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Wide variation in required supply temp

Brad White
Brad White Member Posts: 2,398
Sounds like you have it off to a good start and figured things out well.

You ask if there are standards for variation- Typically, I would size my flow rate for a ten degree delta-T. This will make the floor temperatures more even than say, a 20 degrees. In rooms that have higher R-value floor coverings, the good news is that the floor temperatures are more even than say, tile. The heat is forced laterally as much as up, so you are less likely to feel "striping".

Many designers use a 10 to 15 degree delta-T. Naturally this means more flow so keep your loop lengths short and use more of them. With thermostatic heads or balancing valves, having more circuits also gives you more fine-tuning opportunities by default.

Some spaces such as entryways can tolerate higher temperatures and may well need them, but keep them below 90. Not for barefoot or long term occupancy. An exception, definitely.

Generally, each temperature requires it's own mixing valve and circulator- that is how the temperatures are generated.

The other things that define manifold/mixing/circulator assemblies are logistics -where they are relative to the floors served, (the distance and loop-length thing) how you are zoning the house. But temperature comes first.

Now, with the temperatures you have, I might think about combining the 142-144 range if they are geographically near one-another. If not, give them their own. The other temperatures are far enough apart (more than three to four degrees and depending on floor covering is my own rule), to require their own setups in my opinion.

As you know and are finding out, the floor temperature/water temperature/room temperature relationship is very precise. Temperature drives it but flow does not affect temperature the way most people think.

Flow as a variable is very forgiving or intolerant depending on what you are trying to do: If you need more heat at a fixed temperature, you can double the flow to get a measly 10% more. On the other hand, if you are short 25 percent in flow, you will still get about 95% of design capacity. This is why temperature is so important and those relationships should be closely guarded.

My $0.02

"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

-Ernie White, my Dad


  • [Deleted User]
    Wide variation in required supply temp

    I'm working on an old farmhouse to install (I hope) under floor radiant heat.

    With the help of people in this forum I have determined the required supply temps. There are 6 rooms and the required supply temps for a room temp of 68°F are 91,112, 118, 129, 142 and 144°F.

    Floor temps are from 76-83°F.

    Floor covering R values are from .95 to 2.54 and BTU/sf from 16-30.

    Because of the house layout heat loss is not proportional to room area.

    Questions -

    Are there standards for amount of variation in floor temp other than the 82-83°F high limit?

    Will I have to use 6 separate mixing valves and circulator pumps?
  • tim smith
    tim smith Member Posts: 2,752
    Different water temp needs

    Just a thought, depending on the type of tube installation, maybe for the higher temp areas use extruded plates w/ tighter centers to lower temp required and for the lower temp requirements, use suspended tube on more normal centers. Maybe after calculating, this will get some of the disparity of temps closer. My .01 Cents worth. Tim
  • Brad White
    Brad White Member Posts: 2,398
    Tim has a very good point

    Forgetting that this is an entirely new installation, the means of tubing installation (staple-up with plates, radiant sub-floor, thin-insert sub-floor, gypcrete, etc.) may not yet be determined. All have a bearing on temperatures required and can be tailored to get them to work at more common, shared water temperatures.

    Excellent call, Tim!
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
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