Radiant Floor Heating System – Is a 10°F Single Space - Same Zone Temperature Difference Acceptable?

tom_eric

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?

mattmia2

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.

GroundUp

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.

DCContrarian

That is not acceptable and you shouldn't let the builder make it your problem.

hot_rod

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?

tom_eric

https://forum.heatinghelp.com/discussion/comment/1859316#Comment_1859316

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

tom_eric

https://forum.heatinghelp.com/discussion/comment/1859338#Comment_1859338

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.

hot_rod

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.

tom_eric

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

GGross

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

mattmia2

https://forum.heatinghelp.com/discussion/comment/1859720#Comment_1859720

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.

GGross

https://forum.heatinghelp.com/discussion/comment/1859728#Comment_1859728

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!)

DCContrarian

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.

Alan (California Radiant) Forbes

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.

hot_rod

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 slab

tom_eric

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

mattmia2

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.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1859892#Comment_1859892

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.