Radiant Floor Heating System – Is a 10°F Single Space - Same Zone Temperature Difference Acceptable?
Hi all,
We recently completed a high-end renovation in Madison, Wisconsin, that included a radiant floor heating system powered by an IBC boiler. We’re experiencing a persistent problem: the alcove area consistently remains 10°F colder than the adjacent kitchen during the winter, despite both spaces being part of the same zone and having identical tile flooring.
However, the two areas differ in how the radiant heat is delivered:
- The alcove utilizes piping embedded directly into the concrete.
- The kitchen uses an aluminum heat transfer layer above the slab.
The builder claims that this temperature variation is “within acceptable range” and not a defect, citing the absence of an explicit performance standard in our contract.
We’re seeking input from HVAC professionals, radiant design experts, and others with experience:
Is a 10°F ambient temperature difference in a radiant slab system between adjacent spaces common or acceptable?
Some additional details:
- We engaged an independent HVAC firm that suspects a design flaw, but they’re reluctant to confirm without access to the system drawings, which the original installer refuses to release.
- Floor sensors are present, but their exact locations are unknown due to the limited documentation available.
- On our first night in the house (in December), the interior was uncomfortably cold and required backup heating. The next day, the installer switched the Aube TH114-AF-024T thermostats to “floor-sensing only” mode, disabling ambient sensing. This did not resolve the problem, and they have not returned since.
- To keep the kitchen comfortable during winter, we’ve had to set the floor thermostat to around 78°F. Despite this, the adjacent alcove remains 8–10°F cooler daily.
We are trying to determine whether this persistent differential reflects a design flaw, particularly given that the alcove and kitchen are thermally distinct and yet share the same zone and thermostat.
Have others encountered this issue in slab-based radiant systems?
Comments
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You are talking about air temperature, not floor surface temp, right?
The higher mass slab will heat very differently than the wood floor and unless the slab is a small area that can be heated by the adjacent area would need to be accounted for.
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Assuming you are referring to air temperature, no, that is not at all acceptable. A transfer plate type system will require a higher water temperature to put out a similar BTU per linear foot as concrete, so it's quite probable that the installers combined the two into one and do not have enough tubing/plates and/or are using too low of a water temp for the plate system. Either way, this is a design/install error and needs to be corrected by the designer/installer. They may be able to split it into two zones or even balance the flow to get each system to work as intended, but they may need to turn it into a dual temp system or even tear the floor apart and add more tubing/plates.
Upon reading this again, you refer to "thermostats" and "sensors" as if plural. If there are multiple sensors/stats, there can not be just one zone. Something seems to have been lost in translation.
2 -
That is not acceptable and you shouldn't let the builder make it your problem.
3 -
Does the alcove probably has a high load? 3 outside walls? Lots of glass or doors that are low R-value.
Unless the alcove room can be doored off from the rest of the room, I don't see a 10° ambient being likely. Hot goes to cold, always. Unless there are some infiltration issues also?
The key number to check is floor surface temperature. , vs the load of that various areas. 2 btu/ sq ft for every degree difference between floor surface and ambient.
70° ambient with 80° surface= 20 btu/ sq ft. Oddly enough the cooler the air, the higher the floor output. So if the alcove is 60° and the floor 80 then 40 btu/ sq ft.
Plates over slab? How is that insulated below?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
1 -
Thank you
"Upon reading this again, you refer to "thermostats" and "sensors" as if plural. If there are multiple sensors/stats, there can not be just one zone. Something seems to have been lost in translation." There are multiple zones in the house. Only one in the area I was referencing here, and I do not know where the floor sensor is measuring. The other thermostats are either upstairs or in a room on the ground floor that has its own door
The heating contractor did leave multiple low voltage wire access points around the zone, four to be precise. We do not what they are meant to be used for.
Tom
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The alcove has two outside walls, including one made of glass with a low R-value. It is undoubtedly part of the issue. I measured the floor surface temperature, and the swings are even greater than 10 degrees.
