Geothermal heat pumps are nearly always installed with some provision for back-up heat, supplemental heat or emergency heat. Often, this is electric resistance. This is done because it has the cheapest installed cost for any back-up heat system. Unfortunately, it is the most expensive to operate. There is often confusion about the operation of the back-up system.
To start, I’d like to distinguish some terms. Back-up heat is often associated with older air source heat pumps that suffer significant reductions in capacity when the outside air temperature dropped into the 20 degree area. The air source heat pump either isn’t worth much under those conditions, or it would actually shut off. The controls would then enable the back-up, and the whole heat load would then be on the back-up. Expensive! This is one of the causes of the lingering negative perceptions of heat pumps in general. For this reason, I’d like to discard the term ‘back-up’. Supplemental heat is closer to the mark in describing the scenario for the relationship between a geothermal heat pump and the electric heater. The important point is that with a geothermal system, the heat pump maintains it’s output and efficiency regardless of outdoor temperature and the supplemental is only operated to make up any deficit because of equipment sizing or extreme outdoor conditions. It doesn’t take over, it just adds a boost. The term emergency heat is pretty self explanatory. If something in the heat pump breaks, you can switch over to emergency heat until repairs can be made.
The design of a geothermal system starts with a detailed heat loss/cooling gain calculation. This determines the demands on the system. The heat loss/gain is mostly a function of the size, insulation levels and air tightness of the structure, and the outdoor weather conditions. It’s a simple matter to calculate the exposed area of a structure and assign a thermal resistance to them. Infiltration rates are a little trickier but there are accepted methods. The outdoor design temperature is defined by weather data and represents the 97.5% outdoor temperature. This means, the outdoor design temperature for heating is the temperature it is at or above 97.5% of the time, on average. The same holds for cooling as the design temperature is defined as the outdoor temperature it is at or below 97.5% of the time. Using these parameters, the designer can determine how much heat or cool the structure requires, and can then select the proper capacity heat pump. The most common load calculation method is known as ACCA Manual J and it is the industry standard.
Heat pump selection
It is important to understand that the installed cost of a geothermal heat pump rises with the capacity of the system. Also, geothermal is also not a ‘high output’ system. Another factor is common to any refrigeration system, and most machines in general. That is, once any machine starts, it is most efficient to let it run. Starts and stops introduce inefficiency. Think of your car. Get in and drive to Florida, or drive around the neighborhood delivering papers. The former is much more efficient. We should accurately assess the loads on the system and select the right heat pump to match it. Unit sizing it critical. Grossly over sized units will cycle too frequently (inefficient) and provide poor dehumification in the summer. Grossly undersized units will be too reliant on supplemental heat
One method of heat pump selection is ACCA Manual S. The important aspect of Manual S is that it tries to balance the relationship of the heating and cooling loads with the heat pumps distinct heating and cooling capacities. A geothermal heat pump is one machine that provides both the heating and cooling. There is no flexibility to tailor separate equipment to each load. We work in New England. We are almost always dealing with a heating dominant situation. Manual S limits the heat pump size to 125% of the units rated low speed cooling capacity compared to the calculated cooling load. Often in this area, that will leave the heat pump a little short of heating capacity on the really cold days. The situation is much worse if you are dealing with a single speed unit, but most manufacturers offer a full line of two speed equipment, and that is a very desirable feature. The temptation to ‘make sure you have enough‘ is compelling but wrong. Most often, the correct sizing is something under the calculated heating load and the supplemental makes up the shortfall as required.
Balance point means the outdoor temperature at which the heat pump is running full out, and if it gets a degree colder outside, the interior temperature will also fall a degree. We use +8 degrees as our outdoor design temperature, and 70 degrees for the indoor temperature. This gives us a base of a 62 degree temperature difference. If we were to select a heat pump that would run 100% of the time under these conditions, we would exactly hit the load and +8 degrees would be our balance point. Due to the limitations of Manual S, sometimes we can’t pick a unit that has sufficient heat capacity and our real balance point will be something higher. Outdoor temperatures ranging below the balance point can cause the controls to ask for a shot of the supplemental heat. Other factors are involved such as internal gains. Since the coldest outdoor temperatures occur at night, setbacks can matter. The extreme cold is usually very short lived, and the thermal flywheel of everything in your home may carry you right through.
I would like to briefly discuss how the thermostats control the heat pump and the supplemental heater. The concept is called ‘staging’. We use 3 stage heat, 2 stage cool systems. The first call for heat will bring the heat pump up to low speed. If the call persists, the thermostat calls for high speed, and if the call persists further, it may call in the supplemental. Today’s thermostats use either temperature differential (how far away from the asked for temperature the home is) or a rate of climb calculation to determine when to call in the supplemental. These are also often subject to time delays while some use outdoor temperature sensors to calculate expected needs. On the immediate horizon, we have thermostats that are web enabled and will use expected weather report data to smooth out their operations and anticipate demands. These developments all augur well towards control systems that optimize a ‘right sized’ geothermal system by minimizing calls for supplemental heat..
