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Wood floor surface temp question

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Member Posts: 7
First off I would like to thank the amazing people of this forum as I have learned a lot reading through the threads. I still have lot to learn hence the question below.

I am homeowner remodeling our house and putting in Radiant heat while we are at it. I am doing aluminum plates (stamped) above subfloor with 3/4"plywood and 1/2" engineered wood on top. We are in the Chicago area.

I downloaded loop cad and started running some numbers and the design is calling for 78 degree floor temp (for living room facing north with big window and door to outside, 2 sides facing outward) to satisfy the heat requirement. I understand there is no set floor temp number but I would like to get experts opinion on if 78 is too hot.
From what I understand the floor temp is function of the heat required and heat loss which is based on the building characteristics. So if I want to heat the room at 70 degree I have to heat the floor to 78 unless I am willing to change the building characteristics. Is my understanding right ?

Here is the screenshot from the loop cad numbers

• Member Posts: 7,397
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You don’t want the floor surface to exceed 84* or it will feel uncomfortable.

78* is fine if that meets the heat loss at design temp.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 4,103
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That's quite a large room. Looks like about 30' wide?

The loop lengths are max'd out with tight spacing. With a load of 13 BTU/[], I'm used to seeing lower water temperatures and looser spacing at that design. Did you force any numbers?
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab
• Member Posts: 7
edited February 2023
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Thanks Alan for your input. I didn't force any numbers for the system besides entering required info that its heat transfer plates over subfloor.
The room is 34'6" by 15'9" Room. Spacing is 8 " on center for tubes with some adjustment where needed. Total loop length for the room is 618 (305+311) . With water temp at 110 degree and deltaT forced at 20 degrees this is the output I get from the software.

Edit: I did mess with the supply/return piping a bit which looks like this

I think I may be understanding your logic as looking at the basic formula I should be outputting (.75X500X311) / 311 = 23.79 . Is that true ?
• Member Posts: 22,472
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Looks like room B07 has a 22 btu/ sq ft load, 82F floor surface. Mid 20's is reasonable for radiant staying below 82F surface.

It works out close to 2BTU/ sq. ft for every degree difference between ambient and floor surface.

So a 78 floor surface in a 70 space 8X2= 16 btu/ sq ft.
82 floor = 24 btu/ sq. ft

20∆ is pretty wide, 15 is more common for comfortable floors, at a bit more pumping power.

So tweaking would be tighter delta, tighter spacing, higher flow rate, or higher SWT. High loads areas work better with 6" spacing, if you want to keep SWT down.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 1,572
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Ironman said:

You don’t want the floor surface to exceed 84* or it will feel uncomfortable.

78* is fine if that meets the heat loss at design temp.

Directly from the Wirsbo Complete Design Assistance Manual :Floor surface temperatures are generally designed to remain below 87.5 F for all types of finished floors except hardwood , which has a maximum surface temperature of 80F. See capter 14 for more information about hardwood floors.

On occasions we have run the surface temperatures on TILE bathroom floors to about 90ish since we are not concerned about the off gassing cancer causing aspects..
• Member Posts: 22,472
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Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 7
edited February 2023
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Thanks Bob for chiming in. Ignore room B07 that is stairs to basement plus I am trying to figure out how Loopcad works first before I input all rooms.
According to 2 BTU/SF logic the heat output from the software is not too far off . I can adjust the numbers as you suggested but as per my understanding the fact that I need approx. 13 BTU sq/ft to heat this room stays constant. Since my surface area is limited too it would mean that the surface temp to output that much heat is also a constant (unless the model somehow calculates that at 8 inch OC only x amount of surface area is emitting heat and with 6 inch OC x will increase ).
I can play around with ∆ T or SWT or Flow rate later but spacing is something I am stuck with. since you mentioned is 110 degree considered good SWT or there is benefit to lower it ?
Is 13 btu/hr/sft considered high load ?
What is acceptable head loss ? I am seeing 21 for a manifold with 3 loops of 300 feet. How do I mitigate this ?

• Member Posts: 22,472
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13 btu is an easy load for radiant floors. 78F floor surface should get you that in a 70 space.

A 300’ loop of 1/2” Pex flowing .65 gpm is about 4.5 head. A 250’ loop would be 3.8 so not a lot of incentive to chop 50’ off the loops.

Your program should allow you to select loop delta T, getting down to 15 delta should only cost a couple more feet of head, I suspect? Still well within a small circ capacity.

The head calculation involves all the piping in your longest circuit. You don’t add the head of all 4 loops on the manifold, for example.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 7
edited February 2023
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That's what I thought as I was reading that I can have up to 35 BTUs per sq/ft with above subfloor install , probably with extruded plates though. All the literature I have been reading says with 1/2" pipe I can have 300' foot loop (10% variance) so I am trying to make every loop about that length. I figured when you can have 300 feet then why not try to make loops that length.
I am still not sure about how can I reduce the surface temp. I don't mind 78 as its within acceptable range and if I can get closer to that warm feet feeling without overheating the space may be my better half will forgive me for delaying the floor install.
On the head loss topic I am not sure why the system is telling me that my head loss is so high. I have 2 ~300 feet loops .

Note: I did update the region from Midway to Glenview which increased heat loss to 15 btu /sq/ft from 13 but still within acceptable range.
I am also heating one room to reduce the variables so that its easier for me to understand the numbers.

• Member Posts: 22,472
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Your zone 201 shows two loops in 1800 square feet of space? And a flow rate of 2.16 gpm. That flow rate in 300’of 1/2”tube is why the head is so high, same with all the others on the list.

An 1800 square foot room would be more like 6- 300’ loops, 12” oc, with .50 gpm

.65maybe .75 gpm is max. For 300’ loops.

Your loads sound reasonable, but you zone loops are way out of wack somehow.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 2,511
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All the planning is great, it lowers the chance for errors. When it is all done take an Digital Infrared Thermometer an shoot the floor and make any final adjustments at to SWT and flow. Outdoor reset might be an advantage.

I have found that engineered flooring can experience higher temps than plank flooring.
• Member Posts: 7
edited February 2023
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Here are the updated figures. I filled in all circuits to avoid confusion. I started with one circuit just to get an idea about how the software works and to understand the concepts but since it was throwing people off I added all the circuits. My surface temps have gone up as my load increased due to updating the region and adding more details. I am into 80s now. Granted this design is when the outside temp is 2 degree and inside temp is set at 70. I feel like I should be OK but looking or expert opinions.

Some loads seem out of proportion for me such as Powder room as it has only one wall exposed and has a smaller window. I have updated manual J settings to account for room above, excluded inside walls from envelope, adding window.
Mud room has smaller window than family but still the load is high too. I am wondering if the room size played a factor. My concern is if I am adding too much piping in those rooms especially with tile floor.

I am planning to put in floor sensors to measure floor temp and shut off heat if the temp reaches set temp. Manifold 1 has 3 loops about the same length (~300 feet) and manifol2 has 3 loops about same length (~250 feet). Even though I will have flow meters I wanted to keep the loops same length if possible.

I did a quick load calculations on second floor and they don't look good. We have big windows on both floor but second floor has unconditioned space above. I will tackle it once first floor is settled.
Already done piping in living room so designing as I go. Not ideal but better than going in blind.

Hers is a 3D view of the floorplan to provide sense of what I am dealing with. number 1 corresponds to first 2 screenshots above, number 2 to screenshot 3 and 4 and number 3 screenshot 5 and 6