Radiant Floor Heat Question

[TnWV]

Hey guys, I building a new house that has a full basement (I plan to start a full build thread ASAP). I want to heat the basement using radiant floor heat and have a question about PEX size. The basement is 2100 square feet, so I am thinking 7 loops of 300' makes sense. I can buy 300' rolls of oxygen barrier 1/2" PEX readily. It seems many use 1/2" inch and are happy with it, but I have had a couple of recommendations that I use 5/8" PEX. Problem is, I can only seem to find 5/8" PEX in 250', 500', and 1000' rolls, which makes the layout harder to not waste part of each roll. The basement will not be finished to the same extent as the main living floor, but will be finished enough to be usable for various activities, such as exercise equipment, dog wash room for my wifes dogs, etc. My question is, is 1/2" PEX sufficient? If so, that makes things much easier as far as purchasing the PEX. That said, if 5/8" is the correct size, I will deal with the size rolls I can get. Based on reading other threads here, I plan to staple the PEX to the rigid foam insulation, so it will be at the bottom of the concrete.

Thanks for any insight!

Tony

 

[ovrszd]

Definitely start a thread about your build. We love threads like that, especially with pictures!!!

I used 1/2". I have 16 loops. My loops are approximately 150'. Bought 1000' rolls. Rule of thumb is 1' of pex per square foot of floor space. You seem to already understand that. Our house/garage is 2,480 sqft. Our system uses a Propane fired tankless heater. We move water at the rate of 2.8 gpm. We are living on our concrete stained and sealed.

In your case with a warm house on top of the basement it will heat very easily. Will take some adjusting to get it so it doesn't overheat. Will also make your house easier to heat because your floors will be the same temp as your basement floor. I would suggest a dual control thermostat that measures air temp and slab temp. This will aid in not overheating.

I can't help with the 5/8" versus 1/2" debate. I've never talked to anyone that used 5/8". It would add gallons of water to the system. But I'm not sure if that's necessary?

 

[TnWV]

Definitely start a thread about your build. We love threads like that, especially with pictures!!!

I used 1/2". I have 16 loops. My loops are approximately 150'. Bought 1000' rolls. Rule of thumb is 1' of pex per square foot of floor space. You seem to already understand that. Our house/garage is 2,480 sqft. Our system uses a Propane fired tankless heater. We move water at the rate of 2.8 gpm. We are living on our concrete stained and sealed.

In your case with a warm house on top of the basement it will heat very easily. Will take some adjusting to get it so it doesn't overheat. Will also make your house easier to heat because your floors will be the same temp as your basement floor. I would suggest a dual control thermostat that measures air temp and slab temp. This will aid in not overheating.

I can't help with the 5/8" versus 1/2" debate. I've never talked to anyone that used 5/8". It would add gallons of water to the system. But I'm not sure if that's necessary?

ovrszd,

I followed your build, and all the other builds that have been posted for the last several years, and I they are my favorite threads. I will definitely start a thread soon, cant seem to stop working on the house long enough. Poured walls are in and the wife and I have worked until after dark most days getting all the drains and gravel in place. Floor will be poured on the 17th, so I have to get everything in place by then.

If 1/2" works well for you, I don't see why it will not for me as well. I will be using a natural gas fired tankless heater in my system. To measure slab temp, due I need to put a sensor in the slab?

Thanks!

 

[dlctcg]

Tony - IMO 1/2" Pex in the slab over 5/8"... easier to work with, cheaper & if spaced correctly will perform just as good if not better. (5/8" is usually used for larger commercial areas.)

Here in New England we usually plan 8" - 9" spacing with 1/2" PEX, 12" spacing being max or you will wind up with heat striping in the slab (this will also be effected by the Foam Board R-value you use). So for 8" spacing SqFt x 1.5..... 9" spacing SqFt x 1.34 & for 12" spacing SqFt x 1.0 will give you the approx feet in 1/2" PEX tubing. You are 100% correct on 300ft max for 1/2" lop length.

Are you heating the rest of the house with radiant also? If not with what? I see you are in WV but I don't know the climate...

Depending on the construction / building envelope you may have a very small heat load; with a very tightly constructed home with the floors above the basement heated already the basement heat load may be very small. If I was building a new home I'd have a proper Heat Load Calculation done for it, it will take the guess work out of the design for both heating & cooling based on location, construction & materials used... & you can then size the systems correctly...

 

[dlctcg]

On the projects we have done... For the slab sensor instead of embedding it, they usually put a conduit into the slab in which the sensor can be slid into later.... It allows sensor replacement if ever needed.

 

[TnWV]

Tony - IMO 1/2" Pex in the slab over 5/8"... easier to work with, cheaper & if spaced correctly will perform just as good if not better. (5/8" is usually used for larger commercial areas.)

