HPAC Magazine

Mechanical system stands the test of time

March 24, 2019 | By Robert Waters

The radiant floor tubing is all 1/2 in. hePEX.

Twenty-two years ago, my wife and I built a house in the north end of Guelph, ON. At the time we had been married for almost five years, had one child with another soon to arrive and we wanted to build a home to raise our family in. I had been working in the hydronic heating business for over eight years and I was a total wet-head.

I had worked for Wirsbo for six years designing and teaching people how to use radiant floor heating, and at the time of my house build, I was working for Viessmann as its Academy Instructor. These European companies had exposed me to the benefits of radiant floor heating, low temperature boilers and hydronic controls. All of these products were relatively new to North America in the early 90s, but had been used extensively in Europe for many years prior to this. When we were planning our new house, I managed to convince my wife to let me design a totally hydronic heated house, with radiant floor heating on all levels.

I am not sure exactly how I did this, as this was something that almost no one else was doing at the time and it was going to be a much higher upfront cost for the mechanical system than what the builder was proposing. I give my wife a lot of credit…she took a leap of faith with me and agreed to have me design the mechanical system my way. And here we are 22 years later, still living in our family home, having enjoyed many years of warm, comfortable radiant living. I want to take a look back at the challenges we encountered, the details of the mechanical system we installed, and the lessons learned.

Overall, we have loved living in a hydronic radiant heated house for all these years and have no regrets about the choice we made many years ago. The benefits that I preached about radiant heating in many classrooms turned out to be bang on. The house is always a warm comfortable environment no matter where you are in it. I do not ever wear slippers or thick socks.

When I get up in the morning I am greeted by a warm bathroom floor. All the floor surfaces, ceramic tile, hardwood and carpet are always warm and comfortable. The house is very quiet… I never hear the noise of the heating system coming on and off. There is very little dust in the house and nothing to blow it around.

Is the mechanical system perfect? I don’t think so, as there are some things I have learned and some things I would probably do differently if I were to start over.


Tubing was stapled to the top if the wood subfloor and embedded in 1-1/2 in. gypsum overpour.

When we built the house, we were dealing with a custom home builder. We told them upfront of our mechanical plans, but it was still a challenge as it was all so new to them. It was going to change their standard building procedure and scheduling. I think they thought we were a bit nuts with our mechanical plans, but they humoured us as we were the clients. They certainly were not very generous when it came to backing out the cost of the traditional mechanical system, which would have been a forced air system. But we forged ahead with radiant heating, and I acted as the mechanical system designer and subcontractor.

The system I designed incorporated the following items:

• High mass radiant floor heating on three levels.
• Low temperature hot water boiler with a stainless steel indirect DHW tank.
• High velocity air conditioning system with air handler in the attic.
• Central heat recovery ventilator with separate intakes and exhausts.
• Pex potable water plumbing system, which was very new at the time.

The high mass radiant floor system consisted of two different floor construction techniques. It was fairly straight forward for the basement with tubing installed into the 4 in. poured concrete slab. The only additional item required out of the ordinary was for 6 in. x 6 in. steel wire welded mesh to be laid down so that the tubing could be tied down in the correct pattern.

Cool air is supplied through 2 in. flexible insulated ducts.We also added additional foam insulation to slow the downward heat loss. Since the upper two floors used standard wood frame construction adding high mass radiant heating was much more challenging. The technique we chose was to have the tubing stapled to the top of the wood subfloor and embedded in 1½-in. gypsum overpour.

We had the builder add an additional 2 in. x 6 in. wood plate under all the walls when he framed the first and second floors. By doing this the finished wall and door heights ended up all being standard heights after the 1½ in. gypsum was poured on top of the floor. This later simplified the work of the drywallers and trim carpenters. The self-leveling lightweight gypsum turned out to be great to work with, as it dried quickly and allowed the rest of the construction process to proceed easily.

There was certainly a sigh of relief on my part after the gypsum was poured, all the tubing was encased, and the rest of construction could proceed without me being blamed for any more delays.

I designed the tubing loop layout drawing with manifold locations on each level of the house, with all the upper floor manifolds located inside closets. The radiant floor tubing is all ½ in. hePex with loop lengths generally 250 to 280 ft. The manifolds are connected to the mechanical room with ¾ in. hePex supply and return piping.

Since all three levels of the radiant system were high mass construction it simplified the hydronic design. I was able to use only one pump for the entire heating system, and all levels of the house are fed with the same supply water temperature.

One of the benefits I taught about radiant heating was how easy it was to have individual room zones by mounting zone valves on the radiant manifolds to control individual loops. I took this to heart and went a bit zone crazy, with a total of 10 zone thermostats planned for the house. I never actually ended up installing all those thermostats, as I left a couple of areas as wild loops with continuous circulation. These areas only get variable water temperature from the outdoor reset control.


