Radiant Floor Sensors

Mike T., Swampeast MO

You might want to investigate FHVs (floor heat valves) from Danfoss and others. They're proportional flow devices similar to TRVs (thermostatic radiator valves). The main difference is that most have an integral flow setting device so that you can balance the head loss of loops of various length. This is important because without the flow setter shorter loops would overheat relative to larger loops if the user significantly raises the temp setting of the operator. By ensuring that each loop has nearly the same head loss and using a reasonable reset curve, the possibility is greatly reduced or eliminated.

They're available in two forms: one senses and modulates based on slab temperature (a special TRV operator with floor temp sensor is used); the other senses and modulates on room temperature (a standard TRV operator is used). All operators are user adjustable of course.

The slab temp sensing/modulating version is recommended for spaces no larger than about 10 square meters (107 sq.ft.) and are most often used in bathrooms where a minimum floor temperature (regardless of room air temperature) is desired.

The air temp sensing/modulating versions are generally limited to about 10,000 btu/hr design load for the loop(s) served. Multiples can be used in the same room, but be aware that especially during periods of low to moderate load, not all of the loops may heat evenly.

FHVs and TRVs are wonderful for use with condensing/modulating boiler using full reset and true constant circulation. By "true" I mean that heated water is circulated at any time heat is needed in the structure and if such water is heated just to the "barely adequate" level efficiency will be maximized. Other advantages of both is that a single circulator can be used to drive MANY TRVs or FHVs and no other valves, thermostats or wiring are required. FHV valves generally mount in the wall at a convenient height for adjustment and many have an integral bleeder. Connectors for all sorts and sizes of tube are available.

Floor temp sensing "thermostats" of any sort are generally only used when a steady floor temperature is desired, the most common use in a residence is for bathrooms. If used in "normal" spaces, be aware that a floor temperature does not equate with a room air temperature--the room air temperature will vary with the amount of heat loss experienced.

If you don't use FHVs everywhere, you really might want to investigate them for use in bathrooms--you can run multiple bathrooms each with their own FHV on the same "zone" and still have individual control. This greatly reduces the chance of "micro-zoning" problems (often requiring buffering) for the boiler that can <I>easily</I> occur with highly zoned systems using on-off control via circulators or valves.

While TRVs certainly seem to be growing in popularity here in the U.S., I've yet to hear from anyone who has used FHVs system-wide.

Ron Hartzheim

Radiant Floor Sensors

I am currently installing a 9 zone radiant system in concrete and would like some suggestions on what brand of floor sensor stats that everybody uses?
Ron in Virginia

ALH_4

FHV's

If you do decide to use FHV's, lay out the tube accordingly. A spiral, or embedded loop pattern is better suited to the lower flows seen much of the time with FHV's. Otherwise, the temeperature of the floor may be somewhat uneven. However, it will still heat the room no matter the pattern of the tube layout.

I see no reason for floor temperature limiting thermostats if the boiler is on outdoor reset.

Mike T., Swampeast MO

Andrew,

Have you used FHVs system wide or nearly so?

Unknown

tekmar 508 w/o79 sensors

used undreds of them. they work GREAT!

hr

tekmar or Uponor

version of the same control. It will watch BOTH air and floor temperature to prevent the room from overheating.

Be sure you install the floor sensor in a condiut for replacement down the road. They do fail from time to time.

A simple setpoint control can be used as a floor sensor control, but they will not limit room temperature, without the ambient air sensor onboard.

hot rod

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Unknown

have you run into that yourself andrew?

I have heard from two installers who have done constant circ systems who say the perhaps expected ultra high dTs never materialize if the reset curve is set well. Since it's constant circ, I can only explain that by theorizing that the front end of the loop heats up and then draws less heat from the pipe, allowing the overall temperature to stay higher longer.

Unknown

If you want indoor feedback (high mass, mild climate, it's nice!) then the teknet 4 541 thermostats also can take the floor sensors.

they do work great. floor sensors are standard now on nearly all of our systems. almost never for high temperature limiting, but for minimum temperature maintenance. cheap enough to use on every zone without really jacking up the system cost as well... no good reason not to use them, IMHO.

Andrew Hagen_2

Thoughts

I do not have much personal experience with FHV's, but spiral and embedded loops are generally accepted to provide a more even floor temperature, are they not? I guess time is the big variable, and with a properly set reset curve time to equilibrium tends to be less important. Even a 10°F dT will result in one side of the floor area covered by that loop being cooler than the other. Reducing the flow rate has to increase the dT compared to identical conditions at a higher flow rate. How high does the dT have to get before it is perceptible in a radiant floor with serpentine loops?

My opinion of FHV's is that they are most applicable in "hybrid" systems with primarily TRV'ed radiators and radiant in the bathrooms and kitchen, but I am open to creative ways to pipe these valves in larger zones with multiple manifolds. When I thought of ways to use FHV's in Thermofin systems, it seemed inevitable that the cost and hidden piping would increase as the use of FHV's became more extensive. The FHV's I have seen have compression fittings which make me nervous when they are hidden and connected to a valve that can be moved or bumped.

No doubt I need some education from someone more experienced in their use.

Unknown

Well, I don't know how high the dT needs to be before it's perceptible. I design around a 20 degree drop standard, nearly all serpentine these days and no one has noticed yet. I picked this up from Quik Trak for which it has to be standard, and there are a lot of quik trak jobs out there without noticeable variance... so I wondered why not the rest of install methods? and found out there isn't a reason why not...

You're right that under identical conditions, reducing flow rate increases delta T. But a constant circ circuit is not identical to any normal operating characteristic. But I have the same mental burp at this that you do... choking flow infinitely should result in half-heated floors. But, according to the two fellows we all know who play with this that doesn't happen, plain and simple.

The real question is why?