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New Radiant Heating Install Question

TDio Member Posts: 1
edited February 2022 in Radiant Heating
Hello All
We are almost complete with a major home renovation that included relocating our kitchen and better distributing our room sizes in our 2,600sq' ranch style home on Long Island, New York.

For the 900sq' that had the bulk of the renovated space, we choose between the joist hydronic radiant floor heating to heat the space and are in the process of tuning it and I had a few questions on how to properly control the temperature in the space.

I will list all the details i think you might need to properly direct me to a solution. I'm sure I will forget something so please ask any questions you feel would help

- 900 total sq' of radiant heating space over full basement. 4 circuits using bluefin manifold.
- Navian combi boiler heating a total of 4 zones with zone pumps (1 radiant, 3 hydronic baseboards)
- the radiant zone uses a mixing valve to maintain proper water temps
- Honeywell TH6320wf2003 t6 pro thermostats remotely mounted in boiler room with aprilaire 8051 room air sensor in each zoned area of the house.
- 1/2" pex installed into uponor joist trak below subfloor
- 16" on center floor joists with one full loop (two strips of trak within each bay)

Now comes the fun part......

- Subfloor 1/2" plywood from the 60's with 5/8's plywood laminated to the 1/2" with loctite adhesive and screws (new)
- 5/8" engineered unfinished white oak rift & quartered character grade trowel applied bona adhesive and nailed down.
- Currently we are running 95F water through the radiant system. Been testing with temps from 85 - 95 degrees

My questions to you all are how I should be maintaining room temps. I don't feel using air temp is the correct way, but i could be wrong.
- Should I be using floor temp to maintain the radiant rooms?
- How would I install a floor sensor in hard wood? Drill a hole from underneath the floor just deep enough to push in a sensor without penetrating the surface?
- The Honeywell thermostat can accept a wired room temp sensor; shouldn't I be able to use a "slab" sensor in it's place?
- Based on the density of the flooring, what temps should I be targeting? I read some people look at water temps, some floor surface temps, some subfloor temps etc. I know it's not an exact science, but it would be nice to know the "not to exceed" numbers.

While we are living in the house during the construction, we aren't spending any time in the space as it's not complete, so my testing is being done by feel and by looking at the thermostat data. We have the thermostat set to 68 degrees based on air temp. During the day it seems to maintain the 68 degrees with most, if not all the water temps i've tested with (85-95 degrees). In the early morning, i can see the room temp is 66 degrees even though it's set to 68. I don't have any schedules to change the setpoint. It's always set to 68. Once the sun rises, the room quickly heats up.

My suspicion is maybe the air temp method of maintaining isn't correct due to the density of the floor. What I'm thinking, is the house heats up to the desired 68 degrees air temp and shuts off the circulator pump. The floor is still heating everything around it and come nighttime, there is a call for heat, and it takes a long time to heat everything back up which maybe is why it's 66 in the AM, and eventually gets back to the 68 degree setpoint.

I can see run times are anywhere from 24 hours straight to a minimum run time of 16 hours/day.

Sorry for the rambling, just trying to give as much info as possible so i don't waste any of your time.



  • Ironman
    Ironman Member Posts: 7,376
    You need to set the floor up with a smart mixing device that uses Out Door Reset which varies the water temperature according to ambient outdoor temperature.

    It can be done with a smart mixing valve like Taco’s Iseries or with a Tekmar 356 which uses variable speed injection mixing.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • hot_rod
    hot_rod Member Posts: 22,157
    The 900ft is a zone of it's own with a dedicated thermostat?
    Uponor, tekmar, Ecobee, Honeywell, many others have thermostats with floor sensor options. Strap the sensor to the bottom of the floor with a piece of the aluminum joist track. Or any metal that gets you a good connection to the floor between the two tubes.

    A few way to use floor sensors. Either just the floor sensing, but the room may over-heat from internal gains.

    Air temperature and floor sensing, The stat will read air temperature and the floor sensor can be programed to not exceed a certain valve.

    Or the floor sensor can be programed to not drop below a certain temperature. I've used this for bath floors. But it can overshoot, csuse the room temperature to go too high. And it is possible for the floor to be heating while the AC is running :) I've had customers that want warm bath floors year around, so....

