Heat pumps and other hydronic heat sources

Hana and Peter had a question Grace hears more every year: should they skip the gas boiler and install a heat pump instead? Their neighbor had put in an air-to-water heat pump the previous fall and talked about it constantly. Grace did not give a quick answer. "A heat pump can heat this house," she said, "but only if the radiators can do their job with cooler water, and only if we plan for the coldest nights. Let's work through it properly." This lesson explains how hydronic heat pumps work, what limits them, how they are combined with backup heat, and the other heat sources a hydronic system can use.

How a heat pump moves heat

A heat pump does not make heat from fuel. It moves heat from a cold place to a warm place, using electricity to drive a refrigeration cycle. A refrigerant evaporates in a coil outdoors, absorbing heat from the outdoor air even on cold days; a compressor raises its pressure and temperature; the hot refrigerant condenses in a heat exchanger, giving its heat to the hydronic water; and an expansion valve drops the pressure so the cycle can repeat. Because most of the delivered heat comes from outdoors, a heat pump delivers more heat than the electrical energy it uses.

The ratio of heat delivered to electricity used is the coefficient of performance (COP). A COP of 3 means three units of heat for each unit of electricity. An electric boiler, by comparison, has a COP of 1. The COP is not fixed: it depends on how far the heat pump has to lift the heat, from the outdoor air temperature up to the water temperature it produces. The bigger the lift, the lower the COP.

The right side of the equation is the ideal, or Carnot, limit, with temperatures in kelvin, multiplied by a factor η that real machines achieve, often around 0.4 to 0.5. The equation makes one point very clearly: lowering the water temperature the heat pump must produce raises its COP, just as lowering the return temperature raises a condensing boiler's efficiency. Low-temperature emitters suit both.

Capacity falls as it gets colder

An air-source heat pump also loses capacity as the outdoor temperature drops, just when the house needs the most heat. Modern cold-climate units with variable-speed compressors hold much of their capacity down to around 5 °F and keep running well below that, but their output and COP are lower. The temperature at which the heat pump's capacity equals the house's heat loss is the balance point. Below it, something else must supply the difference.

There are three common arrangements. A heat pump sized for the full design load needs no backup but is large, costly and oversized for most of the season. A heat pump sized for most of the season with an electric element as backup is simpler but runs the element on the coldest days. A hybrid system pairs the heat pump with a gas boiler, which takes over below a changeover temperature chosen for comfort and operating cost. Hybrid systems suit retrofits where the house's radiators need hotter water than the heat pump can efficiently produce on the coldest days.

# Balance point: where heat pump capacity equals house heat loss
def load_btuh(t):      # house loss, zero at 59 F, 36,800 Btu/h at 2 F
    return max(0.0, 36800 * (59 - t) / 57)
def hp_btuh(t):        # heat pump capacity, straight line through the two rated points
    return 22200 + (30700 - 22200) * (t - 2) / 16

for t in range(-5, 46, 10):
    short = max(0.0, load_btuh(t) - hp_btuh(t))
    print(f"{t:4d} F  load {load_btuh(t):7,.0f}  heat pump {hp_btuh(t):7,.0f}  backup {short:6,.0f} Btu/h")
lo, hi = -20.0, 50.0
for _ in range(50):
    mid = (lo + hi) / 2
    lo, hi = (mid, hi) if hp_btuh(mid) < load_btuh(mid) else (lo, mid)
print(f"balance point about {mid:.0f} F")

  -5 F  load  41,319  heat pump  18,481  backup 22,838 Btu/h
   5 F  load  34,863  heat pump  23,794  backup 11,069 Btu/h
  15 F  load  28,407  heat pump  29,106  backup      0 Btu/h
  25 F  load  21,951  heat pump  34,419  backup      0 Btu/h
  35 F  load  15,495  heat pump  39,731  backup      0 Btu/h
  45 F  load   9,039  heat pump  45,044  backup      0 Btu/h
balance point about 14 F

Water temperature is the deciding factor

Most air-to-water heat pumps produce water up to about 130 to 150 °F, but their COP is best below about 115 °F. The bungalow's cast iron radiators were sized decades ago for 180 °F water from an oversized boiler. Grace measured each radiator and estimated its output at lower water temperatures, as Lesson 8 shows. After air sealing, most rooms could be heated with 120 °F water on the design night, but the back bedroom needed more: Grace proposed adding a larger panel radiator there. With that change the whole house could run on 120 °F water at the design condition and much cooler water most of the season.

