Heating a frozen slab (radiant)

[Jon_E]

OK this is probably a new one for most of you, so I am looking for ideas.

I have been 'bench-building' my radiant floor manifold and zone loop for my garage heat. It was my intention to install it in the next week to ten days in my garage, which is completely unheated to this point and mostly uninsulated in the upper part of the walls and the ceiling. I am in the northeast US so it has been below zero for a few days and is expected to dip back down into the minus-teens again this weekend.

I realized that there is a good chance that the slab itself is well below zero, which would likely do one of two things depending on the temperature of the water entering the system: either the water will freeze up in the PEX tubing shortly after being introduced to the floor loops, or crack the slab badly (and possibly damage the PEX) if it is too hot. I think that even running cold groundwater through the system to start the process, will run a significant risk of freezing in the lines.

My options appear to be heating the building for a few days with a propane salamander to get the slab thawed out, or wait until warmer weather. I can take slab temps with an IR thermometer but that's only on the surface.

I am not sure what to do, but I do NOT want to risk damaging the slab or loop tubing.

 

[Hilltopmasonry]

I would wait til it’s warmer out...but I don’t know much about radiant heat but if you freeze up the lines you could have real issues




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[larry4406]

No experience with radiant either but knowing you don’t have insulated walls or ceiling I would punt.

Run the salamander and insulate these other parts first buying time for better weather.

 

[quattro_sinko]

Closed loop radiant? Non-potable? I am not a heat tech (or a concrete guy) but...if it was mine, I'd finish insulating, and then I'd run a glycol mix, and bring it up to temp slowly.

 

[finn]

I’d go with glycol also.

 

[Lelandwelds]

Closed loop radiant? Non-potable? I am not a heat tech (or a concrete guy) but...if it was mine, I'd finish insulating, and then I'd run a glycol mix, and bring it up to temp slowly.

I’d go with glycol also.

I thought all systems used ethylene or propylene glycol to keep the funk from growing and protect the metal parts?

 

[Firebrick43]

I thought all systems used ethylene or propylene glycol to keep the funk from growing and protect the metal parts?

Antifreeze in heating systems is actually not used that much in the grand scheme of things. While it has one big plus(keep things from freezing) it has many smaller cons.

1 it take ~5%+ more energy to pump depending on concentration

2 it carries ~5%+ less heat depending upon concentration

3 it must be checked regularly to maintain ph, it will turn acidic as it uses up its additive package and must be replenished

4 if over heated (250+) it will turn acidic right away. More of a problem in solar collectors/systems

5 it will leak/seep in places water would not due to surface tension.

6 harder to purge air.


If a system is leak free and one is not adding make up water, and has a good air eliminator, oxygen level will be very low. Let me add if you use oxygen barrier pex

To grow "funk" and have corrosion you have to have oxygen. This is of course if you use acceptable fill water.

Never use ethylene glycol(car antifreeze) is in a heating system. It will sludge.

 

[finn]

I thought all systems used ethylene or propylene glycol to keep the funk from growing and protect the metal parts?

Neither my house nor shop have glycol in them.

I’m upgrading the shop boiler to a high efficiency condensing boiler next week, and will go to glycol at that time as an experiment.

Habitated buildings here seem to generally specify water, although antifreeze is not uncommon.

 

[Bretny]

I would run some boiler antifreze in it just for the anti corosion properties. But heating with a salamander may be tour best bet. What is $40 in the grand scheme of things anyway.

 

[yao]

Hi I just start my radiant flood 4 days ago....I live in Québec Canada and it was -34°C. my garage was heated by propane at 5°C. but the slab was way colder then that....dosent really change anything that you heated up the garage its only heated the surface of the slab.. I use water in my system no glycol. I fill up the system but I leave the end drain open that way the cold water heated up the tubing to about 5°C. make sure that water is always moving. After I start the pump but not the boiller. anyways you have to make all the air come out. after start boiler and crank up the heated slowly it took me 2 days to reach 17°C to my slab....but it was -34°C.

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[Jon_E]

My biggest concern would be that even if the water was moving, it would still be cold enough to "slush" the moving water and freeze it in the floor. I don't want to use glycol in my home system - I use it at work in a very large geothermal loop and it's a pain to use. Only really any good down to about 18°F anyway.

