Open radiant heat system not circulating through water heater

GemGem

I'm currently renovating a school bus into a tiny home. We've done all of the work ourselves (which at times we've been annoyed about, with good humour)

This also applies to all our plumbing (working perfectly!) and our under floor radiant heat, which to date is my only 'can't figure it out' issue.

Because of space we went with a open system using a tankless hot water heater. So the water flows through our underfloor pipes then out through domestic hot water should a faucet be opened. In theory that is.

I filled the system successfully and hot water is making it's way into the 2 runs of pipe, the pump is running and it sounds as if water is running through the system.

However the tankless WH is not firing except to initially send hot water into the pipes. The thermostat on the intake is reading hot, but on the out pipe is cold to touch with the thermostat reading room temp.

I'm concerned maybe an airlock is preventing the pump from working properly so it is not circulating the water.

With an open system I should just be able to open a faucet somewhere in the system to allow the air to escape, however this does not seem to be working.

When I open a faucet it is not pulling water from the radiant heat system but from the regular water supply. When I close off the water supply then open a faucet, it drains my radiant heat system and I can see and hear the air bubbles escaping before the water flows through.

So obviously I have made a mistake somewhere here and I'm hoping someone can help me isolate the issue please!
I'm feeling a bit stumped as I researched and worked on my plan and layout for months. I also checked it with a radiant heating company and a person who had also done their own open radiant system (it actually is almost identical to their plan which works) So I really feel like it should be working

Any feedback or assistance is REALLY appreciated. It's getting colder and my wee wood stove is not quite keeping up through the night.

Alan (California Radiant) Forbes

On-demand water heaters have a high pressure drop through their heat exchangers which means you need a bigger than normal pump to get decent flow. What is the make and model of the on-demand and pump?

Jamie Hall

I will start right up front: I don't like, and can't recommend open combined radiant heating with domestic hot water. Just too much chance for contamination. OK. I've said that.

Now. How, exactly, is this thing piped? Can you provide a drawing of exactly how the pipes and pump and heater and valves are laid out? We'll help you if we can, but we need that.

GemGem

I understand many people have concerns about open systems, but as we won't be consuming our hot water, tankless gas fired WHs are the best way of killing bacteria and our temp is set over 130F, I do not hold these same concerns. We will also be emptying the system in the summer.
I really do appreciate the feedback and help though!

I have been looking into head pressure today actually wondering if that is the issue.

I have a Rinnai V65i WH and the pump is a Grundfos UPS15-35SFC

have included a drawing of my plan below, its a rough one I knocked up now because I CANNOT find my notebook, so lemme know if you need anymore info or anything.

Thanks gang!

GemGem

oops! upload didn't work - here's the plan!

Ironman

I seriously doubt that small 15-35 can overcome the high head of the tankless to induce enough flow. Add the high Cv of the mixing valve and it gets even worse.

We see this scenario just about every week on here. The only solution short of getting rid of the tankless is to pipe it primary/secondary with a large circ on the tankless side.

GemGem

I'd be inclined to agree with you about the head pressure, however when I was filling the system and opening a faucet in the system the air did not purge from my RH then either.
I could only get the water to come out of the radiant heat when i cut the water supply to other fixtures SO I am worried the copper manifold split I have done is also an issue....basically there is a circulation issue even without the pump running.

The check valve is placed there to prevent cold water from entering the RH system where it T's into the cold water in to the WH. The water can only run towards the WH. Is this an issue?

GemGem

Thanks

OK so bigger pump added as primary and can the small one still be a secondary do you think or is it pointless?
When you say circ on the tankless side you mean before the mixing valve? Where do you think best placement is in regards to my plan?

GemGem

Youngplumber said:

Sorry Young Plumber I gotta be honest here and say I do not know what I'm looking at

Ironman

Look up p/s piping. It requires two circulators.

Jamie Hall

As I look at your sketch, I'm tryin very hard to make sense of it. This is what I see:

Starting at the lower left corner, there is a pipe coming in which I am going to assume is the cold water feed from your water system. This then comes up to a T where it is jointed by a line from the radiant loops, which we will come back to. It then goes on up to the tankless heater. The water comes out of the tankless heater, and I sincerely hope that there is an error here, as you show it going through the pressure relief valve. If it really does go through the pressure relief valve, honestly there's no point in my trying to help further. That is so elemntarily wrong.

However, let's suppose that that's an error, and the pressure relief valve is properly piped and that pipe is actually coming out of the hot outlet from the Rinnai. Then there is a section of pipe with, presumably, the takeoffs for the domestic hot water. Then we go along and come to a mixing valve. I won't ask why that valve is there... but it is. It isn't labeled, but presumably the pipe we've been following comes in the hot inlet to the valve, and there is a connection to the cold water supply, as labelled,, to the cold inlet. At least, that's how it should be piped -- if it isn't... I can't think what it's doing. Then from the mixed outlet of the mixing valve we go to a pump with an expansion tank on the way, and then to the radiant floor loops and a check valve and back to the cold water supply.

If the above description is correct, yes you will be able to get domestic hot water out of it. If the radiant system were a closed loop and you got rid of the mixing valve and got a big enough pump (if the one @Youngplumber posted is yours, it's has nowhere near a high enough pressure capacity) the radiant floor would work. But the way that is piped? No, and I'm not even sure where to begin to make changes...

hot_rod

I'm not a fan of open/ combined systems either.

I suspect when the heater fires and the hot port will see say 120F or whatever the mixing valve is set for, the hot port will close down or off, so you really don't have a flow path that I can see.
How does the radiant return get to the mix valve to modulate the temperature?

A simple piping to try would be this, see if the pump can provide enough flow to trigger the heater without flowing any DHW.

Im built a 20' tiny and used a small electric boiler, 2000W element. The load in a well insulated tiny is, well tiny.

Jon_blaney

Check the Journal of Light Construction site, plumbing. Article on closed loop install.