The design of the radiant heating system was the easy part for me, as this was what I had been doing and teaching for the prior eight years. The house had requirements for air conditioning and ventilation and since I was heating the house entirely with radiant, there was no central forced air ducting. This forced me to look at alternative methods for AC and ventilation. I chose to have separate self-contained systems for these two loads.

The upper floor manifolds are located inside closets.

The ventilation system is a self-contained heat recovery ventilator, ducted with air returns located in the kitchen and all the bathrooms. The fresh supply air is ducted to two central locations on the first and second floors. Each bathroom and the kitchen have a control with timer to activate the ventilation system.

The air conditioning system installed is a high velocity system that incorporates an attic mounted air handler. The return air for the whole house is located just under the air handler, at the highest point of the house in the ceiling of the second floor. The cool air is supplied through 2 in. diameter flexible insulated ducts. The ducts are snaked from the attic air handler and were roughed in down through interior 2 in. x 4 in. walls to feed the first floor. All of the air outlets are located in the ceilings with small round 2 in. openings.

We have been very happy with the way this air conditioning system has performed over the years. Since the thermostat is located on the second floor, this keeps the bedrooms cool and comfortable all the time. I believe we have the best heating and the best cooling system. Heat comes from below, and cooling comes from above.

Unlike a forced air system we are not trying to push heat down into the basement, or trying to push cool air up to the second floor. The basement is always warm in the winter and the upper floor bedrooms are always cool in the summer. Hot air rises, cool air falls, this is just physics.


What have I learned from living with radiant for the last 20 plus years? I have learned that there are some unique characteristics of a high mass radiant heated house. One of the big things is overheating of the indoor space temperature is inevitable at certain times when you live with high mass radiant.

Overheating happens primarily when there is solar gain from the south facing windows and during the shoulder seasons when there is a cold night followed by a warm day. I tried several different techniques to minimize overheating, and some helped, and some did not.

One thing I found that did help was to move the outdoor temperature sensor for the boiler control system from the north wall where the manufacturer recommends it be installed, onto to the south facing wall. I found that this really helped to knock back some of the overheating on sunny days, as it would ramp the boiler temperature down considerably. There were a few other minor adjustments, but primarily I have just learned that warmer temperatures in the house during sunny spring days are not that big of a deal. If necessary, I just open a window and let some of the free solar radiation energy back outside.

Another lesson learned is how uniquely suited a properly-sized modulating condensing boiler is to a high mass radiant system. I sized my boiler very close to the actual heat loss and did not add any extra capacity for DHW. I have found that the boiler runs almost continuously during cold winter weather, perfectly matching the heat requirements of the building.

The boiler I installed has a burner run hour counter built into the control system and I have kept track of the run hours for many years. I have found that the boiler runs on average almost 5,200 hours/year. When you subtract summer operation, when the boiler is only heating DHW, I found that the boiler runs almost 85 per cent of the time during the heating system (typically end of September to end of June).

The boiler just does its thing to match the load, constantly modulating the burner from 30 to 100 per cent input, and changing the output water temperature based on the outdoor temperature. Most trips to the mechanical room in the winter find the boiler quietly running at a low firing rate, with output water temperatures in the range of 90 to 110F. This certainly enhances the boiler operation by almost eliminating burner short cycling and the burner start and stops associated with poor combustion.


There are a few things I would do a bit differently with the design if I was starting today. There are many new innovative products available today that were not when we built. One of the first things I would look at is a different way of integrating the air loads–air conditioning, ventilation and humidification–into one air handling system. This would simplify the installation and maintenance of all the equipment required to do these functions.

Next, I would consider using low mass radiant on the second floor of the house. This would likely simplify the house construction process and reduce the cost a bit. I believe that low mass radiant would be better suited for the second floor as this area gets the brunt of overheating on sunny days. Also, with all bedrooms and bathrooms on that level, it would be easier to regulate the temperatures lower in this area.

Radiant manufacturers have developed many different techniques I would now consider to integrate low mass radiant into wood joist floors. Finally, I would look at rationalizing my zoning. I would use less thermostats and break the house into bigger zones. Due to the southern orientation of the house, I would likely break the house into northside and southside zones to help alleviate some of the overheating.

Two decades plus of radiant living has been everything I hoped for. I will often be at friends’ houses with forced air heating and think to myself, these floors are cold. Fortunately, I do not have to experience cold floors in my own home. Warm comfortable floors are something everyone should have. If I was ever to start building again, I would not hesitate to install hydronic radiant heating. <>

Robert Waters is president of Solar Water Services Inc., which provides training, education and support services to the hydronic industry. He has over 30 years experience in hydronic and solar water heating. He can be reached at solwatservices@gmail.com.



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