HPAC Magazine

Comfort vs. Thermodynamics

John Siegenthaler notes that achieving and maintaining thermodynamic equilibrium within a conditioned space does not imply comfort.

October 21, 2020   By John Siegenthaler


(credit: Adobe Stock)

For decades, the basic approach to designing space heating systems has been:
1) Determine the “design” heating load of the building, (e.g., how many Btu/hr the building requires to maintain interior comfort during the coldest outside temperature);
2) Select a heat source capable of creating at least this many Btu/hr of heat output;
3) Design a distribution system that can divide up the heat output from the heat source and send the required rate of heat loss to each space within the building.

From the standpoint of thermodynamics, a building can be considered as a “control volume” — a simple hypothetical box that has heat flowing into and out of it. If the rate of heat input happens to equal the rate of heat loss, the temperature inside the control volume remains constant. This is called thermodynamic equilibrium. Any combination of building materials, dimensions, heating hardware, etc. that balances heat input with heat loss could attain a thermodynamic equilibrium.

There’s a Difference

Achieving and maintaining thermodynamic equilibrium within a conditioned space does not imply comfort. Even a room that maintains a stable 20C at the thermostat could have drastically different interior surface temperatures, strong drafts, a high degree of floor to ceiling temperature stratification, and millions of suspended dust particles in the air.

Still, if the space maintains a 20C air temperature at the thermostat some occupants might think the heating system’s performance is acceptable.


There are quantifiable characteristics associated with human thermal comfort. They include operative temperature, mean radiant temperature, radiant asymmetry, air temperature stratification, air velocity, relative humidity and others. Unfortunately, most of these parameters are neither calculated as part of the system design, nor measured during commissioning. Instead, the sole proxy for human thermal comfort during the heating season is being able to establish and maintain an air temperature in the range of 20C at the thermostat.

I call this attitude toward comfort “Btus in a box.” For many it doesn’t seem to matter how the Btus go into the “box” (e.g., living space) as long as the inflow rate equals the outflow rate.

This attitude is applied to a wide spectrum of building types and architectural paradigms. Perhaps it’s not surprising to see it in utilitarian buildings such as business rental properties, hotels or fast food. The intended function of these buildings—generating revenue—treats human thermal comfort with minimal regard. Design to satisfy the thermostat, end of story.

What Matters?

In modern North American culture, energy efficiency is often touted as having high value, something we all should aspire to. Fair enough. But achieving energy efficiency at the expense of comfort will, in the long run, fail as a business strategy.

The desire for comfort is autonomic. We all continually seek it. The lack of comfort has physical and physcological consequences. It determines, in part, our productivity, our attitude and our physical health. The need for comfort is not something we can escape, trivialize or appease by saving money on inferior heating systems.

Space heating products that promote energy efficiency sometimes make claims of “comfort,” but seldom offer quantifiable justification.

Consider, for example, a ductless air-to-air heat pump. Under standard AHRI rating conditions these heat pumps operate with impressive coefficients of performance. However, ductless air-to-air heat pumps used in cold climates require periodic defrosting of their outdoor evaporator coil. During this process heat from the occupied space is sent back outside to melt frost off the coil. This causes noticeably cool air to be blown into occupied spaces.

Although the cycle may only last a few minutes, it’s unpleasant to anyone in the path of that air stream, especially on a cold winter night. Defrost can occur several times each day. How do repeated blasts of cool air into occupied spaces equate to comfort?

Even air-to-water heat pumps supplying hydronic distribution systems have to perform defrost cycles. The difference is that the required heat comes from a buffer tank, or perhaps a high thermal mass heated slab, rather than room air. The occupants will have no perception of when a defrost cycle occurs. Comfort will not be compromised.

Some advocates of ductless heat pumps also suggest that only two or three interior “high wall” cassettes are needed. The caveat is that interior doors should remain open to allow warm air to disperse throughout the house. If this constraint can’t be met, some suggest setting up small electric space heaters to keep the air temperature acceptable in rooms with closed doors. To me, that’s a major compromise in how the building can be used.

Efficiency + Comfort

As the world moves steadily away from using fossil fuels, electrically-operated heat pumps are sure to gain market share as heat sources. This presents a major opportunity for the North American hydronics industry.

To capitalize on this trend hydronic professionals must leverage the high efficiency of air-to-water or water-to-water heat pumps with the superior comfort of low temperature hydronic distribution systems.

Those that do will likely receive support from “allies” they may not have anticipated. Most electric utilities actively promote heat pumps. So do many governmental energy agencies, often with generous financial incentives.

Advocates for renewable energy and a low carbon future generally recognize that electricity—increasingly produced from large scale renewable sources—is the “fuel” of the future. Builders and architects specializing in low energy or net-zero buildings generally promote the use of heat pumps.

What you bring to the table is the track record of superior comfort afforded by low temperature hydronic systems. It’s up to you to tell this story to customers, architects, builders, politicians and anyone else who will listen.

They all know that efficiency is important. Few of them understand the consequences associated with stark differences in comfort between forced-air and hydronic distribution systems.

If you plan to be in the hydronics business in the coming years, you need to know how to combine heat pumps with hydronics. I’ll do my best to bring you continuing information on this winning combination. <>


John Siegenthaler, P.E.

John Siegenthaler, P.E., is a mechanical engineer and a licensed professional engineer. He has more than 35 years experience in designing modern hydronic heating systems. His latest book is Heating with Renewable Energy (see www.hydronicpros.com for more information).

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