Slab heat not working correctly

bubby8178

I built a 400 square foot greenhouse and installed a 4 loop 1/2” oxygen barrier pex at 125’ lengths. I spaced the loops about 6-8” apart at the bottom of 4” pad. I’m using a 240 volt water heater with 2 4500watt elements. My supply temp is around 100 degrees and return is 80. I’ve messed around with the gpm flow and it doesn’t seem like it’s helping heat. I have a reznor as a backup heat. The walls and ceiling are a poly plastic. It doesn’t hold heat in very well at all! What gpm should I be running? What supply and return temps should I be trying to achieve? Thanks

Ironman

That water heater only produces 15.3k btus. It’s probably not sufficient.

Big Ed_4

20* not bad , it will give you a more even temperature across the beds, laying out the tubing with the same thought in mind is the other valuable in even distribution of heat ...

Dave Carpentier

At the bottom of the 4" slab, sitting on what.. dirt, like a slab on grade ?
Thats gonna rob mucho btu, if so.

bubby8178

Yes on gravel

bubby8178

It’s in the concrete, but at the bottom not at the top. No vapor barrier or insulation board

kcopp

Thats asking a lot.
Your loops are short thats good.
However you have heatloss all over. No insulation under the slab and none in the walls.
What temp are you trying to get to?

bubby8178

Would like to maintain 65-70. Higher would be ideal. My 45,000 btu reznor is propane and super expensive to run. Just not sure what gpm I should run and what supply temp? Is a 20 degree drop ideal? First time with slab heat so looking for a little guidance 

Hot_water_fan

Your issue is unrelated to GPM or Delta T. You’re undersized and electric resistance is expensive. 

Radiant23

Without thermal break in the slab under the tubing you are heating the ground. Sounds like a hassle but if you are 400 square feet you might as well put a good thermal barrier, 4 new loops and pour a new slab on top of the old one. Need that heat up not all over. 

OldHouseNewOwner

I agree with what others said. Just wanted to show some calculations. 

Looks like the formula for BTUs put out by your water heater is:
System Delivered BTU = 500 x GPM x System Water Temperature Drop

@Ironman  said the max BTU of the water heater is 15.3k. I think he meant per element, because 9 kW is around 30k BTUs ( 9,000 * 3.41214)

So your flow rate is:
30,700 / (500 * 20) = 3.1 GPM

Is that about what you would expect? 

I agree with others that your water heater can’t put out enough heat. I’m assuming the set point is not 100 degrees, so it’s already struggling to maintain temp. And a 20 degree drop is pretty good. 

Even with a second water heater added, you’re mostly heating the ground. If you don’t want to start over, you could try setting up a properly spec’d heat pump in the space, and insulate the top of the slab, completely abandoning the slab heat.

https://www.contractingbusiness.com/residential-hvac/article/20867342/how-to-calculate-water-system-btu-measurements
https://www.unitconverters.net/power/watt-to-btu-it-hour.htm

Dave Carpentier

What about laying new pipe on the top and just pouring 2" over it ?
Concrete on concrete would still be fairly strong, wouldnt it ?

Could the original pipes, now 6" down, be used for a rudimentary chiller with a small air handler in the summer ?

bburd

Most dual element electric tank water heaters are wired for non-simultaneous operation. Only the upper or the lower element can operate at any given time, to avoid overloading the circuit. The upper element has priority to reheat the water in the upper part of the tank quickly in case of a heavy draw.

Hot_water_fan

I’d install some baseboard or similar space heaters - cheap and quicker to respond than a bunch of rocks. Lets you avoid a lot of moving parts, leaks, glycol, etc. Keep it simple! 

GroundUp

Turn the water temperature up. If it doesn't get any warmer, you need more BTUs. Perhaps the tank can be hot wired to run both elements simultaneously to double your output, but you're still losing the majority of your heat to the earth below. That should have had a minimum of 2" of foam below the slab to prevent conduction into the gravel. Even if your power is 5 cents per kwh, it's still more expensive to dump half of it into the ground versus buying LP

bubby8178

Update. Heat was maintained at 60 last 24hrs! No propane! Circ pump in plugged in continuously, supply is 110 return is 90 slab is 73, gpm is .4. It was high 30s to low 40s outside. LP reznor usually comes on for 10-15 minutes and comes back on every 15-20 minutes in this weather. So the radiant is working but still needs some tweaking. Thinking a timer for the pump? I have a thermostat and switch board for it but not sure if I timer might be better in this situation? Water heater is set at 130 and supply doesn’t reach that with pump running constant.

