HVAC Engineering: Chilled Floors + Humidity

On the Integrated HVAC Engineering discussion board, expert Robert Bean answers questions about chilled floors and humidity considerations and air mixing…

Student 1:  

Chilled floors seems like a great idea, and if you’re going to do a radiant system, you might as well consider cooling and heating.

The humidity, however, seems to be a big concern. Vapor barriers are all well and good, but in many of the homes you’ve shown us, there are operable doors and windows.

How do you deal with this issue? Are humidity sensors part of the controls? Are these reliable enough? Maybe you can just set some default dehumidifcation process in place and the system will balance itself. However, cold spots on floors or in corners might have low temperatures, and maybe the air mixing dehumidification won’t be enough, or in time.

Robert Bean:

RE: If you’re going to do a radiant system, you might as well consider cooling and heating.

Yes (and sometimes no).

RE: The humidity, however, seems to be a big concern.

Yes. As it should be… but not just for radiant cooling. Humidity should be a big concern for all HVAC system types. I like to remind all designers that 100% of all condensation problems in buildings conditioned exclusively with air did not have radiant cooling panels to blame. Another way to say this is, most moisture and mold related problems in buildings exist in structures conditioned with 100% air systems. So, the focus needs to be on moisture control regardless of system type.

RE: Vapor barriers are all well and good, but in many of the homes you’ve shown us, there are operable doors and windows.

I don’t believe you would get any argument from the radiant crowd that it takes a certain client to own and operate a home conditioned with radiant cooling. A young family in Atlanta, Georgia with kids in a mass produced carpeted starter home wouldn’t be the ideal client. Chances are it wouldn’t be offered by the builder, nor would the client likely have the budget for the necessary system and controls. Even if they did – the condensation risk is inflated due to the odds of windows and doors getting left open (which increases proportionally with how many friends the kids have). Good practice would suggest radiant floor cooling would not be an ideal choice for that client in that circumstance in that geography. 

Yet mature, financially established clients constructing a high performance home located in southern Alberta or Northern California finished with low VOC conductive floors such as tile, or slate are an ideal candidate. They have less distractions in managing their property, a low humidity geography, building performance designed to deliver low cooling loads, interior design choices based on IAQ and thermal performance, and the financial ability to invest in the necessary systems and controls.

However, in all buildings, moisture must be controlled for biological concerns (bacteria, viruses, mites and molds), for hydrolysis and the control of VOC emissions, (formaldehydes, phthalates, terpenes etc.) for the dimensional stability of hygroscopic materials (woods),  for the preservation of materials, and for occupant respiratory and thermal comfort. It is the control of moisture which enables the use of cooling systems– including radiant panels.

RE: How do you deal with this issue?

By delivering lean supply air to the space. As we discuss in the radiant chapter- typical supply air will be 55F @ 50%RH and limiting the panel surface temperature 2F to 3F above dew point at the upper humidity ratio at design conditions. 

RE: Are humidity sensors part of the controls?

Yes. In the discharge air plenum and with dew point sensors typically mounted to the supply manifold, or a vertical supply pipe at the manifold.

Re: Are these reliable enough?

Yes- these are reliable. 

RE: Cold spots on floors or in corners might have low temperatures.

In the case of floors, the surface temperature is regulated for thermal comfort as per ASHRAE 55 and dew point control as per the load calculations. As you will learn in week 8 and 9, the fluid temperatures for floors are frequently in the 55F to 65F range with floors surface temperatures limited to 66F and higher. 

The coolest location is a function of load, floor coverings, back gains, manifold location relative to exterior walls, spacing, depth of pipe and tube patterns…all items discussed in week 8 and 9.

RE: Maybe the air mixing dehumidification won’t be enough, or in time.

With radiant based HVAC systems the coil in the air handler is selected for dehumidification priority, with the sensible load on the radiant panel established by deducting the sensible portion of the cooling coil capacity from the total sensible load. With regards to time and condensation, Google search, “Mumma, DOAS, Condensation, Time”. You will find papers discussing the time it takes to actually form water droplets on a chilled panel.

About instructor Robert Bean – Pres., ICC INc.

Robert Bean, R.E.T., P.L.(Eng.) is a registered engineering technologist in building construction and a professional licensee in mechanical engineering. He is president of Indoor Climate Consulting Inc. and director of www.healthyheating.com. He is a volunteer instructor for the ASHRAE Learning Institute and serves ASHRAE TC’s 6.1, 6.5, 7.4 and SSPC 55 Thermal Environmental Conditions for Human Occupancy; and is a special expert on IAPMO’s new Uniform Solar Energy and Hydronics Code committee. He has developed and teaches numerous courses related to the business and engineering of indoor climates and radiant based HVAC systems.

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