Here in New England we usually plan 8" - 9" spacing with 1/2" PEX, 12" spacing being max or you will wind up with heat striping in the slab (this will also be effected by the Foam Board R-value you use). So for 8" spacing SqFt x 1.5..... 9" spacing SqFt x 1.34 & for 12" spacing SqFt x 1.0 will give you the approx feet in 1/2" PEX tubing. You are 100% correct on 300ft max for 1/2" lop length.

Are you heating the rest of the house with radiant also? If not with what? I see you are in WV but I don't know the climate...

Depending on the construction / building envelope you may have a very small heat load; with a very tightly constructed home with the floors above the basement heated already the basement heat load may be very small. If I was building a new home I'd have a proper Heat Load Calculation done for it, it will take the guess work out of the design for both heating & cooling based on location, construction & materials used... & you can then size the systems correctly...

dlctcg,

The rest of the house will be heated with a conventional natural gas HVAC system. Exterior walls will be 2x6 with R19 and I will be using ZIP sheeting/tape. House is one story over the basement, so attic will get 24" of blown in insulation. Probably not the tightest house, but will be well insulated overall. Climate isn't too cold here, but we can have single digit temps for a few weeks in January/February at times. IF'300' is max for 1/2" am I better off going down to 250' and 8 loops? Actually, that leaves about 100 sq. ft. left, so maybe 300' is still the best option for even loops.

Thanks.

 

[dlctcg]

Yeah you will be fine with 300' loops, if the basement is all one zone try & keep the all loops to similar lengths from supply manifold to return manifold...

I know many will say tubing at the bottom of the slab, but none of the contractors we use on our projects do that.... I'll let you do your own research but look at the PDF & do some research... most of our guys are trying to get it closer to mid slab... Just an Idea...

View attachment Common Hydronic Radiant Questions.pdf

 

[TnWV]

Yeah you will be fine with 300' loops, if the basement is all one zone try & keep the all loops to similar lengths from supply manifold to return manifold...

I know many will say tubing at the bottom of the slab, but none of the contractors we use on our projects do that.... I'll let you do your own research but look at the PDF & do some research... most of our guys are trying to get it closer to mid slab... Just an Idea...

View attachment 661737

Definitely one zone for now, not really walling a lot of the basement off, just a bathroom, dog wash room and mechanical room for now. I may add a couple of loops into the attached garage, and that will be on a separate zone.

I will take a look at the PDF and research PEX placement a little further. Thanks for the information.

 

[BrokenTrack]

I would put my pex down closer to the bottom, only because if you ever add walls at a later date, you can Hilti-Gun in the sole-plate of the wall to the concrete without fear of nailing a pex-tube. Either way, because of the way radiant works, it is not going to change your heating effectiveness any.

I would also insulate the outer part of the slab, maybe going in by 8 feet or so, but then not insulate the center of the floor. That is where you can really gain some heating advantage by using geothermal heat. Your slab is going to lose heat out against the outer portions of the slab where the cold is located, depending on how your walls are back-filled, but in the center, it will be a constant 57 degrees, or at least in Maine, it is a constant 57 degrees. That is what I did and I got some great results. I also added 400 ton of rock under my slab for added heat sink ability, and the efficiency is staggering.

Last year we lived in a different house, but still owned this one, and I watched the temps in the house. It got down to -7 degrees below zero (f), and the house stayed at 44 degrees. That was with no heat all winter, and no one living in the house. For a 2600 sq foot house to self-heat just by geothermal heat was pretty amazing I thought. It is too uncomfortable to live in for sure, but it means I can leave the house for extended periods of time without worrying about draining water lines and such, even if it is unheated.

As for tube size, I would not worry too much about 1/2 inch or 5/8" because radiant floor heating works by constant, low amounts of heat, unlike baseboard and other forms of heating. You get your heating efficiency by controlling the dwell time in the loops, and not so much by size. But I would add flow-controls to each loop anyway just because you can balance a radiant floor so much easier that way, and adjust that all important dwell time. Get everything right and you will not shock your boiler on the return, and heat your basement really effeciently.

 

[TnWV]

I will certainly be adding walls, and will need to anchor them to the floor. There is a footing under the weight Bering walls and I will not go under that wall with the pex.

Yesterday I used a skid steer to fill in around my plumbing and footings, and converted the whole floor with somewhere between 2-4 inches of gravel. The outer 2 foot is so around the footings was closer to 10 inches. It’s all packed in and ready for vapor barrier and insulation.

I never thought about not insulating the center of the slab, never saw that it any of my research, but it’s definitely an interesting idea. The outside back fill is all river gravel up to where the top soil will be. The over dig is 2 feet or so all the way around. I’ve used 75 tons so far and still working on it.

My next question is, open or closed system? I’m getting varying opinions on that. My initial thought was closed and keep potable water system separate, and will likely do so, but I’ve heard a couple good arguments for an open system. Not having a water heater doing nothing a lot of the year is one, and makes sense. In the end, I don’t want to complicate things any more than necessary.

Floor will be poured on the 17th, and I hope to get it mostly ready this weekend.