Radiant floor heating/domestic hot water check valve needed

[2002sheds]

Hi,
I have an "open system" that supplies hot water for both radiant floor heating and domestic hot water. Our needs for the building have changed, and there is now a small kitchen and bathroom that receive hot water. Since the system is open (no check valves to prevent water from coming from the radiant tubing and back into the system after the heater), I would get slugs of cold water along with heated water at the 2 faucets and from the shower.
I have isolated the issue and confirmed that there is cross over via the radiant tubing (the structure has 2 stories so gravity is feeding at least some of the water from the tubing back into the line, post-heater) by shutting off the supply and return valves to the radiant tubing. Once I did this, hot water arrived quickly and consistently to the fixtures. We are in a temperate climate and it is summer anyway, so there is a little bit of time to solve this without having to turn the radiant system back on right away.
The hot water line has two short runs (like a capital F), so that the top short line in the F supplies hot water to the radiant tubing, while the other provides domestic hot water. My idea is to simply insert a check valve between the two, so water from the radiant tubing cannot back into the domestic side.
Is this stupid? Correct? Other?


Thanks,
David

 

[Dana]

I have an "open system" that supplies hot water for both radiant floor heating and domestic hot water.

Is this stupid?

Yes- "stupid" is about right.

The amount of time the heating system water spends stagnating at pathogen-growth temperatures is large. This type of system is expressly banned by code in many states, and in some other states (such as mine) there is a minimum duty cycle requirement that has to be met even when it's 100F outside (not just when you need the radiant heat to be working) to run potable water in the heating system.

It's more expensive than a check valve, but the right solution is to install a potable-compatible plate type heat exchanger and a second circulator pump to run on the heating system side of the heat exchanger whenever the potable side circulator is running. The heating system side will need it's own expansion tank too.

This schematic shows a tank type water heater as the heating source, but with minor adjustments it works with tankless systems too:

 

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[Jadziedzic]

Hey Dana, from a long-time (mostly) lurker who enjoys reading the "why" behind how things are done - thank you! Your explanation was really informative!

 

[2002sheds]

Hi Dana,
Thanks for the response. The system was installed by a licensed pro that has set up many of these, so it must be allowed here. In fact, here is a line from the
2013 Building Energy Efficiency Standards - Reference Ace
"The other type of hydronic heating uses a water heater as a heat source. The water heater provides domestic hot water as usual." The use of one appliance for both functions is mentioned in a few more places, so it must be allowed. In our case, if there is a minimum amount of cycling to prevent stagnation we would easily surpass it, since the building is and has been in constant use. Obviously, there are no chemicals added to the hydronic system for freeze prevention, etc.

Still, I understand what you are saying, and I will investigate some more to be sure of the code here in California, and also how to move towards the setup that you describe. Another option that I have been looking into is to separate the two systems entirely by installing a domestic water heater closer to the new bath and kitchen, allowing the tankless heater to serve the radiant only.

In the meantime, my original question remains: will a check valve help with the issue I described above?



Thanks,
David

Yes- "stupid" is about right.

The amount of time the heating system water spends stagnating at pathogen-growth temperatures is large. This type of system is expressly banned by code in many states, and in some other states (such as mine) there is a minimum duty cycle requirement that has to be met even when it's 100F outside (not just when you need the radiant heat to be working) to run potable water in the heating system.

It's more expensive than a check valve, but the right solution is to install a potable-compatible plate type heat exchanger and a second circulator pump to run on the heating system side of the heat exchanger whenever the potable side circulator is running. The heating system side will need it's own expansion tank too.

This schematic shows a tank type water heater as the heating source, but with minor adjustments it works with tankless systems too:

 

[Dana]

I'd need a better schematic of the plumbing, pumping and valving schemes than I can draw in my head from the description in the original post to know if a check valve would solve the immediate problem.

There are many variations of using a water heater as a combi-appliance for space heating and such systems are widely allowed (indeed I'm not aware of any US states that expressly disallow it), but nothing about it requires an open system running potable water in the fan coil or radiation.

Just because the building is in constant use doesn't guarantee that fresh water will consistently purge all branches of a pumped hydronic heating system, though in simpler systems it's possible to design it such that it does. Shutting off the supply and return valves to the radiant guarantees stagnation, and it means the system is plumbed in such a way that purging of the radiation water from normal water use is only incidental, and of unknown volume when the valves are left open.

 

[2002sheds]

Hi Dana,
I will try to upload a few photos, but the simplest version I can describe is 3 loops per floor, 2 floors.

I think I know what you mean by "the system is plumbed in such a way that purging of the radiation water from normal water use is only incidental, and of unknown volume when the valves are left open." Hopefully, the photos confirm or deny that. What is a better way to ensure that the water is purged correctly?

(Above) the left side of the plumbing for the hydronic and domestic water.

(Above) The right hand side of the plumbing for the hydronic and domestic water. Not seen is a Taco control box for the upper floor part of the system.

(Above) The outside plumbing. It is not super easy to see, but the hot water for the radiant system is the furthest pipe that goes through the wall on the lower "F" looking assembly, and the cold is the furthest out on the upper "F". The closer part of the "F" on each level is for hot or cold domestic water. There is also a tee that has been soldered in for domestic water to a small bathroom that was added later.

I totally agree about not leaving the hydronic system shut off for any length of time, if possible. I hope to figure out this riddle soon!


Thanks,
David

I'd need a better schematic of the plumbing, pumping and valving schemes than I can draw in my head from the description in the original post to know if a check valve would solve the immediate problem.

There are many variations of using a water heater as a combi-appliance for space heating and such systems are widely allowed (indeed I'm not aware of any US states that expressly disallow it), but nothing about it requires an open system running potable water in the fan coil or radiation.

Just because the building is in constant use doesn't guarantee that fresh water will consistently purge all branches of a pumped hydronic heating system, though in simpler systems it's possible to design it such that it does. Shutting off the supply and return valves to the radiant guarantees stagnation, and it means the system is plumbed in such a way that purging of the radiation water from normal water use is only incidental, and of unknown volume when the valves are left open.

 

[2002sheds]

Hi Dana,
The short-term plan is to install a check valve on the inlet side of the hydronic system. There is not a lot of room in the current plumbing setup, so I may be capping the point past the domestic hot water inlet and re-routing the last few inches to the pipe leading to the hydronic system.
If you have suggestions as to how to better purge the water in the pex loops, I am definitely interested. I also want to thank you again for your advice so far. It is likely to be the way that I will proceed in the near future.



Thanks,
David

Hi Dana,
I will try to upload a few photos, but the simplest version I can describe is 3 loops per floor, 2 floors.

I think I know what you mean by "the system is plumbed in such a way that purging of the radiation water from normal water use is only incidental, and of unknown volume when the valves are left open." Hopefully, the photos confirm or deny that. What is a better way to ensure that the water is purged correctly?

View attachment 54765
(Above) the left side of the plumbing for the hydronic and domestic water.

View attachment 54766
(Above) The right hand side of the plumbing for the hydronic and domestic water. Not seen is a Taco control box for the upper floor part of the system.

View attachment 54767
(Above) The outside plumbing. It is not super easy to see, but the hot water for the radiant system is the furthest pipe that goes through the wall on the lower "F" looking assembly, and the cold is the furthest out on the upper "F". The closer part of the "F" on each level is for hot or cold domestic water. There is also a tee that has been soldered in for domestic water to a small bathroom that was added later.

I totally agree about not leaving the hydronic system shut off for any length of time, if possible. I hope to figure out this riddle soon!


Thanks,
David