Next week is supposed to be in the 30's again, I'm thinking that I'll try to heat the building with propane and then start circulating well water (in the 50's) to thaw out the slab, then I can put the heat to it in a few days. Also, I will not have the option to put the heat in gradually, it will be fed through a mixing valve to 105°F max but still that's pretty hot water for a cold slab. I may have to fill & circulate and then stop and let it sit for a couple hours, over and over until the slab temp rises up into the 50's.

 

[86turbodsl]

I'd be concerned about your slab temp too, depending on how fast you move water through it and the temp differential, you could get into a freezing situation. due to the cost to fix it, i'd be real cautious about doing this. I am in a similar situation, and my solution is a heat exchanger between heat source and floor loop, and running a 25% methanol / water mix in the floor only. Keeps the flammables away from the flame and protects the floor, and methanol doesn't degrade into acid, and doesn't cost too much to pump. The rest of the system can be drained easily in event of a failure.

 

[Jon_E]

Should have mentioned that I am using both a heat exchanger AND a mixing valve. The mixing valve has a 95° to 105° range.

 

[larry4406]

What’s the long term plan?

Once it’s up an running keep it that way the rest of the winter while you address the thermal loss from missing wall and ceiling insulation?

I think once you introduce fluids you are committed.

 

[Redraptor]

What’s the long term plan?

I think once you introduce fluids you are committed.

What he said. I think the ship sailed on that for this season. What if you have other unforeseen issues?

 

[86turbodsl]

If he uses a antifreeze in the whole system he could fire this year. But i agree he will have to stay on it if water only.

 

[yeldogt]

Would avoid using antifreeze unless you will be turning the system off or doing partial zoning where a zone will be off ... antifreeze adds an additional item that requires continuous monitoring.

One concern not mentioned -- the return water to the boiler What's making the heat ? Cold start up requires maintaining return water.

Also -- does you heat loss work with 105 water ?

 

[86turbodsl]

I am a big advocate of antifreeze and/or heat exchangers on garages. I lost a pump on my house garage on a cold winter day the first year i had the house. On the weekend, and the contractor couldn't make it for a couple of days. It was cold of course, and since the garage was not as well insulated as the house, the slab temp dropped a lot faster than the house. We ended up losing one of the two loops in the floor. This is on a fully insulated garage with slab insulation and it still didn't make it the 2 days it took to replace the pump. My experience. YMMV.

 

[meathooker]

I'm a big proponent of glycol as well.

You'll still need water treatment if it's plain water.

And I have a spare pump in stock at the house in the event we loose one over the weekend. It's cheap insurance to have on hand.

 

[yeldogt]

In the mid Atlantic, have never had the need for glycol ... but single digits are rare and we don't lose power like many areas. We also don't need any water treatment

I have a used pump at each property -- just in case. At 8 years I swap out my primary and DHW pump. I keep the primary as a spare.

 

[jack stand]

This may be extreme, but I rented a "ground thawing" rig, trailer mounted. It's basically a oil fired boiler with hundreds of feet rubber lines on a spool & a circ pump. A radiant heating system on a trailer that you lay out the lines just like a slab and cover them with concrete blankets or hay. The hay actually works better than the blankets. I was thawing frozen ground (about 24" of frost) but it would heat up your slab prior to introducing your water.
Iirc it was around $5-600/week + fuel (diesel or #2).

 

[Jon_E]

The long-term plan is to keep it running once it's started. I have 8 pumps in my system and I keep two spares in case anything dies on a weekend or holiday.

More than likely I am going to just wait until March or April for a 'warm-up', 3 to 4 days in the 40's or 50's, and then start the system up at that point. Just too much to risk by trying to thaw a frozen slab at this point.

 

[willymakeit]

Why don't you temp heat the space first, then check floor temp. When it's at 35+ start your boiler and slowly raise the floor temp.

 

[Vicious_Cycle]

I used RV anti-freeze in my system and haven't any problems with it yet. It has been in service for more than 10 years. I agree with others who have suggested preheating the slab.

 

[jayoldschool]

You don't run water in a zone that can go below freezing. You run glycol through a heat exchanger.