    Floors with surface temperature above @ 82F start to feel uncomfortable to most barefooted people. Sweaty feet syndrome.

    Find which control logic works best for you, that is why they build them with choices.

    As @Ironman mentioned, a reset control can help smooth out cycling, especially when used with dual sensing t-stats.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Paul Pollets
    Paul Pollets Member Posts: 3,656
    The Vitodens 200 comes with a sophisticated computer with outdoor reset that is fully capable of handling 2 different system temperatures plus an indirect tank. It needs the Viessmann mixing valve package if 2 temps are desired and the low loss header. If only 1 temp is being used, the mixing valve is not needed, nor is the LLH, as long as flow rates are below 6gpm. Use the Vitotrol 300 thermostat with sensor enabled to maintain system temps.
  • Micmann
    Micmann Member Posts: 6
    edited February 2022
    Couple thoughts. Part 1

    With the between joists below sub floor system your install has a good deal of mass for the system to warm. Looks like about 2" total of wood. Not a bad thing, but that will play heavily into setting the system. A system that had piping above or in the 5/8" sub-floor may have been better, but not a big deal. The system water will need to run a little hotter and need to be dialed in with as little on/off running as possible. It's not about turning on and off, but trying to run most of the time and let the water temp in the system adjust through outdoor reset. That is the basic principle of radiant systems anyway. (sorry for telling you things you probably know).

    If you don't have it already, a must is to insulate under the joists. Spray foam if you can. Or install foam board cut to squeeze in between the joists and sandwich the radiant system between the foam board and floor. 1" foam board min. Then "can" spray foam any gaps. Or least best option install pink insulation. A floor with that much mass to work through will bleed heat the wrong way if not insulated. This will help with temp swings as well. Also insulate system pipes. Low water temp systems like radiant need to keep all the heat they can until in the floor.

    As another poster said, use a control with outdoor reset and actuator speed control. The Tekmar 360 Control works really well. It also allows temp settings to raise the heating curve to heat through the mass you have. It will take some tweaking to figure out how warm the water needs to run to keep up on the coldest days.

    Keep in mind, not all flooring is acceptable to run over radiant. Engineered typically is, but most natural flooring, as well as the engineered, cannot be heated above 80 degree floor temp. That's the floor temp not system temp. And it's top layer temp not bottom sub floor temp. You may need 120+ water temp to hit the 80 max floor temp. Especially with the underside system. This is where a floor sensor is necessary. Ideally it would be embedded horizontally into the underside of the engineered floor. Or embedded into as close to the top surface of the sub floor as possible. Since it seems your floor is already installed you will need to carefully cut into the underside of the floor as you mention and get as deep as you can. You are trying to pickup the temp of the engineered floor. That will be the most accurate reading. A hand held temp gun pointed at the top of the floor will also help to know if your floor temp sensor on the thermostat is reading similar to the surface temp when dialing in the system.

    A thermostat that reads both floor and air temp is necessary. Also one that learns the rhythm of heating. Not occupied/unoccupied times, heating rhythms. With floor mass heating a typical thermostat will turn off when air temp is satisfied. Heating the air is not the goal of radiant. Radiant is more a feeling of warmth. So when a typical thermostat finally shuts the system down after air temp being satisfied, the floor will still be radiating heat and will over shoot the air temp possibly for another hour. If the target temp is 68, the therm will shut down at 68, but the room may reach 70 or 72+ (depending on other factors of course, i.e insulation , outside temp, etc) because the floor is still warm. After overshooting, the floor system will be off for a longer period and pass the ideal run temp as it cools. Then the system needs extra time to re-heat the floor to satisfy the room temp on the up swing. Then overshooting room temp again. This causes a never ending overshooting cycle that is very difficult if not impossible to balance. Throw in thermal sun heating and its a mess that will make you pull your hair out every heating season. A thermostat with Pulse Width Modulation control is ideal. It learns how long the system needs to be on or off in high mass systems like yours to hit the target temp. I personally like and use the Tekmar 552. It has learned the swings to match my floor mass. It also has input for the floor sensor. Thinking how radiant heats is another world of thermostat control to be done right. Do not use setbacks, they only throw the system off. If your boiler has setback water temp control, that can be used. Some boiler controls can drop the system water heating curve at night a few degrees. This cools everything slightly without major floor temp swings. If your system has this control, program the boiler to at least an hour ahead of your target temp and time to begin warming your mass. Ideally it will not have cooled a great deal anyway over night. Simply get used to one target temp 24hrs/day. Its the best for radiant systems.

