We have a 2 yr old radiant floor heating system and have overheat issues that the installer has not resolved after numerous attemts. Recently we had a different contractor perform the annual maint & inspection for the boiler. During his inspection he removed the T-stat covers to insure the dip switches were correctly set. He found that in all 5 zones the slab sensors were coiled up inside the covers. He states he has never seen that before. These are Tekmar 508 T-stats and Tekmar 079 sensors which are connected (plugged into) to the T-stats. We wonder if this could be the cause of the constant overheating. Why wouldn't the sensors be installed in the floor? Why confine the sensors behind the T-stat covers? Any help will be greatly appreciated.

well, they are called slab sensors, one would think that they belong in the slab. I also think that if they are measuring the room air temperature that the system is going to overshoot. Anyway, I don't think much of your original installer, and if you are going to want this thing to work you are going to have to find a way to put the sensors where they belong.

Do you see any PEX ends in the tstat boxes? We took lengths of PEX piping and trapped it in the slab before the pour so we could slide temperature sensors down it. Makes replacement easier.

Luckily, the sensors you have are very slim. They can even be incorporated into the floor covering, either wood or tile without too much destruction after the fact. They don't have to go into the slab.

Our affiliate, PexSupply.com, sells these Tekmar thermostats and sensors, and you can find Q&As and install/user manuals here:

Tekmar Thermostat

Tekmar Slab Sensor

ICF is correct that these sensors can be easily placed in a finished floor although a competent installer would have installed a stainless tube into the slab at a location mid-way between the PEX circuit spacing BEFORE the pour. This allows easy replacement of a failed sensor and also provides representative slab temperature sensing for the associated zone upward heat flux with minimal temperature feedback lag to the controller (i.e., your simple thermostat in this case). There are many designers/installers who know little about control system fundamentals and create large over/undershoot problems where there should be none.

However, given the lack of competence of your installer, I don't know if this will solve your problem. Once you get the slab temperature sensors installed, carefully read and follow the thermostat installation directions because the wrong indoor/outdoor/slab temperature settings can certainly create an overheating situation as is clearly explained in the directions. If you are lucky, it may well be that your installer who didn't properly place the slab sensors also didn't properly set the thermostat settings either.

If you are interested in why so many hydronic radiant floor heating so called "professionals" create so many problems, read the instructions for our free DIY hydronic radiant floor heating design software sometime:

Borst Hydronic Radiant Floor Heating Design Software

Thank you very much for the great feedback. I shall keep you updated with events as we go forward.

the TEKMAR Slab sensor has a 3 position dip switch,
Slab temp only
air temp only
salb/air combined.
set them to air for now, this is controled by the dip switches in the t stat.
later when you have a chance to, get the sensor in to a position where they can measure floor temp as well as air.
A simple scrap of pex will work same as a 1/2 stainless or type L copper pipe, in your case you may need to get creative now to get in the slab.
Avoid placing the sensor directly on the pipe.
Good luck.
Dan

We design systems for DIY "installers" all over N.America and rarely specify slab sensors, as most radiant systems that overshoot the thermostat settings are victims of poor design temperature control not electronic control. Whether the system will benefit from a slab sensor is dependent on many factors most of which can not be divined by any "DIY" program, free or otherwise. We do specify and install the 508 but only where a clear need is established.

I am seldom disturbed by floor sensors coiled up in a box but can't get over outdoor reset sensors left in the box or missing from condensing boilers that come complete with control and sensor all ready to go and not used.

Where over-shoot is concerned we look to the delivered water temperature as the root cause of this common problem and in most cases use the Tekmar 508 to those who treat symptoms instead of taking the time to find the root cause.

As Dan helpfully points out, make sure that the 508 is set to "air only" for the best results.

"DIY design, always the first mistake".

The order of importance of temp control feedback for hydronic radiant floor heating systems is slab, outdoor air, and indoor air.

Room convective heat gain and associated affect on indoor air temp is a secondary effect and not well governed using only indoor air temp control feedback. Room radiant heat gain and associated affect on comfort level is the primary effect and is entirely governed by slab temp control feedback. This is why there is a "radiant" in hydronic radiant floor heating system.

Outdoor temp control feedback is used in the simple Tekmar thermostat as a crude means to determine the current rate of building heat loss (which is a function of both the indoor and outdoor temps) and attempt to compensate for this. For a hydronic radiant floor heating system to avoid over/undershoot problems and maintain a uniform comfort level, it is important for the controller to accurately determine/forecast the rate of building heat loss and use this lead compensation feedback.

Bottom line, for good hydronic radiant floor heating system temp control performance, one needs to use slab, outdoor air, and indoor air temp feedback. This why the Tekmar thermostat has these temp feedbacks. One also needs to carefully read the installation instructions for the thermostat and properly set all the settings. As I indicated previously, there are many designers/installers who know little about control system fundamentals and create large over/undershoot problems where there should be none.

"Hiring an incompetent HVAC company, the most common reason for having HVAC problems."

It would be interesting to see temperature logs/graphs of slab and indoor air temps for the various options. But I didn't see any in a google search.

Exactly right, time transients of the actual outdoor temp history, along with the desired/actual slab temp and indoor air temp profiles, should always be logged to validate that the control system feedback gains are indeed correct. More sophisticated controllers log this data continuously. This data can be transmitted and the controller feedback gains can even be adjusted over the internet if needed. However, most controllers don't need much feedback gain adjustment because the control algorithms are designed using sophisticated frequency domain analysis techniques to ensure adequate feedback gain margin for most installations. However, there is no good reason for not installing and using the required temp feedback and this only occurs as a consequence of laziness and/or incompetence which usually go hand-in-hand.