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Community and Q&A

Can my hydronic radiant heat system be saved?

skierpage2 | Posted in Mechanicals on

Hi! I’ve read a lot of the posts and essays here and they’ve convinced me that hydronic radiant floor heating is not the best way to go. But what if you already have a radiant floor heating system?

In 2006 as part of a major remodel of a two-story 2,300 square foot house in the San Francisco Bay Area (climate zone 3), I spent a lot to go low carbon. To summarize what the subcontractor came up with: Solamax solar thermal collector tubes on the roof would heat three domestic hot water tanks and also heat the storage tank for a two-manifold multi-zone hydronic radiant floor heating. As backup for cloudy days, a Unico Unichiller RC air-water heat pump would heat our domestic hot water and provide radiant heating. Twelve pumps, two flat plate heat exchangers, lots of Tekmar and Taco controllers, endless pipes and wires.

It never worked. At best, in the middle of summer, we could enjoy endless scalding hot baths for free. Come winter the solar thermal would struggle to provide hot water, let alone space heating. The solar thermal loop broke and I got an estimate of $3,800 to fix it; clearly a waste of money (much better to add more solar PV panels). Instead we’ve used the electric strip heater in the last DHW tank for hot water; there are only two of us so not too bad.

The heat pump never worked to provide domestic hot water, but it was able to supply 125°F water to the radiant heating loop and for a few winters we enjoyed comfortable zone heating and warm floors. Hooray! Unfortunately the contractor set the radiant heating system to call for a boiler temperature of 130°F, so the first winter the heat pump ran 24 hours a day (cue Scotty of Star Trek voice “Cap’n, she’s doing all she can, she simply cannae produce any moor heat!”). Maybe that’s why after three years the heat pump required $1000 repairs every winter until I stopped throwing money away. We’ve made do with portable space heaters the last few winters, which isn’t comfortable.
So, it was an poorly engineered over-complex one-off system, but it was zero carbon (we have some solar PV and pay for renewable grid electricity). My problem is what to install to replace it. I’m about to order a Mitsubishi ductless mini-split to heat and cool our living room. If it works well I could extend this with another external unit and four more indoor units to cover most of the house. I’d still have to use space heaters in the two bathrooms.

But I had a working radiant floor heating system! I just need a carbon-free source of heat for it. I’m surprised that this is so hard and options seem so few in the USA. Daikin Altherma is not available in the USA. Sanden sort-of permits you to use its CO2 water heater for space heating, but I’m not sure the existing outside piping to my broken heat pump is rated for potable water. Remember, I’m in temperate California, temperature only drops to 50s on winter days and rarely to freezing overnight.

Although I don’t think it’s a great expense to run a strip heater in our DHW tank, I wonder if I could/should combine DHW and space heating. It doesn’t seem rocket science. The air-water heat pump sends back hot water. Have it flow into a big DHW tank, either directly if it’s potable, or circulating through a coil in the DHW tank. Then have a radiant heating coil in the tank for space heating. I think you can buy such combined space and water heat pump systems in Europe, but in the USA HVAC installers are from Mars and plumbers are from Venus.

Another wrinkle is our modest need for space cooling. Every few years it’s hot enough to want to cool our house. The Unico heat pump could be switched to produced chilled water. I’m aware of the cautions about condensation and mold, but just to run some tepid water through the floor seems like a no-brainer; however, the Tekmar/Taco controls are all set up for radiant heating, not cooling. Does anyone make radiant heating system controllers that understand cooling as well as heating? Or is this a hopeless endeavor?

Thanks for any suggestions! I would be happy to pay a consultant to figure this out, are there any good ones in Northern California?

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  1. GBA Editor
    Martin Holladay | | #1

    I'm sorry to hear that you fell in with a bunch of thieves and got mugged. Welcome to GBA.

    It sounds like you spent tens of thousands of dollars on inappropriate hardware that never worked properly.

    If any GBA readers wonder which articles might have saved Skier from his fate, here is a list:

    "Solar Thermal Is Really, Really Dead"

    "All About Radiant Floors"

    "Air-to-Water Heat Pumps"

    I can't think of any reason why you would want to salvage your radiant-floor heat distribution system, but if you do, here are your alternatives: 1. Install a natural gas boiler, or 2. Install an electric-resistance boiler and balance out the high energy use with more PV.

    What would I do if I were you? Install a few ductless minisplits and call it a lesson learned.

    I hope you will become an active member of the GBA community so that you can share your experience with other homeowners who may be tempted down the path you took.

  2. Expert Member
    Akos | | #2


    Radiant heat properly done is a great thing, I would not give up on it so easily.

    What I would do is find somebody that know radiant and remove all the junk, controls, pumps and tanks of the previous system. For a two level house, the only thing you should have is two thermostats and either two pumps or a pump and two zone valves. My two cents is that domestic hot-water should be on its own. If you don't like restive heat get a heat pump water heater when your existing tank dies.

    If you want to keep the radiant for comfort, the best way to go is as Martin suggested, add in an electric boiler.

    If you are worried about electricity usage, you can get a larger mini split for your main floor and use it as main heat source with the hydronic making up the remainder to even out the temperature in the house.

    Best of luck.

