any of these issues will adversely impact the heat pumps performance and result in undue reliance on the auxiliary heat. However, they can all be fixed.