Open loop radiant not enough flow to trigger my tankless HW heater

RangerDanger

I have radiant floor heating in 2 bathrooms in an open loop system fed by a HW tank heater.

The tank failed and I replaced it with a tankless. There was no check valve on the radiant loop and the installer didn't have one on his truck, so he left the valves to the radiant loop closed and told me how to install one. I installed a Sharkbite check valve on the hot water feed to the radiant loop (it was much easier to access than the return), opened the valves, and turned up the thermostat. The tankless shows that there's flow, but insufficient (<0.4gpm) to start heating.

The sharkbite check valve is a spring valve with 2 psi drop @ 0.5gpm. Is it worth trying a swing valve with a lower drop?

The circulating pump is a Taco 008-SF6, spec'ed up to 16 ft pump head. I followed an online calculator and figured I need at least 9ft @ 0.5gpm to overcome the pressure drop in ~200' pex and the check valve. I can't find a spec for pressure drop in the HW heater (Noritz EZ111).

Any suggestions?

mattmia2

does this also supply dhw or could you have used an actual boiler for this?

EBEBRATT-Ed

Tankless hot water heater have a much higher pressure drop than a tank type heater. If the system heated with the tank type heater and dose not heat well now and that is all that has changed that could be the issue. You Taco pump may have a check valve in it.

Mad Dog_2

Can you better explain what you mean by "open loop?" Mad Dog

hot_rod

Not too many plumbers here are fond of combined DHW/ radiant systems.

The installation manual does show piping a combined system with an air handler

It indicates 10' at 2 gpm for recirc, and shows a pressure drop chart for the HX.3 gpm max. for AH.

The Taco 008 should be enough pump, depending on you loops pressure drop. Is it a stainless or bronze pump?

If it is piped and pumped properly, you could have a plugged strainer somewhere?

Kaos

As Hot_rod said, likely you have a clogged strainer. There is a small one on the inlet port of the tankless.

That check valve is also too high of a pressure drop, you want something with lower cracking pressure. You can try a swing check (make sure in the right orientation as not all work on a horizontal pipe) but I've had issues with them clanging when turning on taps. A low cracking pressure spring check is a better option.

Taknless units usually have a min fire of around 10000BTU, that means you want to heat at least 300sqft to avoid short cycling. If you less than that, you might need a buffer tank. Also make sure to program the pump to run at least 15m even in the summer to avoid the water in the loop stagnating.

Also not all tankless units like very hot return water, generally if the unit doesn't allow for recirc, it won't work.

hot_rod

I asked a Rinnai trainer about feeding a tankless warm pre-heated water. Early tankless didn't always perform with solar pre-heated water. Which is why Caleffi developed the diverting valve setup years ago specifically foir the tankless manufacturers requests.

Rinnai has changed the control algorithms as explained by this Rinnai trainer.

RangerDanger

Thanks for all the thoughtful replies!

The system also supplies DHW.

The Taco pump is stainless and does not have an integrated check valve.

I used the Sharkbite check valve because it was an easy install and pretty much the only thing I saw with push on fittings. I can't find specs for other check valves. Can anyone recommend a part #?

Meanwhile, I'll check the strainer, though flow seems fine at faucets.

hot_rod

It is nice to have a serviceable check, dirt particles can hold them open.

And it needs to be an approved low lead with domestic water. These have a low "pop" pressure.

You can get crimp or expansion pex connections.

Larry Weingarten

Hi, A little sideways thinking for you. Plumbers generally don't like to mix DHW and space heating as it can encourage bugs like legionella to grow and get into the tap water. Next point is the one Hot Rod made about tankless needing cooler supply water to be able to fire. Next is about the flow restriction through tankless heaters. Putting all of this together, I'd do a heat loss calculation for the bathrooms and get an electric tank-type heater to cover that load. Plumb it separately from the domestic hot water, so you have two systems. Now, you'll have decent flow, no bacterial concerns, and will be able to heat the bathrooms… probably without ever needing to change that check valve.

Another question or perspective is, what will you do if replacing the check valve with a low cracking pressure valve doesn't fix the problem? 🤔

Yours, Larry

EdTheHeaterMan

Another discussion about using a water heater to do the job of a boiler. I wonder if anyone ever gets them to work?

I have installed a tank type water heater for this application. I would never use a tankless for this application. It is the wrong piece of equipment for the job.

Can you sell the tankless and put the tank type back in? Or better yet, use a separate tank like @Larry Weingarten suggested. It is only 2 small bathrooms compared to using a boiler to heat the whole house. So the small electric tank is the best idea for you IMHO.

RangerDanger

I cleaned the strainer at the tankless input and it seems to be working OK now. There wasn't much to clean, so I think it's right on the threshold of working.

I'll revisit the swing valve if the flow rate drops too low again.

Ironman

A simple question: is the check valve even necessary?

If all you have is a small radiant loop with nothing else connected (excluding domestic), then there’s no need for the check valve.

I agree with Larry that you should put the radiant separately on a small tank heater.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1849742#Comment_1849742

Is the check valve to prevent pulling water through the AH when the pump is off and you are running DHW?