water heated slab floor

I am working in a greenhouse that has a concrete slab which is plumbed to cycle hot water throughout the floor to maintain a minimal temperature over the night. Its not currently achieving that goal. The current system is a simple large home hot water heater with the cycling pump on a timer. We were trying 15 minute alternating on/off, then we tried 30 minute cycles, then 15 minutes on/1 hour off, now today we just tried turning on to continually cycle. We will find out if that works.

Does anyone have any experience or ideas about  how to improve this system? Our dream was that we would have a slab of concrete that would get up to 75ish degrees and more or less stay there, helping the  whole greenhouse stay in the 60s throughout the night. Does that even seem plausible?

Truly answering the question requires math and knowing things like the BTU rating of the water heater,  mass of the concrete, type of pipe and its diameter, flow rate of the pump, etc.  

That said, lets try.

Back in the day in Wisconsin I knew someone with a radiant floor in an aircraft hangar.  This was an insulated 40x40 (or so) building, but it would stay in the 50s through the winter.  I don't recall any timers on the system - he just turned it on and allowed the thermostat int he water heater to modulate as needed.

That said ... a quick look doesn't show water heater thermostats that go below 90F.  I'm sure they exist.

Another point ... losing heat to the atmosphere is a given.  Anything you can do to build up a battery of heat in and under the slab the better.  Foam insulation is nasty but if you can make a vertical wall 12-36" it will do wonders.

It may pay to keep a record of what is happening for a while.
Collect temps inside / outside
Details of the heating cycle
measure everything a few times a day consistantly at the same time.
Record weather pattern for the day.

That maybe better than anybcalculations.

If you need to upgrade, full details of everything will be needed to get the Mathmatics going.

Eliot Mason wrote:Truly answering the question requires math and knowing things like the BTU rating of the water heater,  mass of the concrete, type of pipe and its diameter, flow rate of the pump, etc.  

That said, lets try.

Back in the day in Wisconsin I knew someone with a radiant floor in an aircraft hangar.  This was an insulated 40x40 (or so) building, but it would stay in the 50s through the winter.  I don't recall any timers on the system - he just turned it on and allowed the thermostat int he water heater to modulate as needed.

That said ... a quick look doesn't show water heater thermostats that go below 90F.  I'm sure they exist.

Another point ... losing heat to the atmosphere is a given.  Anything you can do to build up a battery of heat in and under the slab the better.  Foam insulation is nasty but if you can make a vertical wall 12-36" it will do wonders.

There is a thin layer of foam insulation under the slab (I wasn't there when it was poured this is just what was reported to me so I don't really know what 'thin' means), I could get the water heater stats but it is just a standard household water heater (maybe 100 gallon) purchased new 2 or 3 years ago. I believe all of the plumbing is 1/2" clear plastic tubing and i believe there is a tube every 8" (half running away from the heater and half returning, not sure on their lay out)

I am not sure that the clear tubing will last long with hot water. That maybe something to check.

John C Daley wrote:I am not sure that the clear tubing will last long with hot water. That maybe something to check.

It was installed by a professional about 3 months ago. I am assuming all the constituent parts are suitable for the use they are being put to and that the problem is in the operation

Our house has a concrete slab foundation with insulated base and sides and embedded hydroponic heating which is heated by our wood fired stove. Concrete slab has 45mm earth floor on top. I don’t know anything about the calculations but I do know that from cold we have the fire on for 2 solid days to bring the slab temp up and then it is maintained with the usual fire each evening. Thermal mass takes aaages to warm up. This may or may not be useful to you 😁

Karen Ward wrote:Our house has a concrete slab foundation with insulated base and sides and embedded hydroponic heating which is heated by our wood fired stove. Concrete slab has 45mm earth floor on top. I don’t know anything about the calculations but I do know that from cold we have the fire on for 2 solid days to bring the slab temp up and then it is maintained with the usual fire each evening. Thermal mass takes aaages to warm up. This may or may not be useful to you 😁

Thank you Karen, that's super helpful. I suspect that is our problem, we have never let the slab heat up fully. Hopefully running the water continually for a couple of days will make a difference

So...:

1) "clear" tubing.  Um, if its truly transparent then that's vinyl hose, not pipe.  I'm sure that's not what you have because that's used for fish tanks and such, not for in floor use.  If its "translucent" white, then its almost certainly pex which is the totally correct product for this application, and is probably 1/2" pipe.

2) Karen Ward makes an excellent point about bringing that mass up to temperature.

3) any sort of insulation or thermal-break under the slab can be helpful.  Adding a strip of insulation around the slab would really help insulate it against the colder dirt

4) And I should have been clear - even if we had all those details I still couldn't solve the problem!  

Let us know how just running it wide open goes.

Eliot Mason wrote:So...:

1) "clear" tubing.  Um, if its truly transparent then that's vinyl hose, not pipe.  I'm sure that's not what you have because that's used for fish tanks and such, not for in floor use.  If its "translucent" white, then its almost certainly pex which is the totally correct product for this application, and is probably 1/2" pipe.

2) Karen Ward makes an excellent point about bringing that mass up to temperature.

3) any sort of insulation or thermal-break under the slab can be helpful.  Adding a strip of insulation around the slab would really help insulate it against the colder dirt

4) And I should have been clear - even if we had all those details I still couldn't solve the problem!  

Let us know how just running it wide open goes.

1) I believe it is 1/2" pex, I have only seen the 9 or so inches that stick up out of the slab to the manifold but I think translucent would be a better description than clear.

At this point I am hoping that Karen has hit the nail on the head. It has been wide open since yesterday afternoon and my boss reset the min/max on the thermostat today, so tomorrow I will be able to see of 24 hours wide open made a difference in the nightly minimum temperature

Well 4 days into continuous cycling of the water heater seems to be solving the problem. The temp of the floor has slowly risen into the 70s and the nighttime temperatures are getting up into the middle 60s.

I think tomorrow I will actually turn the temp control on the water heater down and start to try to dial in the stable temp that way.

The more I think about it the more sensible it is to have the continuous cycling of water. Any idea how continuous use will affect the pump?

Some bronze pumps are designed to run continuosly with hydronic heating systems

Great it seems to be working now. Large amount of thermal mass takes so much time to heat up but by the same token it also takes a long time to cool down again ( which is why it’s brilliant to have as a house floor for winter ) Once it’s up to temp you won’t need to heat it all the time, just a daily maintenance dose. Even the sun shining on it keeps it really warm in my experience. Good luck with it.