The alcove has the tubes embedded in the slab, with the tile directly on top of that. The other areas of the ground floor have aluminum plates between the slab and the tile. I am not sure what type of insulation if any is present. The colder readings are from the alcove, where the tubes are in the slab.
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If it is an option some tubular thermal shades over the large glass area will cut down on the heatloss in the alcove.
It could be there is not enough tubing in the alcove to cover a high heat load room. About 82°F is about as warm as you can comfortably run a tile floor.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
I measure the tile on one of the colder days. The thermostat was set to 78, the floor tile was reading 61. Obviously, the sensor being used by the thermostat was not in the alcove
0 -
Sounds like you need more zone control. You had mentioned that the installer made some thermostat wiring locations available for future use? My guess is that the routed the loops so you could add telestats to loops after the fact to better zone each area. Is the installer able to provide you info for which loops go to which rooms so you can add telestats and thermostats to the problem areas?
If the loops are routed to each room in a reasonable manner you may be able to accomplish your goal, or very close to it by setting flowrates to each loop, assuming there are flow meters on your manifold
1 -
I am not entirely clear on the law, but I think if you have someone create intellectual property specifically for your project, you own that intellectual property. I think that they have to give you the system documentation but I am not a lawyer.
1 -
Not sure about the law and I could see that being very messy and specific to contracts but I don't think it really matters. I can see no reason why, after completion, someone would not share this information with the owner if it were available. I design well over 100 radiant jobs each year and I rarely have any contact with the building owners, I let all my customers know that they should print a full size copy of the radiant plans and include them for the owner as part of the project along with a digital copy for future reference. As far as I'm concerned that info should always stay with the house, I'm very grateful for the plans and notes that were left from previous owners for my house (along with the box of breakers for my now obsolete panel!)
2 -
I really think the expert you need to talk to is a lawyer.
The fact that the builder and installer are acting this way makes me think they know there is something fundamentally wrong. If there were an easy fix they would have done it and moved on.
If the design or installation is fundamentally flawed, it could be that the only solution is to jackhammer out what's there and do it over. Depending on what the finishes are, that could be tens of thousands or even hundreds of thousands of dollars of work.
Once you start trying to fix it, you own the problem. I'll pass on some advice a lawyer once gave me. If you find yourself in a situation where it's looking like you're heading to litigation, think forward to the day when the trial is ending and both sides' lawyers are offering their summations, what do you want the facts to be? Because now is the time you can control what those facts are going to be. You want the jury to be hearing that you gave the contractors every opportunity to fix it, they stonewalled and only when you threatened to sue did they even try, and then they failed. You were the reasonable one, they were the unreasonable ones and the incompetent ones.
What you definitely don't want the jury hearing is that you went on the internet and did some research and changed a bunch of things and then the system didn't work.
2 -
If the contractor is no longer responding, have your lawyer send them a letter. That will usually grab their attention.
I'd like to know if they can safely raise the water temperature to heat that room properly. Is the boiler currently operating on outdoor reset or just running up to a set temperature?
How many square feet is that alcove? If it's on it's own, dedicated loop, install a thermostat in the room; wired or wireless. Easy.
Or get supplemental heat installed. A small radiator if you can get piping to and from that location. Or even an electric radiator. More difficult.
Change the doors and windows to a higher R-valve. Even more difficult.
Re-do the whole project. Tighter tube spacing. Very difficult.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
Two btu per sq ft for degree difference for a slab2 -
the alcove is a high mass system with tube in the slab the kitchen is low mass with aluminum plates. So the kitchen will heat up quickly . The slab takes longer and if it is the end of the kitchen loop it will get a lower water temperature, so less output
A tubing layout with pictures of the installation would help. It could also show the insulation details below the slabBob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream2 -
Thank you all. We have had lawyers involved for a couple of months. The advice you have all given me is very useful and consistent with what my lawyer is saying. I have brought in another heating contractor, but they are unable to provide an exact diagnosis without the drawings. @hot_rod Thank for your statement. This is a vital point in my opinion "the alcove is a high mass system with tube in the slab the kitchen is low mass with aluminum plates. So the kitchen will heat up quickly . The slab takes longer and if it is the end of the kitchen loop it will get a lower water temperature, so less output" The contractor put them on the same zone, which does not seem logical given their different performance
0 -
If you are just trying to make the floor warm in the alcove then they could be on the same zone. If you need the output of the tubing to heat the alcove then it needs to be at least somewhat separately controllable.