This is the general rule for any retrofit: find out the lowest water temperature the existing emitters can work with, then choose the heat source. Emitters, not the heat source, decide whether a low-temperature system will work.

Buffer tanks, defrost and minimum run time

Heat pumps need a minimum flow and a minimum run time, and air-source units periodically run a defrost cycle that briefly takes heat from the hydronic water to melt ice off the outdoor coil. A buffer tank, an insulated tank of system water between the heat pump and the zones, provides the volume that smooths these out. Without enough water volume, a heat pump serving small zones short cycles, and defrost can chill the zones. Manufacturers state a minimum system volume, and when the zones cannot guarantee it, a buffer tank is added. The buffer tank also acts as a hydraulic separator, letting the heat pump's circulator and the zone circulators work independently.

Refrigerant work and who does it

Monobloc air-to-water heat pumps keep all the refrigerant inside the factory-sealed outdoor unit; only water piping runs into the house, so the plumber can install the hydronic side without opening the refrigerant circuit. Split units have refrigerant lines between indoor and outdoor parts, and any work on the refrigerant circuit requires EPA Section 608 certification and the appropriate state or local license. Under the federal phasedown of high-global-warming refrigerants, new residential heat pumps have moved to lower-impact refrigerants, some of them mildly flammable, which brings its own handling rules.

The outdoor unit of an air-to-water monobloc contains water, so freeze protection matters. Manufacturers protect it with antifreeze in the loop, with freeze protection valves that dump water at near-freezing temperatures, or with a heat exchanger and glycol on the outdoor side only. The manufacturer's method must be followed exactly; a frozen outdoor unit is an expensive failure.

Other heat sources

Ground-source heat pumps, also called geothermal heat pumps, take heat from buried loops or wells instead of outdoor air. The ground stays at a steady temperature, so their COP is higher and steadier, but drilling or trenching the loop field costs much more. Water-to-water ground-source units pair naturally with radiant floors.

Solar thermal collectors heat a glycol loop that transfers heat to a storage tank, usually for domestic hot water with some space heating. Wood and pellet boilers serve rural houses with their own fuel supply and need large storage tanks to run efficiently. District energy systems deliver hot water from a central plant to many buildings through buried mains, with a heat exchanger in each building separating the district water from the building loop.

Domestic hot water is often part of the same plant. An indirect water heater is a storage tank with a coil heated by boiler water; the boiler treats it as a priority zone. A combi boiler heats domestic water on demand. Heat pumps heat domestic water in tanks with large coils, at lower temperatures, and periodically boost the tank higher to control Legionella bacteria, as PLB-130 discussed.

The recommendation for the bungalow

Grace wrote up three options for Hana and Peter. The first was a condensing gas boiler alone, the lowest installed cost, with fuel use far below the old boiler's. The second was an air-to-water heat pump with an electric backup element, the lowest emissions but the highest cost to install, with the backup running on the coldest nights. The third was a hybrid: the heat pump for most of the season and a small condensing boiler for the coldest weather and as backup if the heat pump ever failed.

For each option she listed the installed cost, the expected yearly energy use, the space needed indoors and outside, the noise of the outdoor unit near the bedroom window, and the work needed on the electrical service, because a heat pump with a backup element can need a larger electrical panel. She did not tell the owners which to choose. She explained the trade-offs and pointed out the one decision that applied to every option: the radiator changes and air sealing made any heat source work better and cost less to run.

The owners chose the hybrid, with the heat pump as the main source and the boiler taking over below about the balance point. Grace noted that the boiler would also heat the domestic water through an indirect tank, while the heat pump handled space heating. The rest of this course follows that system as it is designed, installed and commissioned.

Kingsway, by contrast, kept its gas boilers. The building's engineer had studied heat pumps for the plant, but the existing fan coils were sized for warmer water and the roof had no room for the outdoor equipment. The engineer's report recommended reviewing the option again when the boilers reach the end of their life, a common conclusion in existing buildings.