This is the second article of a two-part guest post by David Butler of Optimal Building Systems in Arizona and a frequent commenter in the RESNET BPI group on LinkedIn. I've known David for a couple of years now, and I can attest to his expertise in the field of HVAC. When an HVAC question stumps Chris and me, we call on David because he'll most likely know the answer and will explain it in great detail. If you're a home energy pro, make sure you download and read his paper, The Elephant in the Room.
In Just Say No to Furnaces, part 1 in this 2-part series, I made the case for why a furnace may not be the best choice for heating a high performance home.
Heat pumps and/or hydronic forced air heat are more appropriate since system capacity can be aligned more closely with the load. This ensures good air mixing at design conditions, an important prerequisite for comfort. Deciding which is best for a given project depends on climate and relative energy costs.
In warm climates with average or better electric rates, the heat pump is hands-down the best heating system. It costs less to operate than a furnace (most markets), and in tight, well insulated homes, a heat pump is actually more comfortable. The lower supply temperature, once considered the bane of heat pumps, is actually preferable in well insulated homes.
Since heat pumps produce less heat as the outdoor temperature drops, supplemental heat is needed in all but the warmest regions. In high a performance home, the need for supplemental heat is dramatically reduced, with a typical balance point (BP) in the mid-twenties. One concept that’s important to understand is that performance doesn’t fall off a cliff below the balance point. At five degrees below the BP, a heat pump will still carry about 75% of the load. If the number of hours below the BP is small, electric supplemental heat is the best option, given its low first-cost.
In moderate to cold climates where a heat pump requires significant supplemental heat, a natural gas-fired hydronic coil is preferable to electric strips. A high-efficiency tankless water heater, such as the Rinnai Ultra, is the ideal heat source for hydronics. The coil, which looks like a radiator, should be sized to handle the full design load. It’s installed between the air handler and the supply plenum (the heat pump coil is on the return side). As with electric strip heat, the hydronic pump is energized when the thermostat senses the heat pump can no longer handle the load.
Most people don’t realize that heat pumps are still relatively efficient at cold temperatures. My 15-SEER Lenox has a COP of 2.8 at 23°F (the BP for my home). But with natural gas prices the lowest it’s been in decades, it’s important to determine the economic balance point for hydronic heat – the temperature where it becomes more economical. Thermostats are available that can lock out the heat pump below a given temperature.
In areas with high electric rates, the economic balance point may be sufficiently high to warrant using straight hydronics. In this case, the air conditioner can be matched with an air handler with a hydronic coil. If there’s no cooling requirement, radiant heat begins to look attractive. Note that baseboard radiators cost much less to install than in-floor tubing, especially in situations where gypcrete is required.
In cold climates without access to natural gas, the choices are more difficult. On a per-btu basis, propane costs about as much as electric resistance heat, and prices tend to be far more volatile. Fuel oil may be more economical, but is decidedly un-green. Ground-source heat pumps may be worth considering in this scenario, but the high initial investment may be better spent on additional insulation. In cold climates without access to natural gas AND high electric rates, you're up the creek without a paddle. Can you say PassivHaus?
Here’s an interesting fact that surprises many people: Extremely cold climates experience far more hours above than below the BP. For this reason, heat pumps are an excellent choice in markets like Minneapolis, where 75% of the heating hours exceeds 25°F. If you don’t believe me, just check the TMY2 BIN data published by NREL.
When comparing the operating costs for heat pumps vs. hydronics or other types of heat, it’s necessary to estimate how much of the annual load will be carried by the supplemental heat. Unfortunately, annual heat loads are notoriously difficult to model due to the impacts of solar gain and thermal storage. Modeling supplemental heat usage is more dubious. It requires judgment prefaced by a thorough understanding of the numbers. This area is fodder for another article, but one thing I’ll warn you about now: Forget about HSPF when comparing heat pumps with other types of heat. That rating is only marginally useful when comparing one heat pump with another. Rely instead on COP (typically at 47°F), blended with your best estimate of supplemental heat usage.
A final word about heat pumps. The contractor must do the following four things to ensure maximum comfort and economy:
- Locate supply registers so air can be directed away from beds and seating areas.
- Install the air handler and duct system inside the thermal envelope. (I would say this is a prerequisite for calling a house high performance.)
- Provide an override switch for electric supplemental heat (instruct occupants to leave it off until it’s really needed).
- Set the thermostat auto-changeover ‘dead band’ to 8 degrees, or better yet, deactivate that feature in the installer menu (applies to any HVAC system).
Read Part 1: