Hydronic Floor Heat Issues

blckolds

Hey everyone.

Im at my wits end and I need help from experts! I recently had a 40x60 shop built and for heat I decided to go with hydronic floor heat. I ended up designing and installing the system per Loopcad program’s specifications. There are 2 zones, one with 10 loops and other with 2. Loops are roughly 220 feet long and are within about 10-15 feet of one another. They are oxygen barrier 1/2 type A pex pipe.

According to the setup, it called for 80k BTU heater so I decided to 200k Rheem tankless water heater and pre-build radiant water distribution panel from getfloorheat.com (Sorry I couldn’t attach picture of the panel). Panel has 2 circulating pumps (UPS15-58FC, 3-Speed Circulator Pump, 1/25 HP, 115 volt).

I was able to prime the system and purge the air from the manifold. Inlet gauge at the distribution panel is at 18 PSI and return gauge at the bottom shows 22. So now for the problem I’m experiencing, I can’t seem to get zones to circulate higher than 1 Liter per minute. Even if I isolate the manifold with 2 zones and shut off one of the loops, it barely gets to 2.5 Liter per min. I don’t know if that’s sufficient but it doesn’t seem to circulate the water. Inlet temperature is 130-140 and return is 60. Shop just doesn’t heat up at all.

Could there be an airlock somewhere that’s preventing the water from circulating? Or do I need a larger circulator pumps?

Thank you in advance for any help!

Zman

How have you attempted to purge? What type of manifold do you have? Can you isolate the loops one at a time? If you feel the loops with your hands are the temps consistent? Your loop sizes are correct and the circ models seem reasonable.

dopey27177

Try jacking up the water temperature to 160. and make sure that the pumps are at the the highest speed.

Hot water heaters are not the best way to use in any water heating system.

You probably should have used 3/4 pex.

You are pumping about .66 gallons of water per minute, not enough flow to heat your floors.

Jake

Ironman

A a tankless water heater is not a boiler and it’s NOT the right tool for the job. It’s not designed, controlled or approved for space heating. The heat exchanger in it has way too much resistance to flow (head) for hydronic circulators.

 Why did you choose a 200k btu when the load is 80k?

You may be able to get some functionality by piping it p/s with an over-sized circulator on the heater, but it’s gonna have a very shortened life expectancy. I’d get the right appliance now (an 85k btu mod/con boiler) and avoid having to do it all over correctly in the near future.

Think about it: if a tankless water heater could replace a boiler, there’d be no need to make boilers.

blckolds

 Thank you for the feedback. 

GroundUp

Your problem is the tankless water heater. They have far too much pressure drop in the heat exchanger to work in systems like this piped direct. As mentioned above, it's possible that you could get some decent performance temporarily by repiping it to a primary/secondary arrangement with a high head circ on the tankless, but it's still the wrong tool for the job- and 3-4x oversized unless you forgot to insulate the building.

Rich_49

Rheem states the minimum pressure for these heaters is 15 PSI . 15 x 2.31feet head = 34.65' . That is a PROBLEM

HomerJSmith

Would a buffer tank help you?

Ironman

HomerJSmith said:

Would a buffer tank help you?

I’m curious. How would that help compensate for the high resistance of the heat exchanger in the tankless?

delta T

So, just looked at the panels from that site that you listed and they are piped in a primary/secondary arrangement. I agree with all that has been said above, the on demand water heater is the wrong tool for the job. However, before we go too far with this, I want to check something. Is the P/S fitting (bottom right of the panel according to the pictures, looks like two very closely spaced tees) set such that the valve between the tees is open? If not, then open it and check your flow rates. If your flow starts to behave as it should (Loopcad should give you the flow rates required), then we know there is nothing wrong with that side. I suspect it'll be fine. It will certainly not heat anything, as now there will be no flow going through the water heater. It should not even fire.

Now, as has been said before, you will need to either install a real boiler, or in the mean time install another pump that can handle the high head pressure requirements for the water heater on the primary side of the P/S piping. I assume the only pumps you have are on the panel. Opening that valve will hydraulically separate the the system piping from the heat source piping. This is advantageous for a variety of reasons and I would recommend it even on a boiler, especially considering the huge discrepancy in load size between your two zones. If the system is piped in the proper P/S arrangement, that valve should always be open unless you are purging the system. That is its only job.

HomerJSmith

What I was thinking off hand was that the tankless would heat the water in a buffer tank and the buffer tank would feed the manifold. It would provide hydrolic separation and differing flows between the two sys, too.

delta T

@HomerJSmith Given the size of the water heater, that may not be a terrible idea. Would certainly cut down on the short cycling, still need a high head pump to make the heater reasonably efficient. Kinda like lipstick on a pig though

JUGHNE

Bickolds; I have never been near a tankless water heater, therefore wondered why they would not work as a heating unit.
Then Mr. Obvious kicked in......these things need house pressure of 40-70 PSI to push the water thru the small passages. Small heating pump will not move the water fast enough without overheating and/or short cycling.

hot_rod

A hydraulic sep would be an easy fix. High head pump for the tankless, proper sized circ for the tube. A small electric tank piped a a buffer would eliminate some cycling

HomerJSmith

Delta T
@HomerJSmith Given the size of the water heater, that may not be a terrible idea. Would certainly cut down on the short cycling, still need a high head pump to make the heater reasonably efficient. Kinda like lipstick on a pig though. SO!

I kinda think that a hydrolic sep would not prevent short cycling which I would expect.
It would separate the flows, but the heat energy produced by the tankless has to be dissipated to the heat emitters and I think it would be unbalanced, more heat produce than can be used that is why I like the buffer tank idea.

An electric w/h would certainly function as a buffer tank. The only thing is that stratification might be an issue, but that may be inconsequential.

I would have to look up the flow rate and temp output of the tankless to be confident in my suggestion.