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Government planners, taking their queues from politicians and advocacy organizations, have steered the future of energy supply away from fossil fuels and toward renewably-sourced electricity. It’s happening on every level from the feds right down to local city councils.

One of the major targets for this transition are heating systems, most of which currently operate on natural gas, fuel oil or propane. One of the challenges is how to migrate “legacy” heating systems that currently operate on these fuels over to heat pumps. Such migration is particularly challenging when it comes to hydronic heating systems.

`Imagine a boiler-based system that was installed in the 1950s, and has undergone numerous modifications, expansions, or component replacements, many of which were done by different installers.

Although there’s a chance that such modifications were done in a coherent and professional manner, experience has shown that there’s a better chance that the system has been “buggered up” to an extent that the next heating pro who gets called to “fix it” would rather just turn it off, and convince the homeowner to go with a plug and play solution the offers heating and — let’s get excited here — cooling!

In many cases that replacement system is going to be a ductless mini-split heat pump system. Just get out the 3 inch hole saw, fasten some plastic raceway to the outside of the building, run the refrigerant line sets and wiring through that raceway, commission the refrigerant line sets, and hand the remote controller to the owner.

It’s hard to argue that ductless mini-splits are an attractive option in comparison to diagnosing and fixing some cobbled up assembly of piping, heat emitters, circulators, controls and heat sources. However, those little white boxes on the wall do not, and never will, offer comfort comparable to a properly designed and installed hydronic system.

Homeowners don’t have to “settle” for a ductless mini-split in order to bring heat pump technology into a home with an existing hydronic system. Air-to-water heat pumps offer an alternative that’s comparable in price on the basis of heating capacity, but offers far more benefits than ductless. Here are some of the key differences.

Comfort matters

In North America, the vast majority of new home heating systems are selected by builders rather than the eventual occupants of the home. When planning for new construction, there so many modern amenities competing for consumer attention that the choice of heating system often ranks just above that of a septic tank or a sump pump. These days a Wi-Fi thermostat garners more interest than the remaining 99% of the heating system. Most consumers, who don’t think much about how their heating and cooling systems work, are easily convinced that a smart thermostat knows how to best manage the system. A key selling point is that such thermostats can reduce energy use by lowering air temperatures. That’s true, but that savings comes with the assumption that the resulting change in comfort is acceptable. Occupants insulated from their interior surroundings by Hoodie Footies tap on their smart phones to review the energy saving stats reported from their WiFi thermostats. If only data could sooth the thermal soul.

Prospective homeowners who appreciate how superior thermal comfort will affect their health and enjoyment of the new home, and who’ve taken time to research the differences in heating systems, will typically choose hydronic distribution systems over forced air. Air-to-water heat pumps, rather than boilers, can be the heat sources in such systems. They can also provide cooling, which for many decades, was the missing part of the hydronics offering.


Ductless mini-splits can be configured with high wall cassettes, 4-way ceiling boxes, or wall consoles that try to mask themselves as picture frames. Those terminal unit options all involve aesthetic compromise. Other than heating geeks, who really wants to “look” at the devices that heat their interior spaces? Those same options also involve fans that emit unwanted sounds, and thus represent some degree of acoustical compromise.

Hydronic systems can be designed with a wide range of heat emitters such as radiant floor, ceiling, or wall panels that are completely out of sight and operate silently. When the completely out of sight constraint isn’t a priority, additional options include panel radiators and towel warmers equipped with thermostatic radiator valves that allow individual comfort control in each room. No smart phones, apps or Wi-Fi needed. No batteries or wires either. Just simple, silent, and stable comfort management.

It’s also possible to adapt an air-to-water heat pump to an existing forced air distribution system. An air handler with a generously sized coil replaces the furnace, A-coil, and outdoor condenser.

You might be thinking — why not just use an air-to-air heat pump for such a retrofit? Read on, you’ll see that air-to-water heat pumps offer more possibility beyond forced air heating and cooling.

Domestic duty

Every house needs domestic hot water. Hydronic heat pump systems (e.g., air-to-water or water-to-water geothermal) can be designed to provide domestic hot water as well as space heating and cooling. The DHW load is an excellent match for such heat pumps because much of the energy needed occurs at lower temperatures allowing the heat pump to operate at high efficiency.

When ductless mini-splits are selected for space heating and cooling, domestic water heating is typically provided by a separate heat pump water heater. This combination works, but it’s not as efficient as using a single air-to-water heat pump for both space heating and domestic water heating. For more information on why this is true, check out my column from the January 2021 issue of Plumbing & Mechanical.

