Lower Temperatures for Efficient Hydronic Heating

Load temperature is second only to heat pump selection in determining hydronic heat pump system efficiency. Load temperature is the temperature of the water that is distributed via the hydronic system to the heat-emitting devices, e.g. baseboard, hydronic air handlers, radiant floor systems. The lower the required maximum load temperature to that set of devices, the more efficient the heat pump system is. It’s not uncommon to be able to reduce annual operating cost by $300-500 (or more depending on the size of the system) by reducing the required load temperature by 20F.

Our designers reduce load temperature requirements by designing or specifying efficient heat-emitting devices. This will likely increase the cost of the system a bit, but the return on that incremental investment generally pays back quickly – a couple years is typical.

Here’s are a some illustrative examples:

  1. The heat distribution system is comprised of radiant floor heating in the basement slab, a ducted air handler in the attic that services the 2nd floor, and a couple of ductless air handlers on the 1st floor. We know that radiant heating with a slab requires relatively low water temperatures to maintain comfort, in this case less than 80F in the winter. However, required water temperatures to the air handlers are highly dependent on the size of the air handler. If we were to specify a couple of ¾-ton air handlers for the 1st floor and a 2-ton in the attic, the required water temperatures would be 125F in the winter months for each of the air handlers. Instead, we’ll specify larger air handlers – more powerful blowers and heat-exchange coils with more surface area – to enable those devices to deliver the same rate of heat to the areas they service at a water temperature of only 100F. While we’ve increased the total cost of the system installation by $800, we’ve reduced the annual operating costs for heating by $350 – a good return on that additional investment.
  • The entire house – under construction – will be heated by radiant floor. There will also be two ducted air handlers, one each in the basement and the attic to provide air conditioning to the 1st and 2nd floors, respectively. As above, we’re not concerned about the water temperature requirements for the radiant slab (in the basement); instead, we’re focused on the design of the radiant floor systems for the suspended 1st and 2nd floors and minimizing their required water temperatures. With suspended floors, the first consideration is whether to place the radiant tubing above or below the subfloor. Placed above, the heat distribution system has less distance to conduct heat to the space and, consequentially, requires lower water temperatures; however, it can be significantly – thousands, not hundreds, of dollars – more expensive and may impart additional requirements on construction as it can add a couple of inches of height of the finished floor above the subfloor and affect door clearances and ceiling heights. If those issues are acceptable, we’ll go with that approach. If not, we’ll install the tubing beneath the subfloor and consider using the air handlers servicing those spaces for air conditioning as supplemental heating, enabling us to reduce the required water temperatures to the radiant system… and increase the overall efficiency of the heat pump system.
  • We’re converting a home that currently heats with hydronic baseboard… and know at the outset that the baseboard, as is, will not sufficiently heat the space during winter conditions – it was likely sized for boiler water temperatures and the highest heat pumps can reliably provide is 130-140F. At a minimum, we’ll plan to upgrade portions of the existing baseboard panels from single- to two- or three-finned-tube panels to reduce the required water temperature to something a heat pump can achieve, but it may be justified to go further than that to drive required water temperatures even lower. We’ll consider replacing even more of the existing baseboard and/or using hydronic air handlers that may be required to provide air conditioning (like the above example) as second stage heat, thereby reducing the maximum required water temperature for the baseboard to 120F… and increasing system efficiency by 20% and reducing annual operating costs by hundreds of dollars.

It’s important to note that it’s the heat distribution device requiring the highest water temperature that needs our design attention to try to improve operational efficiency. The heat pump will have to deliver that temperature all the time regardless of the requirements of the other devices.

Although it has become common practice, it’s worth noting that any hydronic system should include an outdoor temperature sensor and controls that enable the heat pump system to automatically adjust its maximum load temperature output to optimize its efficiency – so-called outdoor reset. While the maximum load temperature requirement may be 110F when outside temperature is 5F, required load temperature will be proportionally less for milder outside temperatures… and the heating system must be designed to take advantage of that.

By carefully considering the design and/or specification of the heat distribution devices, we can often significantly increase the efficiency of the overall system with a relatively modest additional cost to installation.