An ir camera and some tape will tell you a lot about where the tubing is.
1 -
Your lawyer is the best one to give advice, but I'm thinking this ends with you filing a lawsuit and making a discovery demand for the design documents. And I'd bet dollars to donuts that in discovery you'll learn the reason the contractor is refusing to provide them is they don't exist, and the reason they don't exist is the system wasn't really designed.
It's an extremely common mistake to assume that floor heat systems will "just work." They won't. They need to be designed.
1 -
Update, and a request.
Since July of last year, we have had the engineered drawings. They show the alcove on its own manifold with its own sensor conduit, separate from the aluminum-plate zones. It was installed, but it was never given its own control; it runs off the kitchen thermostat. So the diagnosis several of you offered in July, high-mass slab and low-mass plate sharing one stat, is what the drawings show as well.
The original installer has now proposed corrective work: independent control for the alcove, and balancing and commissioning that was never done. Before that work happens, I want an independent third party involved for two things:
- Write acceptance criteria against the engineered design: circuit flows per the loop schedule, mixing valve supply temperatures, slab sensor placement, and a zone-independence test (alcove holds setpoint while the kitchen zone is satisfied, and vice versa), plus floor surface temperatures in the alcove vs. the kitchen at a given outdoor temperature.
- Visit the site after the work is complete, measure against those criteria, and write a short report.
This is commissioning verification, not expert testimony. It is a paid engagement. Ideally, someone in southern Wisconsin who can get to Madison, but a designer could write the criteria remotely from the drawings and the loop schedule, with a local person doing the measurements.
GGross and @GroundUp , if either of you would consider this, or know someone closer who would, I'd welcome a PM. @hot_rod If Caleffi's Milwaukee people can point me to contractors in this part of the state who do this kind of verification, that would help a great deal.
Thank you all again for the July replies. They held up.
Tom
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I'd love to help you as much as possible, but office work isn't my cup of tea unfortunately. Without actual photos and data of the tubing installation, there is not a whole lot that can be done in the way of a design reference. I am about 6 hours from Madison but don't have anybody out that way who I could refer either. Hopefully you're able to find someone and get this taken care of!
1 -
Dale Ardnt and Son
Cardinal Plbg
Are a couple shops I have worked with in that area
Or Google hydronic contractor in Madison
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
I'm not a lawyer, but I play one on TV. For a 100 smackers pay a lawyer send him a letter. If someone else drew up the plans, contact them and see if they have a copy on their computer.
I would crank the sys up and shoot it with my IR camera to get an idea of tube layout and temps, The design should have taken in consideration, exterior walls and glass. You really need to know tube dia, spacing, and layout.
You may just need a higher flow rate to get the hot water to the end of the layout. You could try that.
"The best laid schemes o’ Mice an’ Men Gang aft agley"—Robert Burns. The best laid plans of mice and men often go awry.
1 -
@HomerJSmith Thank you. All great points and valid. And we have done all of that. We are now at the phase of reconciliation, where they are prepared to make adjustments. We know the tube dia, spacing and layout at this point in time (We did not when I wrote the first note).
Getting a higher flow rate through some balancing may indeed be the solution.
I am looking for an independent who can help validate the plan to rectify.
Thanks for your input.
Tom0 -
I would argue that by trying to validate their plan you are letting them make their problem your problem.
They owe you a system that works, not one that should work.