    I also run a similar system of radiant with hydronic baseboards. Depending our your floor plan and layout they will impact each other. The baseboard is relatively fast heating and can have the adverse effect of satisfying the radiant thermostat and not allow for consistent radiant running. This will take time to balance temps in all rooms. It may be that you will need to lower the baseboard temp a degree and raise the room with the radiant a degree to get all to balance. The room with radiant needs to be the parent system and the other rooms the child rooms. If necessary let the radiant system bleed heat into the other areas not the other way around. You want the radiant running as consistently as possible. It sounds like wasting energy, but because its a low temp system its more energy efficient constantly running at a lower water temp than a baseboard system running on/off at higher water temps.

    A mixed system (radiant and baseboard) control may mean letting existing control run the baseboards and installing a new independent control to run the radiant (like a Tekmar 360). Hard to know without knowing exactly how you current system works. Does it give you all the necessary control for radiant? Sometimes two independent systems can be combined to be extensions of one system, especially if priority zoning is preferred. Try not to let the radiant be affected by priority zoning unless your hot water demand just cannot keep up with radiant running. Priority shuts down the radiant to make hot water and the radiant system need to re-balance every time. Ideally the radiant is not as temp swing demanding on the boiler/furnace as the baseboard. But this situation varies depending on hot water usage and heat demand. If hot water heating is ok without the radiant system shut down by priority keep it that way.

  • Micmann
    Micmann Member Posts: 6
    edited March 2022
    Had to split this in two posts.

    Part 2:

    Do not attempt to set the radiant system water temp to a specific temp. It will not work. Outdoor temp plays a major role in floor temp control. If the mixing valve is manual control get rid of it. The control system must have outdoor reset with a power actuated mixing valve. The control must also allow you to independently adjust the radiant water temp targets based on outdoor temp. If you find on cold days the room is not warm enough you will need to raise the mix temp targets. Raise them incrementally over days until you get the room to your target temp on the coldest days. This will take time and tinkering to do it right. A Caution. If you are raising the mix temp so high that the floor temp passes 80 degrees, but the room is still not warm enough, you have a problem. The floor will not sufficiently warm the room. Other factors need to be adjusted to stop heat bleed. Insulation, better doors windows, etc. Pumping the floor to 90 to get the room temp adequate will run the risk of buckling or cupping your wood floor. Set the thermostat to a max 80 floor temp. This will keep the system from overheating the floor. Also keep in mind 68 in the radiant room is going to feel warmer that 68 in the baseboard room. So you may be able to adjust the target room temp down a degree or two.

    Don't heat based on floor temp. Heat on room temp with the right thermostat (again Tekmar 552 or similar). Heating by floor temp will not account for thermal swings of outdoor temp and sun. You will always be too hot or too cold. The only area to set a floor min temp is the bathroom since it is the room we want warm feet and like warmer than any other room anyway.

    Don't cover the floor with throw rugs everywhere. Everything has an R value and will block the radiant. Enjoy the wood surface as is.

    Glad to hear you install radiant. Its a superb energy efficient type of heat. Sadly its not popular because of its upfront cost, but the payback is over time and well worth it. There are also many radiant systems out there that run, but don't run right. They were installed to run like a conventional system. I've even seen them installed like a base board with no mixing control! The installer ruined that guys floors in the middle of winter and he didn't know why. They are not a one and done like forced hot air or baseboard. They need the right thermostats, controls and understanding. They take tweaking to dial in just right. But when they are, they purr like a kitten and will not need further adjusting.

    Key takeaways.

    Insulate under floor joists.
    Install floor sensor as close to top layer as possible.
    Install thermostats with floor temp sensor inputs and Pulse Modulation Control.
    Do not run surface floor temp past 80.
    Balance radiant to play nice with other heating systems room temps. Let radiant dictate other rooms.
    Install a control that gives you the heating curve and outdoor rest control.
    Start with pre-programmed mix water temps in the outdoor reset control and then on coldest days make adjustments.

    I've rambled enough, guess I wrote a book, hope it helps.