As an engineer, I find hydronic radiant floor heating systems the most fun residential heating system to design – challenging yet rewarding. Done right, they deliver superior comfort for the client in each and every room while working within physical, thermodynamic, and cost constraints.

A recent project was a great illustration. The clients were renovating a single-story two-bedroom bungalow (crawlspace beneath) and decided that it would be a perfect opportunity to convert from hydronic baseboard to radiant floor heating. I couldn’t have agreed more.

Their plan was to enlarge the space by extending the west wall to allow for larger bedrooms and a more spacious kitchen, and to install all new flooring throughout. This also meant that the existing boiler would have to be moved (to enable the kitchen plans) which would alleviate some physical constraints with regard to placement of circulators, manifolds, valves, controls, and associated piping.

With all new flooring, that meant that we could locate the tubing distributing the heated water on top of the subflooring and in close contact with the finished floor surfaces… and avoid “staple-up”.

Staple-up – often effective but not preferred

Staple-up, as it’s known in the industry, secures the tubing in grooved aluminum plates which are tightly attached to the underside of the subflooring between the supporting joists. The plates help to disperse the heat horizontally to provide more uniform surface temperatures. In addition to the constraints on tubing layout due to the joists, this method requires the heat to be additionally conducted through the subfloor – wood is not a great conductor – before it reaches the floor surface. A consequence of this additional resistance is slower response time to changes in thermostat setting and/or heat load (e.g. sudden outdoor temperature change, abnormally high infiltration). While staple-up can be effective for radiant floor heating, tubing above the subfloor is preferable from a heating performance perspective.

Heat Loss Analyses and Zoning

Once the clients’ objectives and the physical constraints were understood, the next step was to do a room-by-room heat loss analysis – Manual J is the generally accepted method by both building code enforcers and the industry. Each room will lose heat at different rates requiring customized rates of heat application in order to achieve and maintain optimum comfort. The rate of room heat loss will depend primarily on how much of the wall, floor, and ceiling surfaces are exposed to outdoor temperatures; how well each (including any embedded windows or doors) is insulated; passive solar heating (through windows); air leakage to/from the outside; any heat contributed by devices within the room (e.g. lights, stoves); and the outdoor temperature.

Of course, the outdoor temperature is constantly varying. When we design heating and cooling systems, we design to what is commonly referred to as the outdoor design temperature (or simply design temperature) which, for heating, is defined as the outdoor temperature that is exceeded during 99% of the hours of a typical weather year. In the Finger Lakes region and north to southern shore of Lake Ontario, that usually ranges from 0 to 6F depending on location.

Initially, the clients wanted to divide the floor plan into four zones –
• Bedroom 1
• Bedroom 2
• Bathroom, Hallway, and Utility Room combined
• Living Room, Dining Room, and Kitchen combined
– each independently thermostatically controlled. However, the heat loss analyses told us a different story:

The differences in heat loss among the rooms in Zone 104 are small enough that we would probably be able to compensate by adjusting floor tube spacing. However, in Zone 101, the Dining Room heat loss – 52.5 Btu/ft2/hr – is dramatically greater than the other two rooms in that proposed zone, too much to accommodate through tube spacing. Notice that the Dining Room has three exterior walls – more than any other room – and each has a lot of glass, either windows or sliding glass door, so it came as no surprise that its heat loss was much higher than any other. Left in the same zone, the Dining Room would be underheated and/or the other two would be overheated – not an acceptable situation. After discussion with the clients, we decide to move the Dining Room into its own zone – Zone 105.

Designing the Radiant Tubing Circuits

With the zones defined, our next step in design was to locate the manifold and specify each of the room tubing circuits – pattern and spacing. Whenever possible, I like to run the tubing entering the room along the exterior walls first, so that the warmest water is closest to where the heat loss is occurring, then serpentine them back toward the entrance. If there’s an exterior wall with a lot of heat loss, e.g. Living Room east-facing wall with large picture window, I like to reduce the tubing spacing for the first few laps (and, thus, increasing the rate of heat transmission) along that wall. For zones with multiple rooms, I’ll adjust the tube spacing to balance room-to-room differences in heat loss. For example, in Zone 3, the Hallway has lower heat loss (due to mostly interior walls), so I’ve widen the tube spacing there so that it is not overheated (or the other rooms in that zone underheated).