    1. Trevor_Lambert | | #3

      DHW and in floor heating can be combined, but it requires careful planning and consideration. You have to size the water heater such that the space heating load is a small percentage of the overall capacity. I agree that in most cases there's not much point. Otherwise you're likely to get the occasional luke warm shower or bath.

      I think there's some merit to a hybrid system, where you have two water heaters. But instead of just one for space heating and the other for DHW, you have them in series. The space heating taps in between the two heaters. So all of the water from heater 2 is available for DHW, plus the water coming in to heater 2 has been pre-heated by heater 1. This results in either a massive increase in DHW capacity (when the space heating has not operated recently), or a drastic reduction in recovery time for the DHW (when the space heating has been running).

  3. Jon_R | | #4

    If you still want to use radiant, you should investigate the Chiltrix heat pump.

    1. JesseTrinque | | #5

      I had seriously contemplated the Chiltrix. looks like an awesome unit. I have radiant and though I wanted to experiment with the chilltrix, my HVAC contractor talked me out of due to unfamiliarity and alot of the reasons Skier mentioned. At least if I die tomorrow my wife will be able to call someone and have them service the boiler, I wasn't convinced of that on the heat pump. For 'similar' money I ended up going with a top-of-the-line highly efficient Viessmann Vitodens 200 boiler. That boiler is incredible and the advanced controls make the system very simple and elegant. The only problem is they haven't figured out how to make propane from panels on your roof yet! We love the radiant so far, My parents have fujitsu heat pumps and though they are efficient and cost effective, there is no doubt our system is far more comfortable and even, most will say that radiant floors are unfit in an efficient envelope but i disagree. Overkill - maybe, but I'm heating my house with 80-85degree water and that 70 deg return water is condensing like crazy, hoping to make it all year on our 400 gal of propane!

  4. AlexPoi | | #6

    Nordic (Maritime Geothermal, a canadian company) sells an air to water heat pump. You could try contacting them to see if a replacement is possible and worth it.

  5. Expert Member
    Dana Dorsett | | #7

    >"I’m about to order a Mitsubishi ductless mini-split to heat and cool our living room. If it works well I could extend this with another external unit and four more indoor units to cover most of the house. "

    Be very careful about sizing the indoor units proportionally to the load when going with a multi-split. Unlike single zone mini-splits on multi-zone system the heads don't really modulate, and when any one zone is active there is refrigerant going to the others, even when "off", with no blowers active. On rooms with very loads & ludicrously oversized head capacity just the excess refrigerant can overheat/overcool the room when a more appropriately sized head is serving another zone on the same compressor. The better solution is often to go with a single zone (and thus modulating) mini-duct cassette serving 2-4 rooms, tweaking the room to room temperature balance with the duct design and balancing vanes.

    For any heating/cooling solutions it's important to do room by room, zone by zone load calculations, even for a single-zone mini-split. The modulation range is not infinite, with some mini-splits it's only 3:1, and if it's not able to modulate with load it won't deliver the comfort & efficiency that it's capable of when right-sized for the load. A little bit of oversizing is fine, assuming a reasonable modulation range, but hold the line at 1.5x (not more) if you can.

    1. Jon_R | | #8

      > it's only 3:1

      Considering the indoor unit CFM needed for AC comfort (ie latent removal, important in most areas), some are more like 1.3:1.

      1. Expert Member
        Dana Dorsett | | #9

        In the SF Bay area latent loads are usually negative.

    2. skierpage2 | | #10

      Thanks so much for your comment. The local Mitsubishi Diamond Contractor proposed a MLZ-KP18NA ceiling cassette because of appearance and easier installation; for some reason it *requires* a multi-zone heat pump, so MXZ-3C24NA.

      My living room is 380 sq feet but we usually open sliding doors so it opens onto 480 sq feet of dining/kitchen/office with cathedral ceilings. Unfortunately I can't find the Title 24 energy report from my remodel. My impression is the ceiling unit's 18,000 Btu/hr cooling and 21,600 heating is way overkill for the living room alone but not wrong for the entire space. I could delay the minisplit while I try to find someone independent to do the load calculations (and explain "latent loads" to me)... it seems none of you experts in cyberspace are in Northern California to take my money (!).

      1. Expert Member
        Dana Dorsett | | #12

        I agree that for the amount of space and location 1.5 tons of cooling and 21,600 BTU/hr of heating is overkill. Overkill is even worse when using a multi-split compressor, since the cassettes won't modulate. The installer isn't interested in quoting the carpentry necessary to install something more approporiate.

        They're bigger and arguably uglier, but KA series ceiling cassettes have decent modulation range, and can be sized appropriately. But since they don't fit between 16" o.c. joist framing it means it takes some carpentry to install them. The KA series can all throttle back to ~3K @ +47F:

        Run at least a freebie online Manual J (eg: or ) with aggressive assumptions on air tightness & R values (otherwise they'll oversize by more than 1.5x) to get a handle on which makes the most sense.

  6. motoguy128 | | #11

    Just one thought. I wonder if the Unico could be piped to the solar system directly. that way it's efficiency would be much higher since in heating more it makes cold water that the collectors could push more heat into. I think that chiller can operate up to 130F, so could make DHW OK.

    The chiller will be 2-6x more efficient than strip heat.

    I think it can work. It probably lacks storage to match the real world capacity and peak demand.

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