The question you should be asking is, how can I verify that the system works? Short of waiting for a cold winter day I'm not sure how you answer that question.
1 -
@hot_rod Thank you for Dale Arndt and Son and Cardinal. I'm calling both. I've also approached a NEBB-certified testing and balancing firm here in Madison.
@DCContrarian You're right, and I'm not trying to bless their plan. What I want is someone independent who can say whether the finished work meets the engineered design, so the question of whether it works isn't left to my judgment or theirs.
That leaves the question you raised, which is the one I'd like input on. Without waiting for a design-day cold snap, what would you measure to conclude the system performs to design?
What I have so far: circuit flows against the design GPM in the loop schedule, mixing valve outlet temperatures, supply and return at each manifold, slab sensor location, and a test that each zone holds setpoint independently of the other. Plus floor surface temperature against room air in each area, which by the 2 BTU/sq ft per degree rule, gives output that can be compared to the design heat loss for that room at whatever outdoor temperature I happen to have.
Is that the right list? What's missing, and is there anything on it that won't give a meaningful reading in October?
Tom
0 -
The temperature difference between the two might just be a matter of more balancing. Reduce flow to the area with the tube in transfer plates that is overshooting, and allow more to the slab/ alcove area.
However the design should show the required temperature to each area. If the higher load alcove area requires a temperature 10- 12° or more hotter, it should have its own temperature zone, mix valve or some temperature control.
If you increase the supply temperature to cover the higher load alcove area, then the other room on that zone may be too warm. Generally you want about an 82° floor surface temperature. Above that, on a hard tile surface it begins to feel uncomfortably warm to bare feet.
Ideally those two rooms would have their own temperature control.
Other things to check, the system should be set up on outdoor reset, so temperature supplied to the floors would vary based on the heat load. Basically warmer supply as outdoor temperature drops. This increases efficiency and comfort as you get long run cycles and less floor temperature variation.
Assuming the design was done correctly, this info should be in the design printout. SWT and flow to the various loops.
Another indicator would be the heat load of the two rooms in BTU/ sq ft. Rooms with exterior walls, lots of glass often have higher loads. So either tighter tube spacing, or hotter SWT would be needed in the higher load areas.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
@tom_eric : "What I want is someone independent who can say whether the finished work meets the engineered design, so the question of whether it works isn't left to my judgment or theirs."
There's nothing to guarantee that if it's installed as designed it actually heats the space satisfactorily. At the end of the day, you want a heating system that works. Unless you designed it and they built it to your design, a system that works is what they sold you and that's what they owe you.
The question I would ask is, "Is there a way to verify the output of a radiant floor heat system is sufficient other than waiting for a day when the temperature drops below design temperature?" I would start a new thread and just ask that question.
2 -
Closing the loop, and thank you all.
We have a plan with the HVAC installer, and I wanted to say where the heating ends up.
The alcove gets completed as the engineered drawing shows it: its own control point for that loop, with a circulator and a thermostat whose sensor is actually in that room, rather than running off the kitchen stat. Balancing at the manifolds comes with it. We're also looking at a separate zone for the master bath, which is new work rather than a correction.
@DCContrarian, Your point stands, and I took it. I'm not in a position to guarantee the outcome, and building to the design doesn't by itself prove the room will be comfortable at the design temperature. What I can do is have the work measured against the drawing when it's done, and then live through a Wisconsin January and find out. If it doesn't hold, I'll know a great deal more than I did in July of last year.
Which is the real value of this thread. When I first posted, I didn't have the drawings, didn't know where the floor sensors were, and couldn't get a straight answer about why one room ran ten degrees cold. Within a few replies, several of you told me it was high-mass slab and a low-mass plate sharing one thermostat and one water temperature. When the stamped drawing finally surfaced a year later, that's what it showed. @hot_rod , @GroundUp , @GGross , @mattmia2 , @Alan (California Radiant) Forbes, @HomerJSmith , @DCContrarian , thank you. You were right early
With gratitude,
Tom0
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