Impact of Finished Floor Surfaces

The finished floor type (e.g. tile, carpet, hardwood) will impart resistance to upward heat conduction from the tubing to the surface. Carpet, especially with pad, has one of the highest resistances and can be problematic for radiant floor heating, sometimes requiring supplemental heating – typically baseboard or radiant wall tubing – to meet the heat load. In general, 87F is our floor surface temperature target for ideal comfort. However, many hardwood floorings must be limited to 80F to reduce the risk of warping and cracking.

The clients planned for carpeting in each of the bedrooms, but by choosing a low, looped carpet with no or thin rubber pad, supplemental heat was not necessary. Except for the bathroom, their desire was for hardwood flooring in the rest of the house. That turned out to be a bad choice for the dining room as it needed a lot of heat and restricting surface temperature to 80F would virtually ensure supplemental heat would be required. After discussion with the clients, they decided to switch the dining floor to linoleum which has one of the lowest resistances to heat transfer, enabling me to increase the design floor surface temperature. That change increased the dining room calculated radiant floor heating from 60% to 86% of peak heating load. Now the room would only be modestly underheated when outdoor temperatures dropped to single digits; rather than have us install supplemental heat, the clients preferred to accept the infrequent compromise in comfort or winterize the room each season to reduce the heat load.

Water Temperature Considerations

Ideally, the circuits that we design can all be supplied with water at the same temperature which enables a simpler manifold design and lower materials and installation costs. Required supply water temperatures to each circuit are determined by a number of factors including heat loss rate, tube spacing, and finished floor type. In design, we try to adjust those parameters that we can control and have to live with the others. In the end, we may not be able to use a single supply water temperature for each circuit, in which case we’ll have more than one manifold and use additional mixing valves and controls to achieve multiple supply water temperatures. (For this project, we were able to use a single supply water temperature.)

When the heat source is a conventional boiler as it was for this project, we also have an additional constraint regarding water temperature. The temperature of water recirculating from the tubing circuits and returning to the boiler must be high enough to prevent the boiler flue gas from condensing which will result in accelerated corrosion of piping and internals of the boiler and dramatically shorten the life of the boiler. To be safe, water returning to a conventional boiler should be at least 140F. Given that radiant floor circuits require supply water at much lower temperatures, typically 100 to 130F, and, after transferring heat to the room floors, will be returning from the circuits 15-25F cooler than that, our design must raise that water temperature significantly before it re-enters the boiler. We accomplished that by diverting a portion of the freshly heated water from the boiler away from the radiant circuits and blending it with the cooler water returning from the circuits in a motorized mixing valve that is constantly controlled to ensure 140F water at the inlet to the boiler.

If the heat source is a modern modulating condensing boiler or a geothermal heat pump, for example, we do not have to be concerned with flue gas condensation and, thus, the returning water temperature. Design is made a little bit simpler with these heat sources.

A Complete Design before Installation

As is evident from this example, there is a quite a bit of designing and engineering that goes into specifying a customized radiant floor heating system – one that will achieve optimum comfort at the lowest possible life costs. And all of it needs to occur before a project can be accurately quoted and, certainly, before installation begins. For even modestly complex projects, expect to pay a design cost before you get a quote for installation. Conversely, be wary of the contractor who quotes you a price for installation before any design work has been completed – they’re either padding their quote to account for uncertainty in design or planning to adjust the price through change orders after the design is finalized.

If you’re entertaining converting to hydronic radiant floor heating, we invite you to contact us to discuss your project.

Lake Country Geothermal, Inc. services the Finger Lakes and Greater Rochester NY areas, including Albion, Auburn, Avon, Batavia, Bloomfield, Branchport, Brighton, Bristol, Brockport, Caledonia, Canandaigua, Clifton Springs, Clyde, Cohocton, Conesus, Churchville, Dansville, Fairport, Farmington, Gates, Geneseo, Greece, Groveland, Hamlin, Hemlock, Henrietta, Hilton, Honeoye, Honeoye Falls, Ionia, Interlaken, Irondequiot, Kendall, Keuka Park, Leroy, Lima, Livonia, Lodi, Lyons, Macedon, Marion, Macedon, Mendon, Mount Morris, Mumford, Newark, North Rose, Ontario, Ovid, Palmyra, Pavilion, Penfield, Penn Yan, Pittsford, Port Byron, Prattsbugh, Pultneyville, Red Creek, Riga, Rush, Savannah, Scottsville, Seneca Falls, Sodus, Shortsville, Spencerport, Springwater, Victor, Walworth, Wayland, Webster, Weedsport, Williamson, Wolcott, Wyoming.

Design Considerations for Hydronic Radiant Floor Heating

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