# Radiant in slab - Anti-freeze needed? Closed loop with heat exchanger?



## headrc (May 7, 2008)

Hi again to all ....In my continuing pursuit on designing my system ....I talked to one dealer yesterday about my concept with radiant heat installed in a slab.  He stated that I really should use a heat exhanger and put anti-freexe in a closed loop for the piping in the slab.  This is the first I have heard this recommendation ....all along I thought I would just pipe the pex tubing to a manifold with a mix valve and not be using a heat exchanger.  Additionally, I was going to insulate the slab well and was not thinking of anti-freeze in the pex tubing.  Are his suggestions overkill? Necessary?  As always any input would be appreciated.  I could not find another place to put this thread on the various forums ....so I thought this was the place to open this discussion.  Thank you,  RH


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## ISeeDeadBTUs (May 7, 2008)

Is the slab in the basement of your living area? An attached garage? Detached building?

Sorry, just read this is in TN . . . You need heat in TN??? :wow:


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## heaterman (May 7, 2008)

headrc said:
			
		

> Hi again to all ....In my continuing pursuit on designing my system ....I talked to one dealer yesterday about my concept with radiant heat installed in a slab.  He stated that I really should use a heat exhanger and put anti-freexe in a closed loop for the piping in the slab.  This is the first I have heard this recommendation ....all along I thought I would just pipe the pex tubing to a manifold with a mix valve and not be using a heat exchanger.  Additionally, I was going to insulate the slab well and was not thinking of anti-freeze in the pex tubing.  Are his suggestions overkill? Necessary?  As always any input would be appreciated.  I could not find another place to put this thread on the various forums ....so I thought this was the place to open this discussion.  Thank you,  RH



I'm assuming you have a boiler that is open and that's the reason you want/need to leave the slab open. If your boiler is sealed there's no reason not to pressurize the floor tubing.

There's some definite merit to sealing the in floor and pressurizing it. The main reasons would be as follow:
You'll have a much easier time purging the loops due to the ability to pressurize it.
No worries long term about sludge or biological buildup that can occur in an open system.
The benefit of antifreeze is obvious.



That being said, we've done it both ways.  For purging you want to rig up some kind of a high pressure pump (a 1/2hp well pump will work) that will be able to force all the air out of the loops. The system circ you use may not have enough grunt to get all the air out. You also need to monitor the water quality closely, long term. Using a good system cleaner on the loops and manifold piping before actually filling with the final fluid will help out and should be done on any system. You wouldn't believe the amount of sludge a little soldering flux in the system will generate. Do those two things and you should be all right

PS keep the loop lengths at 300' or less for 1/2" I try to design for 250'. If you're using 5/8" you can go up to 400 and not lose to much temp or gain too much head. If you go with an open design you positively want to stay on the short side or you will run into suction head problems with the circ. 1-1/2" blue board underneath works good and remember to incorporate some kind of a thermal break on your edges. You'd be amazed at the amount of heat you can lose out the sides of the slab.

Good luck. There's nothing like a nice 70* slab in the winter.


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## headrc (May 7, 2008)

Right now it will either be a gas fired hot water heater or an OWB as the heat source (yes I know about OWBs but I have been all thru the gasification and why I cannot justify it in Tn.)  Still trying to sus that out.  Yes we need heat in the winter here ...and cooling in the summer of course. 

I hadn't thought about the radiant heat being non-pressurized ...so you are saying it is better to be pressurized?   So now is that a simple matter of using the heat exchanger etc. like this particular dealer recommended??  Or am I now heading into yet another complex type of system??

I am a novice at this of course ....

Thx again,

RH


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## headrc (May 7, 2008)

I gorgot to answer the other question ...the radiant heat will be in a new outbuilding I will build ...about 400 square feet with a six inch slab. Thx,  RH


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## heaterman (May 7, 2008)

headrc said:
			
		

> I gorgot to answer the other question ...the radiant heat will be in a new outbuilding I will build ...about 400 square feet with a six inch slab. Thx,  RH



In that case I would probably recommend the hot water heater. For a well insulated building that size the gas isn't going to cost very much at all. The water heater will handle radiant floor temps (set it at about 105-110*) and the total heat loss will be well within the capability of a standard burner. (35,000btu) Pressurize the whole thing. Get an expansion tank that's about 20 gallon capacity, set the empty pressure at 12-15 PSI then fill it to about 30 PSI. It will have enough capacity to absorb the expansion in your system plus "feed it" make up water should it ever need some. Install a gauge upstream of your PRV to monitor the "feed" side of the system and simply add fluid if you ever need it. No need to connect it to your plumbing. Antifreeze of course would be optional but I would probably recommend skipping it in your case. Very low probability of freezing an indoor slab in your location provided you're in and out of the building a couple times a week. That slab will hold heat for days once it's up to 70-75*.


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## headrc (May 7, 2008)

The reason for the OWB is to also heat the house through a heat exchanger in the plenum of the existing forced air ...so I still need to know about pressurizing the radiant floor in the new outbuilding.  I had talked to several suppliers previously and nobody mentioned pressurizing that floor ....so I guess I need to understand this type of install better.  Thx,  RH


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## WILDSOURDOUGH (May 8, 2008)

All good advice from heaterman !

