Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Any advice before we majorly rearrange our in floor heat system?

We have found a lot of great insight on this site. We have a bit of a unique situation so we would greatly appreciate any thoughts before we majorly rearrange our in floor heat system. Thank you in advance for any thoughts!


We did a new build in the North East a few years ago. We have cold winters so the thought by the original plumber was that in floor alone wouldn’t be responsive enough. Given this we added in floor heat “just to keep the floors warm” and then added two air handlers to pick up the extra demands as needed (and circulate fresh air). The in floor and air handlers all run off a single indirect loop heated by the boiler.


For the floors one zone is in an insulated slab in the basement, and three zones are for the main floor. The main floor zones are staple up piping between the floor joists with no heat plates, just stapled up to the sub floor with an air gap and cardboard underneath, and insulation below that (we were told heat plates weren’t necessary, which was maybe a mistake in hindsight).


The four zones are controlled with floor sensors and thermostats. When calling for heat the boiler turns on, which heats up the indirect loop to a specified temp, which is then circulated through the floors with the four circulation pumps. So it is an on/off system.


Originally the in floor loops were set at 160 degrees “so that the heat would get through the floor” but we worried this might be putting too much stress on the glue down vinyl plank flooring above so we lowered the temp to 130.


The three problems we see are 1) we get cold spots near windows because the in floor circulation is off most of the time, 2) we are running the boiler on and off at what seems to be a relatively low temperature so we worry about boiler efficiency, and 3) we worry about the stress this high temp on/off cycle might be putting on the floors.


Given this we are considering adding an indirect HW tank, and then running the in floor heat off this tank with the four circulator pumps running continuously. The thought is that this should give a more even heat, and also put less stress on the boiler as there is a better heat sink with the tank and therefore hopefully less cycling of the boiler. We would then use the thermostat on the indirect tank to control the temperature of the floors, with outdoor temp info being used to maximize the floor temp without overheating the space. The air handlers would continue to be used to pick up any extra heat needed.


That said we aren’t sure if this is a good idea or not. Any thoughts or advice would be greatly appreciated.

Comments

  • hot_rod
    hot_rod Member Posts: 18,211
    It would be nice to start with a room by room heat load calculation. With that you could determine how much heat emitter or what size is required in each room

    Im a bit concerned that the system was running 160 SWT. That could indicate the room has a high load, or the bare tube method is not transferring enough to the floor, into the room, or both.
     Constant circulation or a buffer will not fix a constipated distribution system.

    I’d hate to see you spend time and money on repiping until you are sure that will solve the issues

    Search for the SlantFin free load calculation ap
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ted_1209
    ted_1209 Member Posts: 9
    Thanks hot_rod. I’m a bit new at this so let me know what you think of this. The system is only meant to keep the floors warm, not to heat the whole space. The floor controller has no knowledge or connection to the air temp. It only has a temp sensor that is located at one spot in the shallow cavity under the subfloor where the in floor heat pex is.

    The 160 temp was set arbitrarily. So the floor thermostat notices the floor sensor temp is below the set temp. When this happens the boiler turns on to 160 (now 130) and stays there until the floor sensor reports back that the air cavity under the sub floor is now back to the floor set temp. It then turns off and waits for the air cavity to drop in temp again. So there is zero control connection between the floors and the air. The in floor heat doesn’t know about the air temp at all.

    Does that make sense? Is that helpful to understand the intent of what we are trying to solve for?
  • hot_rod
    hot_rod Member Posts: 18,211
    Yep, I get it :)

    In shoulder seasons, low load conditions, what keeps the floors from over-heating the space?

    Anytime the floor surface is above ambient, heat is being given off.
    Maybe consider one of the radiant stats that watches the floor sensor and ambient as its high limit to overide the slab sensor call?
    You can set the floor to never exceed a temperature, or never drop below a setpoint temperature. Is it possible the radiant alone could heat the home on milder winter days? With the dual stat the floors would be primary, AH only kicks in when ambient drops below what the floor could provide.

