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Hydronic radiant heat using a single water heater

I recently purchased a house (3 stories, standalone, townhome-style) that has a ~10 year old hydronic radiant heat system, but the system seems a bit peculiar.

From the bit of research I've done, my understanding is that these systems usually need a separate boiler to heat the water in a closed system, but mine uses the same tankless hot water heater that is used for the rest of the home's hot water. As far as I can tell, this has resulted in an overly-complex piping and valve system to ensure that the radiant heat water doesn't flow back into the normal tap. The radiant water is piped to wall-mounted radiators on each floor, except for the first floor, where it seems to be piped through the floor? (Although there are cold patches, so I'm not even sure)

I've had a few problems with this system:
  • Whenever the house has been heating, I've had a harder time getting hot water from faucets, etc, presumably because it's competing with the radiant heating
  • There's a "bubbling" sound that comes from the wall-mounted radiators whenever the system is on, and it's been getting louder over the last few months (I started noticing it after we had a plumber come and do some work to try to fix the hot water tap problem)
I have a few questions:
  • Could the "bubbling" be indicative of a leak? Should I try to get the original plumber to make sure they didn't cause it? Or how else should I address this? (Incidentally, the house also has a leaky roof and bad siding, but that's the exterior - the rest of its plumbing has been fine)
  • In general, what are my options to improve the situation overall? Should I look into getting a boiler for the radiant heat system, to separate it from the water heater? Alternatively, the house doesn't have any kind of A/C, so it's likely that I'll soon purchase something like a few ductless mini-split systems, which can also do heating. Should I consider just getting rid of the hydronic system altogether after I get the mini-splits? If so, what would that involve?
Thank you in advance for any suggestions.

Here's a video of the bubbling sound:
And here are some images of the system:


  • Jamie Hall
    Jamie Hall Member Posts: 19,414
    Ah... can you verify one thing for me ASAP? Find the temperature and pressure relief valve -- I think I see it in the last two pictures there, with a silver lever handle and cover -- and determine where its outlet goes? The picture makes it look as though it's piped to something else, which is an absolute no-no.

    Now. That out of the way. The bubbling sound is probably trapped air in those radiators, and I'm not a bit surprised that there would be some. In your setup, it's going to be somewhere between very difficult to impossible to get rid of and keep it gone.

    I do not care for systems -- however carefully piped -- which mix the domestic hot water with the heating system water. If you do keep the hot water heat, you need to have a separate on-demand water heater, such as you have, for the domestic hot water, and a separate small (but properly sized) boiler for the heat. If both units are sized properly, you will have adequate if not ample hot water, and the heat will be safely separated and also what you want.

    Heat pumps and mini-splits. Well, it depends on where you are located. If your outdoor temperatures rarely if ever get below 10 or so above zero (Fahrenheit), they can do a very good job. Below that, they either don't heat -- or tend to use a great deal of electricity.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • voithos
    voithos Member Posts: 3
    Thanks for your response!

    Regarding the relief valve, if I'm looking at the correct thing, it seems like it's piped down into an outlet that goes into the wall, but is also connected to a second pipe that comes from what looks to me like a different relief valve? (Pictures attached) The entire outlet piping is cold to the touch, which makes me think that there's nothing flowing through it.

    Regarding avoiding mixed domestic/heating water systems -- that makes sense. I assume that would also help with the bubbling, if properly set up?

    Regarding mini-splits, yes, I'm in the Seattle area, so the cold is quite mild here.
  • Jamie Hall
    Jamie Hall Member Posts: 19,414
    Well... back in the day when I was a building inspector (boo hiss) I wouldn't have passed that relief valve setup. You really truly do want to see if the relief valve opens -- and, in your case, you really truly do want to see which one it was. They both should be piped -- separately -- to a safe drain location. Your authority having jurisdiction may be of a different opinion, but that's mine...

    Seattle? You should be able to do very well with heat pumps.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Zman
    Zman Member Posts: 7,375
    The noise sounds like air in the system.
    Honestly, unless there is a place in the building with adaquate space for a true boiler setup, I would consider abandoning and going with the splits.
    Do you know what the note about the exhaust buildup is referring to? Did they write it up in the paperwork?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jolly Bodger
    Jolly Bodger Member Posts: 209
    This type of system is used all the time. It does not mix the heating and domestic water. There is a braised plate exchanger at the floor on the left. This system probably requires a service from a knowledgeable Pro. He will find the best way to purge the air from the hydronic side, make sure all the expansion tanks are good and charged correctly, check to make sure the pumps are working correctly, check the DHW mixing valve, and DHW priority.
    Yes, it is a complex system. But they can work well. Another option before abandoning the Hydronic would be to replace the Rinnai (if it is having problems) with a Combi boiler. This puts the DHW and Hydronic separation and Priority in the boiler. Simplifying the system.
  • voithos
    voithos Member Posts: 3
    Thanks for the replies, everyone!

    @Zman, yeah, that little closet is basically the only place I'd be able to put a boiler - not sure if there's room (I'm assuming the piping would be a tad simpler with the boiler setup?).

    Regarding the bubbling noise, besides it being annoying, should I be worried about it otherwise? Can it cause damage to the system?

    @Jolly Bodger, interesting, I didn't even know that Combi boilers existed, but that makes sense. Something to consider.

    My current thought is that I'll hold off on any changes to the system until I decide on whether or not I'll get some mini-splits (since I'll be wanting A/C anyway), at which point it doesn't seem to make much sense keeping the hydronic system if the mini-splits can take care of the heating anyway.