I can't tell you much about pressurized systems. If you are going to do radiant two- zone, ( the heat exchanger in the house, and in-slab in the outbuilding) I would go with an OWB, and follow the directions of heaterman reguarding underslab insulation and the thermal break at the slab edges. 

Just finished my home in February- two years of work. I used an OWB, 100' out from the house, single zone to 1800' ( 6 loops of 300') of 3/4" Pex in-slab. Basement is 2320 Sq. Ft. as is the main floor. I have no heating fixtures on the main floor !  The basement is HOT- the upstairs stays warm enough- (must be, as I have heard no complaints from the wife or her 91 year old mother !). Thought about the antifreeze thing- did not want to mess with it and from what I've read- It is not necessary with Pex. I read in a plumbing forum about a plumber in Maine, that upon going to a bank foreclosure house that had been vacancy for two years, where he thought all the pipes would have burst, found them all to be Pex, turned on the water and everything was perfect. Convinced him- all he uses now. Had all my plumbing done in Pex- It's the civilized way in 2008 !

Back to my radiant/OWB- LOVE IT. Granted, house is super-insulated, Foundation too- but several times in the middle of this winter I stopped feeding my boiler for 2-4 days- never saw the water in the system get below 65 degrees- even after four days off and COLD weather. I don't think it will freeze- think that the curculators take heat from under the slab and move it back out to the OWB. Does take 12-18 hours to reheat the slab/house, so won't do that much- but it does prove that you can go away, and not worry.

Note: I would recommend two pumps (curculators)- one on the OWB to move it to the slab, and one after the manifold to help push it back to the OWB- thats what I did.


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## headrc (May 9, 2008)

So here are a couple more questions ....how much water would there be in a floor radiant heat system for a 4000 square foot slab?  If it is not a lot ....then probably puttin anti-freeze in would be not expensive.  Also, pressurizing the closed loop radiant floor heat is just a simple matter of an expansion tank aand filing the lines form my water source and then clusing off the valve to it....and then it will be pressurized?  Thx,  RH


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## Willman (May 9, 2008)

Anti freeze is more money than plain old H20 and it doesn't have as good as heat exchange as water. Also it does break down and have to be changed as well as proper disposal. There is a way to figure gallons per tube length x dia. 
Will


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## WILDSOURDOUGH (May 11, 2008)

Yesssss- all those reasons, Antifreeze is nasty stuff ! and costly and how do you get rid of it and it breaks down and, and, and...
There is a way to caculate the fluid in a certain size of pipe... don't remember exactlly what it is but think mine was around 70 gallons,
in 1800' 3/4" Pex and 200' of 7/8"- to and from the OWB.


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## heaterman (May 12, 2008)

Uummmmm?

 I know it's only one zero but it makes quite a difference. One place says 400 sq ft and another says 4000? Which is correct? It would make a taddy bit of a difference in my response.

RE antifreeze. 

Boiler antifreeze (Propylene Glycol) is non toxic and actually breaks down quite easily when exposed to air. The approved method for disposal is to dilute is and pour it down the drain. Ingestion of any normal amount will not cause any ill effects except lubricating your digestive system VERY well....if you get what I mean. 

Regular or automotive antifreeze (Ethylene Glycol) is a completely different animal and should never be used in a boiler system. As you all probably know, it is highly toxic and does not degrade very well. It must always be disposed of correctly at a hazardous water facility and will kill cats that drink it nearly instantly.  Do not use automotive antifreeze in a boiler. Naughty Naughty!

That being said, using the following parameters, this is what you're looking at for antifreeze and total system volume.

1. I figured you'll probably have a maximum of 10 gallons of fluid in the manifold and miscellaneous piping. 
2. You'll have 4,000 feet of 1/2" tube in that slab, assuming 4,000 is the size of the building and not 400. 
3. Your maximum fluid temperature will be limited to 120* and your fill temperature will be roughly 60*. 
4. A 20% solution of antifreeze should be all you'd ever need where you are located so that's what I designed for.
5. I assumed that there is no major elevation difference between the fill point and the slab.

So. Your total sealed system volume will be 48.4 gallons with 38.44 residing in the tubing itself. A 20% solution requires 9.7 gallons of glycol. That should cost you about $140-160. You'll need a tank with an expansion capability of 1.8 gallons. Another $100 or so.

Make sure you use a good cleaner in the sealed part of the system before you fill with antifreeze.

Probably worth the peace of mind.


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## bridgerman (Nov 8, 2008)

You could try a ThermGuard.

www.bearmountaindesign.com

It is a programmable timer that can circulate water through the pipes periodically and prevent them from freezing.  I have several in my house where I have this problem.  It is simple to install and program and it is beign offered with a 10% discount and free shipping to Hearth.com members.  It is only $62.00 so it is a lot cheaper than antifreeze, doesn't break down and need to be replaced, no disposal problems, doesn't use batteries and there is no possibility of contamination with your potable water in your house.

Cheers,
John


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## BrownianHeatingTech (Nov 8, 2008)

heaterman said:
			
		

> Boiler antifreeze (Propylene Glycol) is non toxic and actually breaks down quite easily when exposed to air. The approved method for disposal is to dilute is and pour it down the drain. Ingestion of any normal amount will not cause any ill effects except lubricating your digestive system VERY well....if you get what I mean.



It's tasty, though.  I'd say, "don't ask me how I know," but most everyone here has tasted it on occasion, since it's a common food additive.

Joe


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