    Air handlers kinda ruin the benefit of quiet comfortable radiant floors.

    There are piping arrangements that would give you constant circulation, and still allow temperature control.

    My system works like this. A 3 way zone valve has flow always going around the loops. When there is a heat call the 3 way valve moves to the other position and runs through the buffer or boiler. Flow goes from B to AB, then when there is a heat call B closes and A opens to AB.

    This works nicely when you have rooms with solar gain, it moves that heat throughout the system, to shaded areas. About the most comfortable radiant systems you will get are constant circ.

    I'd add outdoor reset also so the boiler or buffer could modulate based on outdoor temperature.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GGross
  • ScottSecor
    ScottSecor Member Posts: 642
    You mentioned northeast as your location. We need to know if you consider the middle or NJ northeast or the top of Vermont the northeast? The reason I ask is NJ may have design temp of 14 degrees F while Montgomery Center Vt may have negative 21for design temp.

    As you may already know, most radiant heating systems can handle the heating load alone. However in the coldest climates we have seen the need for ancillary heat. My parents built a home approximately eight miles from the Canadian border in northern VT in 2005. The house was very well built (SIPS construction) and very efficient, however they still needed to use HWBB in addition to the radiant heat on the coldest nights (-24F was the record while they lived there).

    I think you may want a thermostat to control the radiant heat. Yes you will get solar (radiant) gain on very sunny days if there is a lot of glass and the stat might get "fooled." But we often design for the radiant to operate throughout the heating season. Hot Rod, is much more knowledgeable than I am with radiant (and most things). It seem to go against the grain to run a warm air system and cycle the radiant system off at the same time.

    I am not sure about this idea, but I believe Dan Holohan taught us years ago at a seminar that blowing warm air across radiant heated floors is a bad idea. The idea of radiant is to heat the objects, not the air. Yes the air temp will be comfortable, but so will the floors, couch, chair, etc. With warm air not so much.

    With regard to your boiler is a cast iron oil fired unit or a gas fired condensing unit, or a wood boiler? It makes a difference to how it operates (and handles relatively cool return water). It also may help us in pointing you in the right direction for controlling the indoor temps.
  • ted_1209
    ted_1209 Member Posts: 9
    Thank you for the thoughtful comments and questions. Here are some answers:
    With regard to your boiler is a cast iron oil fired unit or a gas fired condensing unit, or a wood boiler?

    It’s a Weil-McLain Evergreen 155 Modulating Condensing Boiler. We are near the Canadian border as well so we get quite a few dark, windy days that are negative 10 f and sometimes even a bit worse.

    what keeps the floors from over-heating the space?
    Your question is spot on. Right now we manually dial the floor temperature up and down based on the approximate weather outside (moderate, cold, very cold). If we get a warm snap the space can be too hot all of a sudden. We’re hoping to find something that can automate this. I think we’d rather adjust the circulating temperature if possible, versus pausing circulation. Our thinking on this is that we don’t have any heat plates, so it seems like we should keep the heat on as much as possible as it will take a while to heat the cavity and penetrate the floor. We also want to eliminate cold spots.
    My system works like this.
    This is super helpful to see. Do you think your setup would be better than our proposed strategy of adding a 60 gallon HW tank? The four floor zones are roughly 7,000’ total of 1/2” pex, for a total line volume of roughly 65 gallons. Maybe this is enough of a heat sink to only turn on the boiler to “top up” the circulating temperature by a degree or two?

    Our four floor loops are already heated via an indirect heat exchanger with the boiler, so maybe all we need to do is add a temp sensor to the floor loop such that the boiler only kicks on when the temp of the circulating glycol needs to be increased a degree or two?

    I guess the question is do you think the glycol flowing through the lines would be enough of a heat sink to run the boiler efficiently, especially if we only want to increase the line temperature a degree or two? Or maybe there’s something nice about having a centralized heat sink in a 60 gallon HW tank that the boiler can “see” and heat all at once? 

