Warning: This is another of my more technical posts. There are no pretty pictures.
This post addresses two parts of the HVAC system, the Heating and the Air Conditioning. The other part of HVAC, Ventilation, will be addressed in another post.
In Michigan, most people install a natural gas forced air furnace and air conditioning. Those willing to spend money on a premium system consider radiant floor heating. However, highly insulated houses have some differences from typical houses which can change what types of heating a cooling systems are appropriate. Homes which take insulation and air tightness to extremes, such as those that conform to the Passivehaus standard, usually end up with different types of heating and cooling systems, such as one or two mini-split ductless heat pumps. In terms of insulation, this house falls into a middle ground between typical houses and Passivehaus houses. So, it wasn’t obvious whether our systems should be like typical houses, like Passivehaus houses, or some other choice. (Well, maybe it would have been obvious to someone with less tendency than me to over-think these things.)
Highly insulated houses have a much lower heating demand than typical houses. Why wouldn’t they just use smaller versions of same types of systems that typical houses use?
- Mainstream HVAC equipment manufacturers don’t attempt to serve the highly insulated house market. The range of equipment sizes offered is based on typical house heating loads. In fact, most equipment installed in typical houses is considerably oversized.
- Distributing heat is easier in a highly insulated house since the heat is not escaping as quickly. That opens up some possibilities. However, the warm floor feeling that many people like about radiant floor heating would not be so noticeable in a highly insulated house.
- People interested in net-zero homes prefer all-electric systems, which can be supplied by PV panels.
- Ironically, spending more for highly efficient equipment is less likely to pay off in a low load home. People who have spent extra money for insulation may want to recoup some of that by spending less on heating and cooling equipment.
For any type of equipment, the first step is to calculate the design heating and cooling loads. The heating load includes heat lost through walls, windows, ceiling, etc. due to conduction and also heat lost due to air leakage and forced ventilation. For my location, the design heating load is calculated at 7 degrees F. Although the temperature gets colder than this, it rarely stays colder than this for long periods. The design heating load ignores various internal heat gains like solar gain through the windows, use of the fireplace, appliances, and people. These heat gains, and the thermal mass of the house and contents, sustain the inside temperature when the outside temperature drops below the design temperature. Also, the equipment is usually sized at least a little larger than the design heating load. The design heating load came out to 34,000 btu/hr. The design cooling load, which is calculated at 88 F, came out at 18,000 btu/hr. Design cooling load does include some internal gains.
The first option I considered was a ground source heat pump (sometimes called a geothermal heat pump). Instead of creating heat by burning fuel, a heat pump moves heat that already exists. To move a btu of heat from a cold place to a warm place requires energy but, if the temperature difference is small enough, it requires less than a btu of energy. The advantage of a ground source heat pump is that the heat is being moved from the ground which, in theory, is a constant, moderate temperature. The heat is extracted from the ground by fluid that flows through buried tubes. In summer, the same process is used to transfer heat into the ground to provide air conditioning. Unfortunately, heat transfer from solid ground is not very efficient. A lot of tube must be buried to transfer enough heat, which gets expensive. Even then, the temperature of the dirt near the tubes is not constant due to the heat being extracted. I was dissuaded by a number of articles on Green Building Advisor that found that the extra efficiency of a ground source heat pump relative to a modern air source heat pump is not worth the additional cost.
That takes me to the next option I considered – mini-split heat pumps. These move heat from the outside air. Traditionally, air source heat pumps have been out of favor in cold climates because the capacity and efficiency declines when the outside temperature gets lower. However, technical advances have made them a practical cold climate alternative. A ductless mini-split provides the conditioned air directly to the room as opposed to blowing air through ducts. This improves efficiency but requires equipment on the wall which raises an aesthetic concern. Another issue is how well the heat is distributed from these units to other rooms. A ducted mini-split can serve several rooms though a small duct network. Ducted units give up a little of the efficiency but improve heat distribution. One thing that builders of super-insulated houses like about mini-splits is that they are available in small capacities – as low as 6000 btu/hr. They use electricity instead of natural gas which is great if you want to use PV panels to achieve net zero. In Michigan, however, electricity is much more expensive than natural gas.
A related option is an air-to-water heat pump, such as the Chilltrix system. Whereas a mini-split relies on refrigerant lines between the outdoor unit and the indoor unit, an air-to-water heat pump retains all of the refrigerant within the outdoor unit. The heat is transferred between indoor and outdoor units by water lines. This enables smaller indoor units. Other than this distinction, the advantages and disadvantages of air-to-water heat pumps are similar to those of mini-split heat pumps.
Eventually, a fuel price comparison convinced me that a natural gas solution made more sense than an electric solution. Some builders of low load homes have taken advantage of relatively low natural gas prices by installing a combination space heating and domestic hot water system (sometimes called a combi-system). Domestic hot water is supplied by an efficient natural gas water heater, such as an HTP Phoenix Light Duty. Hot water is circulated through coils in a hydronic air handler to provide space heating. Trying to get local contractors to quote and install a combi-system proved difficult.
In the end, I decided on a conventional natural gas furnace. The smallest size most companies offer in most models, including their modulating models, is 60,000 btu/hr. (An exception is a Canadian company called Dettson that sells modulating furnaces starting at 15,000 btu/hr. I ended up ruling out this option due to unfamiliarity to local contractors.) However, some two-stage high efficiency furnaces are available with an input capacity of 40,000 btu/hr. The output capacity is 25,000 btu/hr on low stage and 39,000 btu/hr on high stage which is a good match for the house’s heating load. The smallest available air conditioning unit is 1 ½ tons which is a good match for the cooling load.
The British say that Americans can be counted on to do the right thing, but only after they have tried everything else. I guess I am willing adopt the conventional solution, but only after I have ruled out all other possibilities.