An analysis of consumption utilizes bin weather data. Bin data is a an hourly measurement of outdoor temperature for a full year broken down into 5 degree hourly ‘bins’. A heat pump sized at 75% of the heat loss will provide 99% of the annual heating requirements, the remaining 1% made up by the supplemental. This is because most of the energy use occurs in the outdoor temperature bin of 30 to 35 degrees, and also the contributing effects of internal gains, (cooking, appliances, light, sun, showers etc.). If we change the parameters to look at a system sized at 100% of the heat load, we find the heat pump provides essentially 100% of the heat requirements (the supplemental uses .15 kwh). Our experience bears this out. We have many customers that have gone year over year with the supplemental heat system disabled. The smaller, less expensive system will perform better on many fronts. It has to move less air (comfort), cycles less frequently and will provide excellent dehumidification in air conditioning, all at no measurable penalty on winter operating costs. The conclusion is that you shouldn’t think that if your geothermal system has less capacity than the calculated heat loss, it is poorly designed. Most likely, it is perfectly designed.
Some common concerns
‘It’s running all the time‘
Heat pumps like to run. The hardest thing they do is start. Just like your car, once it’s going the most efficient thing it can do is run. When the outside air tempeature gets down to design condition, the heat pump runs all the time. Also, since they are multi-speed, low speed will run almost continually at higher outdoor temperatures. In our area, annual run times of 2500 to 3000 hours per year are not uncommon. Since there is only 8760 hours in a year you can see the units run a lot in the winter.
‘If the calculations say i need 50,000 btuh and your only giving me 37,500 btuh, will I freeze?’
People automatically associate the calculated heat loss with what they need. The actual design condition occurs infrequently and for short duration. The installation of the supplemental heater (usually 10kw or 41,000 btuh) provides ample capacity to heat the home well below design temperature.
‘Will I always be running the supplemental and will it cost a lot’
As demonstrated above, the nature of the weather data (very short extreme cold spells), the sophistication of the controls and the helpful benefit of internal gains all minimize the percentage of supplemental heat actually used even when a system is designed at 75% of the heat loss.
A case study where we had to pay attention to supplemental heat
This is a 3454 square foot home built in 1837. The structure is essentially solid rock. We were called in to design and install a geothermal heat pump system. The owner reasonably addressed upgrading all heat loss surfaces, but the stone walls offered no options. We calculated the design heat loss at 99,957 btuh and the cooling load at 53,073 btuh. The home had an existing duct system that could only support the air flow from a six ton heat pump, which had a rated output of 62,400 btuh. This puts the capacity of the heat pump at only 62% of the design heat loss. Another problem was the cooling load was only 53,073, which fit the six ton unit well. We wanted more heating capacity but the duct work would not accommodate it, and we would run afoul of Manual S to install more capacity. Also, a six ton unit is usually the biggest available for residential applications, so to add more capacity would also mean another unit, which is considerably more money. Since we had no options to increase the capacity of the duct work, we were locked into the six ton, undersized heat pump. The balance point of the system would be 31.3 degree outdoor temperature. This is uncomfortably high to rely on electric resistance. The other issue was our comfort level with the calculations. Modern, well insulated homes are one thing, but how should we expect a solid stone home to behave. We chose to not rely on electric resistance for supplemental heat but opted to install an oil boiler controlled as third stage to boost capacity. This configuration affords quick recovery times and operates less expensively than electric resistance. Usually we don’t care about the operating cost of the supplemental because, as shown above, it just doesn’t run much. This house was going to be different. The supplemental may run a fair amount. The oil fired supplemental also provides as a nearly inexhaustible supply of domestic hot water and prevented the need for a significant upgrade in the electric service that would have been required if the supplemental was electric. Our design also required the loop to be oversized. This job required three wells at 440 ft. A more typical well field for a six ton system would be two wells at 375′ or 400′. This was crucial because the system described above will be running long hours and without the extra loop length, the incoming fluid temperatures would go below optimum levels. The system is almost through its second winter season, and all’s well.
The simplest conclusion is that a geothermal heat pump needs to be ‘right sized’. Too big and too small both have negative outcomes. A consumer should not be concerned about some (small) reliance on supplemental heat as it is usually required to balance the heating and cooling aspects of the system. Control of the supplemental heat needs to be well thought out. For replacement systems, existing factors can drive some of the decision making and a thorough understanding of those limitations are crucial.