    We are hoping to circulate a constant 75 degrees in moderate weather and up to 81 degrees in cold weather. Ideally we would get fairly accurate and consistent temperature flowing through the lines as it seems even a few degrees extra can start to overheat the space.

    Any thoughts on this are greatly appreciated!
  • ted_1209
    ted_1209 Member Posts: 9

    Thank you for all the input. Here are some thoughts on your good questions:

    You mentioned northeast as your location.

    We are near the Canadian border as well. We get several days each winter that are negative 10, windy, and cloudy.

    With regard to your boiler is a cast iron oil fired unit or a gas fired condensing unit, or a wood boiler?

    It’s a Weil McLain Evergreen 155.

    What keeps the floors from over-heating the space?

    Your question is spot on. Right now we manually turn up the floors when it gets colder to try to get as much heat as possible from the floors (we love the feeling of warm floors). That said if we get a warm snap I need to rush to turn down the floors as otherwise we overheat the space. Ideally we could find some way to set the circulating temp with the input of an outdoor sensor (or area weather).

    My system works like this.

    This is super helpful to see. In a way our system seems similar, where the four floor loops are heated via an indirect heat exchanger with the boiler. The problem we have right now is that the boiler and the floors blast to full temp when there is a call for heat. Maybe instead we could add a temperature sensor to the circulating glycol and only call for heat when that temp drops a degree or two below what we want the floors circulating at?

    I guess the question is how much of a heat sink do you think the circulating glycol would provide? We have roughly 7,000 feet of 1/2” pex, for a total volume of roughly 65 gallons of glycol. I think we want to control the circulating temp fairly accurately, as it seems we’d want roughly 75 degrees circulating on 50 degree days but 81 degrees circulating on 32 degree days or colder. Do you think the circulating glycol could absorb enough heat to run the boiler efficiently? Or might it be better to have a 60 gallon HW tank that the boiler can “see” and heat all at one time?

    Any thoughts are greatly appreciated!

  • ted_1209
    ted_1209 Member Posts: 9
    Also one more complicating factor: the two air handlers currently run off the same loop as the floors. So if we want to reduce the temp of floor loops then we would need to put the air handlers on a new loop, as they need higher temps.

    So perhaps that is a benefit of installing a new HW tank: keep the air handlers on the loop they’re already on with the heat exchanger, and then move the floors to a new HW tank which is indirectly heated by the boiler.

    Another piece of the puzzle…
  • hot_rod
    hot_rod Member Posts: 18,211
    A two temperature system is what you may consider. Depending on the coil size in the air handlers you probably need 160F or more to get adequate output. Is the radiant tube stapled against the subfloor?  If so 140F would be as hot as you would want to go
    If the  tube is not in contact, basically heating the joist air space, you could run 160F to the radiant. The entire system on one temperature 

    The boiler should have an ODR function built in, need to install the outdoor sensor which should have come with it?

    So on a -10 day you might run 170 or more to the AHs. The temperature would modulate itself down on warmer days.

    If you have staple up radiant, it would see 140F  on the -10 day, via a mixing valve

    Running this logic would handle most everything you ask, the fluid is always in motion, no need to start it from 70F on a heat call(cold start) the boiler is modulating it’s fire constantly to meet the load.

    The dual stats would still be a nice addition, or a two stage with floor sensor. The AH automatically would kick in and out only when needed on a second stage.

    A manual 3 way mix valve for the radiant with the boiler running on ODR. So the radiant temperature also modulates along with the higher AH temperature 

    No need for extra heat exchangers or indirect tanks. Assuming the entire system is glycol?