Hot_water_fan

Thinking a timer for the pump? I have a thermostat and switch board for it but not sure if I timer might be better in this situation? Water heater is set at 130 and supply doesn’t reach that with pump running constant.

What would the timer change? It’s running nonstop because it’s too small. Even if the heater was larger, you might not have surface area anyway. Greenhouses are bad applications for this type of heating - so propane or electric baseboard/unit heaters are needed to supplement

Jamie Hall

bubby8178 said:

Update. Heat was maintained at 60 last 24hrs! No propane! Circ pump in plugged in continuously, supply is 110 return is 90 slab is 73, gpm is .4. It was high 30s to low 40s outside. LP reznor usually comes on for 10-15 minutes and comes back on every 15-20 minutes in this weather. So the radiant is working but still needs some tweaking. Thinking a timer for the pump? I have a thermostat and switch board for it but not sure if I timer might be better in this situation? Water heater is set at 130 and supply doesn’t reach that with pump running constant.

This is the nature of radiant heating. Keep that pump going -- the cost of the electricity is minimal -- and keep the propane heater working as it is. It's giving the system as much neat as it can absorb. You may find that it will run more often as it gets colder and the system cools off faster.

Hot_water_fan

$/MMbtu electric resistance: $/kwh * 293 
$/MMbtu propane: $/gallon * 11 / efficiency - check this for your situation 

bubby8178

If I increase flow rate, will the water return hotter? My water heater is giving off heat quicker than it can replenish so I need to solve that so it doesn’t stay on constantly. A timer would give the water heater time to catch up I would think? The greater the temperature difference, the more rapid the heat transfer. So 130 water would give off heat quicker so starting and stopping with higher temperatures should heat the concrete similar to 100 degree water running for a longer period of time?

Hot_water_fan

Yes, faster would lower the delta T and raise the return temp. That will not change anything about how long it runs in a condition where you’re undersized. This heater will stay constantly on as long as capacity < heat loss. You can fix this by installing more capacity or decreasing heat loss. 3rd option is adjust expectations - maybe 50F is all you get inside this structure. 

bubby8178

Yes I’m ok with temp in structure being lower. I will supplement with LP. I just need the radiant heat to help with heat to offset cost. It’s around $400-$500 per cold month with LP only. And the concrete will loose heat slower than the air inside.

bubby8178

So basically the fear of overheating my slab past 90 is out the window lol. So now I just need the water heater to not run continuously so I need to regulate my Circ pump somehow. So slab probe won’t work, and thermostat won’t because I’ll have my reznor going on and off. So either a timer or supply sensor?

Hot_water_fan

So now I just need the water heater to not run continuously 

Why? Can you state what your goal is here? Are you savings money with the slab? I haven’t seen evidence that you are yet. 

Jamie Hall

I guess I'm confused. If you want to heat your slab, you need to have the water heater running. If your slab is getting too hot, or the building, yes just use a thermostat to turn off the water heater. Don't expect the slab to respond quickly -- it took a few days to get up to temperature, it will take hours for it to drop even a few degrees. And hours to come back up, if you do let it drop.

I really don't understand what you are trying to achieve here.

hot_rod

The actual surface temperature that you can get the slab to will indicate how many btus you are adding to the space

IF in fact you could get the slab surface to 75F and the air temperature is 60f you are adding 
75-60 x 2= 30 btu/ sq ft

400 X30 = 12,000 btu/hr, so not far off from what a 4500w element could produce

But I suspect the Reznor is the reason the air temperature could be 60f

without the Reznor I doubt the radiant will do much, your load is way more than 30 btu/ ft in a building like that

GroundUp

Every second that pump is not running, is a second that it's not heating the space. The water heater can only put out X amount of BTU in an hour, and the building requires Y amount of BTU in that same hour. If Y is greater than X, the water heater will never stop. Simply put, it's too small.