Cool and fresh

Modern air-to-water heat pumps all come with refrigerant reversing valves. When you install an air-to-water heat pump for the purpose of providing heat, you’re also installing a chiller. Pipe the heat pump to a chilled water air handler through a diverter valve, install a simple forced air delivery system, and you've got central cooling. Figure 1 shows one system concept.

air-to-water heat pump system conceptFIGURE 1 [click image to enlarge]

Since there’s a ducting system associated with the air handler, you can also tie in a heat recovery ventilator (HRV) as shown in figure 1. Some modern ECM blowers can be set up to run a low speed 24/4 to distribute the fresh air supplied through the HRV. They can switch to a higher speed when supplying cool, dehumidified air when the system operates in cooling mode. The result is a “total system” that supplies space heating, cooling, domestic hot water, and heat recovery ventilation. It’s expands the scope and profit potential for the installer while ensuring the consumer that a single installer is responsible for all aspects of the HVAC/DHW system installation and servicing.

Easy expansion

Hydronic systems designed around homerun distribution can be easily and inexpensively designed for future expansion. Maybe your client would consider adding a towel warmer in the master bathroom, or a panel radiator in the basement recreation room in the future when finances are available, just provide one or two extra sets of tubing connections on the manifold station. When the time comes, pull a couple of lengths of 1/2-inch PEX from that manifold station to where the towel warmer or radiator will be. Make the connections, open the valves and smile as you watch your client appreciate the added comfort enjoyment you've just provided.

Want to expand a ductless heat pump system? It’s possible, if you happen to have the right combination of outdoor condenser and indoor fan-coils. If not, plan space and money for an extra condenser unit, new wiring, new line set, added refrigerant and another fan-coil.


All air-source heat pumps operating in cool climates must periodically defrost their outdoor evaporator coil. Most do this by reversing the refrigeration flow, effectively putting the system into a temporary cooling mode.

In the case of ductless heat pump, the warmth need to melt the frost is “borrowed” directly from the indoor air. Comfort is temporarily (but noticeably) compromised.

When an air-to-water heat pump is used, the heat required for defrosting comes from the buffer tank, or perhaps for high thermal mass radiant floor. There is no perceptible change in comfort during the defrost cycle. That’s a distinct advantage that potential customer should be made aware of.

Distribution efficiency

Some purveyors of ductless heat pumps, and their commercial cousin (variable refrigerant flow (VRF)) systems, contend that there no need for a circulator, and thus no electrical energy needed to operate a circulator. The refrigerant flow from the outdoor unit to each indoor terminal is driven by the compressor. Both of those statement are true, but they don’t convey the physical fact that the electrical power input to the compressor required to move the refrigerant through the lines sets, as a percentage of the rate of heat transport, is far higher than what’s needed for equivalent heat transport in a well-designed hydronic system. Figure 2 shows a comparison.

Distribution efficiency comparisonFIGURE 2
Image courtesy of Hydronics Industry Alliance.

Adaptability to contemporary utility rates

There’s currently a lot of interest in energy storage on the part of energy planners. Most of it is focused on storing electrical energy using lithium-ion battery systems. That’s fine, but energy planners, as well as HVAC pros, should also evaluate and leverage the thermal energy storage potential of high thermal mass hydronic systems.

Consider a low-energy house with a heated 32 x 48-foot slab-on-grade floor 4 inches thick. The thermal mass of that floor is equivalent to 1,800 gallons of water. A 1° F change in the average temperature of that slab could release just over 15,000 Btu. If the house has a design load heat loss of about 10 Btu/h/ft2, which is very attainable with modern construction methods and materials, that 15,000 Btu could provide one hour of heat input at design load conditions, and two hours of heating at 50% load.

In an effort to better manage existing generating capacity, and avoid building new generation facilities, more and more utilities are offering time-of-use electrical rate. Figure 3 shows an example of how the price per kilowatt-hour varies from one utility offering these rates.

example of how the price per kilowatt-hour varies from one utility offering these ratesFIGURE 3
Image courtesy of

An air-to-water heat pump could be operated at off-peak rates to charge the thermal mass of the slab. The heat pump could be turned off during some or all the ensuing on-peak hours while the house “coasts” using heat released from the slab to maintain comfort.

The exact details of such a system depend on many variables and will require detailed modeling to optimize, but the concept is sound and the parts and pieces needed all exist.

Ductless heat pumps can also be operated on off-peak rates, but they lack the thermal mass needed to decouple heat delivery from heat production while also maintaining relatively stable and acceptable comfort in the building.

Sell the advantages

Heat pumps, in all their assorted “flavors,” are rapidly gaining market share against systems operating on fossil fuels. The global hydronic heating market is steadily working its way towards a new normal where heat pumps will be outselling boilers as hydronic heat sources. That, by the way, has already happened in Germany, arguably the epicenter of modern hydronics technology. Why not ride this wave using innovative hydronic-based approaches that have the potential to provide benefits far exceeding those offered by those little white boxes on the walls.