    If you have a HX with glycol in just some of the system it is more complicated 

    A sketch of the system piping would answer questions for a repipe
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ted_1209
    ted_1209 Member Posts: 9
    Thanks gun_rod. Very helpful as always.
    A sketch of the system piping would answer questions for a repipe

    I’m not too good at drawing, but perhaps I can explain simply. There are two loops coming off the boiler, one to an indirect water tank for DW and one to a heat exchanger, each with their own circulation pump. On the other side of the heat exchanger is a loop that has 4 in floor loops and 2 air handler loops coming off it. The in floor loops each have their own circulation pumps and the air handlers each have their own built in circulation pumps. The in floor zones and air handlers are controlled by Taco control boxes. Everything runs on glycol.

    Is the radiant tube stapled against the subfloor?

    The radiant tubing is stapled to the side of the floor joists just below the sub floor, so it is close but not quite touching.

    If you have staple up radiant, it would see 140F  on the -10 day

    I think our main concern is that this high temp on/off control strategy might be damaging the floors above. We have glue down vinyl plank and all the 4’ long boards have shrunk about 1/8 to 1/4” and will likely need to be replaced. We’re wondering if this might be caused by the in floor heat lines sending high temps, then turning off and cooling down, then on, then off, etc, such that this cycling is causing the vinyl to permanently shrink. This is why we’re wondering if a constant low temp might be better for the floors, while also giving a more even heat without cold spots. But maybe we’re overthinking it. What do you think?

  • hot_rod
    hot_rod Member Posts: 18,211
    Yes the tube stapled to the side of the joist is the issue. Probably good for 10- 15 btu/ sq. ft output. 140F or so is as hot as I would run that if it is touching the wood structure.
    Any outgassing from the adhesives used? If the flooring is moving I think you are over-heating it?

    Systems liken UltraFin can run much hotter as the tube is suspended below the floor, not touching or over-heating any wood.

    Haven you ever measured the actual floor temperature? Really should not run much over 82- 83F on the surface. A point and shoot thermometer would indicate how hot you are running above the tube location.

    The wood, adhesives, flooring all have temperature limits, the manufacturers have that info. 140f seems to be the highest for any wood or wood fiber products. Glue and adhesive, maybe 90- 100F?? The can should indicate some of that info as well as the vinyl flooring install sheet.


    Constant circulation doesn't address the excessive supply temperature? You still need some temperature control.

    Even running lower temperatures it may over heat the space on low heat load days, so a more intelligent thermostat would help..
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • TAG
    TAG Member Posts: 738
    I have built 3 houses with both radiant and heat in the air handlers. In my case they were all weekend places where I did not need to keep the house at full temp when I was not there.

    In my case they all ran off outdoor reset w/ constant circulation .... the boiler would simply run and maintain the proper temp to the floor .... my internal set point was 64 degrees. When we would arrive to the house and want it warmer -- we energized the room thermostat to 72 degrees. On the first one I did -- the thermostat would cause the floor pump to go off and the boiler would then send 180 degree water to the air handlers to quickly bring the spaces up to temp. Just like a radiant floor would go off to heat an indirect water tank. After the house was warm I would raise the target temp on the boiler curve. That system took some trial and error initially to get it all dialed in. The problem you may have is the water temp to your floor may need to be too high at times and it's over heating some areas of the floor. It sounds like you need to have two temps in your system. On cold days the back and forth between the two could cause the air handlers to have the radiant off too much with the curve at 64 .... the people who bought the house used it full time and they basically just used the radiant

    After that first system -- my next two used a gas furnace vs the heat coils. This allowed the radiant to maintain that 64 setting and the furnace raised the temp to 72. Using the ODR of the boiler with no inside feedback the boiler had no idea the house was warmer ..... it just ran on its curve. That was enough to make the floors warm ..... if we were going to be there for an extended time I would raise the floor set point so the radiant was doing more of the heating ... but for a weekend the 64 was a good target.

    It sounds like the ideal situation would be to have two temps .... one lower temp feeding the floor and a second that can feed the coils. This way they can both work at the same time.

