HPAC Magazine
Feature Article

How Low Can You Go?

A look at low temperature systems.

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September 1, 2012 by MIKE MILLER

Living in an era of highly efficient heat sources, including heat pump technologies, low temperature systems have gained tremendous popularity in recent years. Operating systems with low temperatures bring many benefits to the effectiveness and energy management components of traditional heating systems. A low temperature hydronics system operates with fluid running between 60F and 130F (≈16C and 54C). These include systems utilizing radiant floor or even wall heating.

Other heat emitters can run at low temperatures also, if the system is designed for that application. These include radiators, baseboards and fan coils. All of these have a specific Btuh output capacity based on the log mean temperature difference (LMTD), flow rates (resulting deltaT), conductivities and surface areas associated with the heat emitter. If one increases the surface area of a heat emitter, the water temperature needed for the same Btuh output can be lower (e.g. larger air handler coils, longer baseboard, bigger radiators, and so on).

If we understand this, it is reasonable to assume that radiant floors or radiant walls could yield the best results for low temperature systems, as the entire square footage of the floors and/or walls effectively become the radiant surface. The greater the surface, the lower the surface temperature and required water temperature needed, to maintain a space at a desirable setpoint. In turn, the lower the water temperature, the more efficient the fully condensing and modulating boilers can be. Low temperature systems also allow heat pumps to be utilized as a primary source of heat.


Low temperature radiant floor or wall systems are called radiant, but they also provide some convective heat transfer. The convective component varies with orientation (upward, horizontal or downward), surface area and temperature differentials, whereas the radiant component will always be constant following the laws of radiant transfer. It is typically around 0.97 Btu/ft².h.°F (5.5 W/m2K).

With free standing and wall-mounted radiators, since the total surface area is usually significantly less than that of radiant floors and walls, the overall surface temperature of the radiator would be greater compared to that of radiant floors. With forced air or fan coils, the radiant component is ignored. Radiant heats objects (including humans), as opposed to the air, thus increasing comfort levels.

How can we make low temperature systems (or any, for that matter) most efficient? Here are a couple of ideas that can help. First off, at lower temperatures, distribution and transfer losses are already reduced due to the lower temperature difference between the surface of the distribution piping and its surrounding air. Insulate all distribution piping to eliminate losses as much as possible between point A (source) and point B (load). Our European friends refer to it as “Getting from A (source) to B (load), without losing C (Centigrade).”

In order to get the water temperature down, increase the radiant surface, if you can (e.g. larger radiators). For radiant floors or walls, increase the amount of tube you put in(e.g. tighter spacing – 6″ versus 9″ on centre) to lower water temperature for the same Btuh/sq.ft. output. Note once again that greater surface area equals lower surface temperature.

Since systems are designed to heat the space at the worst possible outdoor condition (Outdoor Design), the system needs to be designed for: the right surface area, which could include usage of available square footage on radiant floors and walls; and water temperature (Design Supply Water Temperature) needed on that day. Employ outdoor reset water temperature controls that modulate the water based on outdoor temperature. This will result in greatly reduced water temperatures on milder outdoor days. That is one of the most significant ways of improving system efficiency, while maintaining steady comfort levels. 

From there, add indoor temperature feedback to a system, which is the technology described as fine-tuning the water temperature based on the outdoor and the indoor conditions. If the building envelop is tight, indoor temperature feedback typically results in lower water temperatures being delivered (as opposed to outdoor reset alone), while achieving close to continuous circulation and eliminating any temperature swings in the space. Indoor temperature feedback can also speed up response times of high mass radiant floor heating systems significantly.

Now, let’s look at examples of low temperature systems. The system shown in Figure 1 is a low temperature radiator system with a water to water heat pump. With few exceptions, heat pumps typically have an operating limit (MAX) of about 100-110F. This needs to be kept in mind when sizing the heat emitter. Manufacturers can help you with this information, but as an example in one case, the heat output of a given radiator needs to be multiplied by a factor of 0.271 (about a quarter) when operating in these temperature ranges, as opposed to 180F average water temperature.

What should the designer do in these circumstances? If there is a wealth of real estate, just make the radiator bigger. You would likely find that the heat output changes are similar when using baseboard as opposed to wall-hung radiators. 

Having heard on several occasions that radiators or baseboard cannot be combined with low temperature systems (e.g. heat pumps), it is important to cover this system design option. Everything is relative. If you cannot install radiant floors or walls in a retrofit application, do not forget that there are other choices. Just size accordingly.

Alternatively, the heat pump and storage tank could be replaced with a condensing boiler and the example would remain virtually the same.

What about the system shown in Figure 2 with a combination of condensing boiler and heat pump? During mild days or the majority of the heating season, the heat pump may be able to handle the required water temperature of this system. The boiler only takes over when the system needs increased fluid temperature higher than the operating range of the heat pump. One could size the radiators based on an average water temperature of 140F for the coldest outdoor condition and maximum heat load of the building. The boiler can provide DHW production all year round.

Figure 3 shows a combination of heat pump, condensing boiler, DHW, fan coils for second stage heating (oversized
to operate at lower temperatures) and some radiant floor or wall systems. The fan coils can also be used for air cooling and be tied in with HRVs for ventilation. The same principle applies as in Figure 2 applies. The heat pump can provide heat for the majority of a heating season where second stage heating (air) may not be required, or in the summer, where floor warming can be accomplished while the air temperature is being cooled with the fan coils. The key to this setup is to employ a control system that can enable and disable the heat pumps based on the system’s water temperature requirement to maximize system effectiveness and efficiency.

The examples above may not be  feasible in many circumstances or even commonly implemented. But they do serve as food for thought on what designers can potentially accomplish given the flexibility of hydronic heating systems. <>

Mike Miller is a national business development manager with experience in the manufacturing, distribution and contracting sectors of the industry. He can be reached at mike.miller@uponor.com, Linkedin or @hydronicsmike on twitter.

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