Floor temp v. room temp

Brad White

Then supplement it with other means such as panel radiators or an air coil as a second stage, triggered by a drop in temperature below baseline.

Typically, 30 to 35 BTUH per SF will put you in the 83 to 85 degree floor temperature "limit" for most people. (You can go to 90 in an entryway/mudroom but not a room used for regular occupancy).

If you indeed are at a heat loss equivalent of 40 BTUH per SF, make sure that is not just the assumed floor plate area divided into the heat loss. It has to be the "available for use as radiant floor" area as output.

In other words, say you have a kitchen at 200 SF and 8,000 BTUH heat loss or 40 PSF gross. However, the perimeter has built-in cabinets and an island which take up 30 SF. Now you have 170 net SF. Now you have 47 BTUH/SF and that might be too warm especially when the turkey is in the oven.

[Deleted User]

Floor temp v. room temp

I am planning to install radiant heating in an older Minnesota farm house with a more modern addition.

I plan to use PEX with formed alum. plates under the floor.

I have done a fairly detailed heat load analysis that agrees pretty well with the existing baseboard heat.

I have a fair idea of the BTU/sq.ft. required.

We want to NOT have the baseboard heat when we are finished with the job.

Questions:

1. Are there "rules of thumb" for selecting a floor temp. that will equate to the 68° room temp the family is used to.

2. Is there a standard formula for the temp drop from the water in the PEX to the bottom of the flooring. I think I'm Ok on figuring the temp drop across the flooring.

3. Somewhere I read that a max heat transfer of 25 BTU/hr/sq.ft. is about the max that should be used. One supplier recommended 40 BTU/hr/sq.ft. Who's right?

Weezbo

Fred , that is not a very good way to make a determination,

of anything....

here is why, Not all aluminum plates are created equal.

not all Farmhouses are created equal.

not all floors are created equal.

not all floor coverings are created equal.

not all rooms within an enclosure are created equal.

by guess and by golly doesn't work very well as a design strategy

[Deleted User]

> of anything....

>

> here is why, Not all aluminum

> plates are created equal.

>

> not all Farmhouses

> are created equal.

>

> not all floors are created

> equal.

>

> not all floor coverings are created

> equal.

>

> not all rooms within an enclosure are

> created equal.

>

> by guess and by golly

> doesn't work very well as a design strategy

[Deleted User]

More info



> by guess and by golly

> doesn't work very well as a design strategy




I'm not sure what you are referring to here. Is the below what you are looking for?

I have done a heat load analysis with three different calculators that considered local temps, room size, wall exposure, insulation, window and door size and construction etc. and also sent a drawing of the house with openings, sizes etc. to a vendor for their analysis. All of these methods were in reasonably close agreement. In addition the existing system was sized such that it met the requirements of the above calculations.

I have four different floor situations; plywood and carpet, plywood and vinyl tile, plywood and ceramic tile and plywood and hardwood applied over old ¾' subfloor.

I have determined the R value for each of these floor types and can calculate the temp drop across the floor materiels.

So the questions remain:

1. Are there "rules of thumb" for selecting a floor temp. that will equate to the 68° room temp the family is used to.

2. Is there a standard formula for the temp drop from the water in the PEX to the bottom of the flooring. I think I'm OK on figuring the temp drop across the flooring.

3. Somewhere I read that for under floor radiant heat, a heat transfer of 25 BTU/hr/sq.ft. is about the max that should be used. One supplier recommended 40 BTU/hr/sq.ft. because that's what the heatloss calc. required. Who's right?

Glenn Sossin_2

Lots of variables


Fred

My $.01 per question

1.) ... You design your system to deliver a floor surface temp that equates with the heat load you calculated.

2.) .... There are output charts that you can use to determine surface/supply temps based on heat load and R-Value of the the floor covering. We use computer software - it's just easier. However ...... using the graph, you can easily see the relationship between room temp and surface floor temp. See attachments m21 surface temp.


3.) ..... Refer to the attachment, m21 surface temp. The first variable for each room is the btu load. Let's assume you have a load of 20 btu/sqft at design conditions, and you want the room temp to be 68F. We enter the vertical left axis at 20 btu/hr/sqft go across till we hit the bold diagonal line. Then we go down the find the delta t between room and surface temp. In this example, it would be 10F.

So, at design conditions, if the heat load of the room was 20btu/sqft - and you wanted a room temp of 68F, you would need a floor surface temp of 78F (68 + 10). Works just like the rule of thumb Hot Rod provided - just gives a visual component.

Ok, now I've got the surface temp I need, but how do I translate that to supply temps? Now refer to the attachment joist trak 8 on ctr. This is just one of many references you could use. This chart is from Wirsbo. Based on the delta t you've selected for your heating system, it shows the btu output of their joist trak product with various R-Values for floor covering. For example, lets assume 3/4" Oak with an R-value of .90, a 20F delta T in supply temp, and the 20 btu/sqft load. With these variables, we would need an average supply temp of approximately 117F. Note that this chart is based on a 65F indoor design, not 68F.

It sounds like you've done the first part of this project correctly - you have your heat loss per room. With this, determine the heat load per sqft. Now you have to look at how to deliver this necessary surface floor temp.

Be careful with wood floors. Depending on who you ask, you will usually be told to limit the surface temperature of a wood floor to approx 82F - 85F. Personally, I don't design past 82.5F . I've attached an article from RPA that shows some testing done on different kinds of wood at different supply/surface temperatures. My interpretation, there is alot of margin built in to the 82F surface temp limitation.

40 btu/sqft heat loss sounds on the high side. I assume there is only one or two rooms with this high a load??

I'd look at that number again - make sure its right. Typically, we would see loads that high in areas like a bathroom where there isn't alot of floor space to provide heat with. In those instances, we try to add supplemental heat in the walls of the shower/bath tub or a toe-kick heater.

I would suggest you try to use an injection control with constant circulation if possible. This will allow for very gradual supply temp changes, a very comfortable floor,and allow you to fine tune the system by changing the reset curve it operates on.

Good luck on your project. Hope this information helped you.

hot_rod

Really, surface temperature

drives the panel output. One common rule of thumb is 2 BTU/ square foot for every degree difference.

So at 68 room temperature an 82 degree floor surface would be

82-68 X 2 =28 BTU/ square foot output.

In a residential application with hard surfaces like tile that low 80 degree number is generally agreed to be the max without discomfort or sweaty feet.

This is why many industry experts limit realistic floor output at the 30 btu or less figure.

Run far away when they tell you 50 BTU/ square foot output!. Crunch the numbers... either a very cold room or incrediably hot floor surface to get that output.

Unless you are outdoors melting snow

hot rod

[Deleted User]

OK, thanks for the input and links. They helped a lot. I think I've got the heat requirements under control.

Now I need to get smart on mixing valves and pumps since of the 14 rooms to be heated, 4 are radiant under floor, 4 are radiant and baseboard and 6 are existing baseboard only.

I'd appreciate any suggested reading on that.

[Deleted User]

> output!. Crunch the numbers... either a very

> cold room or incrediably hot floor surface to get

> that output.




Thanks.

It's actually 40BTU/hr/sq.ft. It's a room with lots of windows and exposed on 3 sides to the Minnesota winter.

Mike2

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