    Typically with manifolds you can adjust the flow to get enough little changes depending on the heat load in each area
  • ted_1209
    ted_1209 Member Posts: 9

    This is all really helpful, thank you again. I’m now wondering if we should leave the system as is but see if we can find a controller to automatically set the floor temps based on outdoor temps so as to not overheat the space.

    Have you ever measured the actual floor temperature?

    I have measured with a FLIR and a point and shoot, and the floors seem to consistently get to and stay at the temperature set on the thermostat, so the staple up seems to be able to put out more than enough heat.

    140f seems to be the highest for any wood or wood fiber products

    What do you think would be the ideal temp (most efficient, most “safel) for the staple up in floor zones, the concrete slab in floor zone, and the AHs? We have two Taco boxes so perhaps we could put the slab and staple up on one Taco with 130f heat, and the AHs on the second Taco at 160f. What do you think about those temps? What do you think would be ideal/ most efficient?

    Even running lower temperatures it may over heat the space on low heat load days, so a more intelligent thermostat would help..

    I think you’re spot on. Any suggestions on what we might use to automatically control the in floor so that we get maximum heat from the floors but not so much that we overheat the space?

    Good thoughts as well TAG, thank you. 64 seems a bit low to get that nice warm floor feeling. In a perfect world we would have the floors at 81 all the time but without overheating the space and without wasting energy. I guess that’s the tricky part.

  • hot_rod
    hot_rod Member Posts: 18,211
    edited October 2022
    If you run the system on ODR, the temperature will modulate itself, program in the highest you want to allow it
     to go

    The most efficient would be the lowest temperature that covers the loads 

    If you don’t have a design, it will be trial and error somewhat.Your boiler manual should explain control settings.

    so you have staple up, concrete slab and air handlers? That usually requires 3 different temperatures 

    100 or o for slab. 140 for staple up, 160-180 for air handler 


    https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Weil-Mclain-383-900-006-User-Guide.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ted_1209
    ted_1209 Member Posts: 9
    Thanks hot_rod. From my quick searching it seems like we could safely go up to 140 on the slab. Given this what do you think about putting the staple up and slab on the same Taco at 140 supply temp, and putting the air handlers on the other Taco at 170 supply temp?

    Still doesn’t solve the problem of needing to manually tweak the floor temps to avoid over heating, but I’m thinking I’ll try to make this change first and then solve that next.
  • hot_rod
    hot_rod Member Posts: 18,211
    I don’t think you want to put 140 to a slab? Surface temperature should not exceed 82F or so, If it takes even 110-120swt, it’s time to add supplemental heat in those rooms

    carpet and pad on a slab requires a bit higher SWT but rarely 140 or even 130 That would be. hard slab to wind down once heated with 140, a huge thermal flywheel

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • TAG
    TAG Member Posts: 738
    Ted -- My 64 was target room temp not water temp. This was set on the boiler for the ODR curve .... not the floor temp. 64 was when we were not there full time. The ducted system would bring the house up to 72 when we arrived on a Thursday or Friday night. -- if we were going to be there for a week or so I would bring the boiler set temp to 70 ... if just a normal weekend it would be bumped up to maybe 68. Depends on how cold outside. I like a lot of glass in my builds and when it's cold out the radiant is the best.

    The actual floor temp for any given room temp will depend on the heat loss of the space and building. After some trial and error you normally get the system dialed in and use the outdoor reset curve. Even at 64 degree room temp ... you feel the radiant on arrival.

    My old place in the NJ burbs has a room off the kitchen with a lot of glass and french doors .... slab with limestone. I have to send rather hot water through that system to heat the room .. my guess hotter than wood would like -- but that's what it takes. The floor is warm .... the dogs love it in there.
  • ted_1209
    ted_1209 Member Posts: 9
    Thank you both for your good comments. This forum and your input have been incredibly helpful in wrapping my head around this. It is truly amazing that such a place like this exists with such thoughtful, knowledgeable, and caring people.

    I am going to forgo rearranging my current system and focus on the control side instead. Perhaps I will make a new post. Until then, thank you all.