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Help with supplemental radiant concept

mvoss15mvoss15 Posts: 4Member
I’ve been lurking on the forum for a while, and am hoping to get some advice on a possible radiant system. My goal is to increase the comfort level of our house, yet try to keep things as simple as possible.

The house - we have an old 2-story brick craftsman home built in 1923 in northern Indiana. Most exterior walls have essentially no insulation, and based on my research, short of tearing out the plaster, insulating and adding a proper vapor barrier any sort of blown-in insulation is likely to cause moisture problems, especially with the brick exterior. The attic is well-insulated with blown-in cellulose (R-40+). The 30 large windows have been upgraded to double-pane, Low-E glass, etc. First floor has 9’ ceilings, second floor has 8’ ceilings. I’ve calculated a total heating load of just under 80,000 BTUs (using the tools from I used a design temp of 0 degrees F.

Our home is currently heated with a 95,000BTU 96% efficiency condensing gas furnace. The second floor stays nice and warm, but after 2 heating seasons of playing with the ductwork dampers, the first floor (especially the living room) just never feels warm - especially to my wife!

I’ve been reading for a while about under floor “staple up” hydronic radiant, and am wondering if this may be a solution for adding additional heat (at least making the floors feel warmer) on the first floor of our house. The house sits on an unfinished basement, with complete access to the joist bays. 3/4 diagonal plank subfloor with oak hardwood flooring above.

We already have a Rinnai 160,000BTU condensing high efficiency tankless water heater - while I recognize that a tankless water heater is not as recommended as a mod-con boiler, it’s installed and operating (and I'm not concerned about voiding the warranty). I’m wondering about the possibility of using the water heater, along with a Taco X-Pump Block (plate heat exchanger) to create a closed-system supplemental radiant zone under the first floor. I’d run 1/2” Pex with aluminum transfer plates in each joist bay.

Some details:
- Total area where the radiant would be installed is about 700 square feet. Estimating about 3 300’ loops of 1/2" Pex assuming 2 pipes per joist bay.
- If possible I'd like to include a small fan forced hanging heater in the basement, but this isn’t absolutely necessary. This may require a second zone which likely will drive the cost of the project higher than I'd want. I am trying to keep my overall investment as minimal as possible. We live in a neighborhood where I’ll never get this back out of the house - but a happy and warmer wife is an important too!
- I would create a loop through the Rinnai and the X-Block heat exchanger, coming off this loop for our DHW with a mixing valve down to 120 on the hot water. Rinnai would likely be set to 130-140 degrees, and would target 120 degrees on the radiant side?
- If and when the Rinnai bites the dust, I’d upgrade to a proper boiler.

- Is this even a workable solution? If so, I'll work on drawing out the system for feedback and advice.
- Without breaking the bank, what other options might be better? If any?

Thanks so much for any help!



  • RetrosPexRetrosPex Posts: 56Member
    Matt: I've owned a lot of Victorians, and a couple Arts and Crafts homes. May I make a quick observation you might want to look into before you tackle the radiant loops?

    Almost all my buildings were balloon framed. Buildings like that can get tremendous convection currents going in the walls. The cold air drops in the outer walls, and speeds up as it drops, pushing warmer air up inside the house. And sucking in even more cold air through various leak paths. Even if your attic is well insulated, have you checked to make sure the tops of the wall cavities are sealed off? In our projects we would sometimes be able to reach straight down into the cavities, where we would roll up some insulation and tightly stuff it into the cavity, and then seal the top several inches of it with 2# closed cell spray foam. If the cavity had a plate sealing it off, we would plug any holes, cracks etc, and then spray all along the outside perimeter with the spray foam. After that was done, we would go the basement, and seal it up in much the same way. You just have to be careful near stone foundations and the like to not spray foam in a manner that would allow termites and other bugs to crawl in and eat away without your knowledge. The attic portion is by far the more important area to address.

    Sorry for the long winded explanation. As for your question, I think that doing staple up is a great idea. I don't know if the Taco X Block has provisions for things like expansion, etc. You might want to look for a boiler integration panel, the likes of Hydro Smart, or that ilk.

    You may not need it to warm up your first floor if the first issue is addressed. And check into energy rebates from your local utility. Ours has programs for this type of situation.

    Good luck

  • mvoss15mvoss15 Posts: 4Member
    Thanks for the response! Our house is not balloon framed - the prior owner seems to have done a fairly decent job sealing off the basement, though I just bought some incense sticks today to use to try and identify any gaps allowing air to infiltrate. Prior owner is the one who insulated the attic - and unfortunately with all the insulation that's been blown into the attic there's really no practical way of getting around and checking to see whether any penetrations through the top plates of walls were sealed well before he insulated - there's close to 20" of cellulose in the attic. That said, given the limited number of electrical outlets on the second floor, any penetrations for wiring would be minimal.

    Reading the XPB documentation, it calls for an expansion tank on the radiant side of the system, and possibly on the DHW side if I am interpreting it correctly.
  • mvoss15mvoss15 Posts: 4Member
    I finally found a Youtube video of a similar idea. I'm thinking something like this, however, with staple-up (and transfer plates) rather than radiators.

  • RetrosPexRetrosPex Posts: 56Member
    Matt: Having 20" of insulation up there certainly does makes it harder to check. Although, with all that insulation, it should be helping to keep cold air from dropping down inside the walls. Sealing up an cracks, holes etc on the outside will help too.

    I remember that Taco system now. I was really interested in it many years ago, for one of our projects. You might also want to look at the Caleffi fixed point mixing stations. They integrate a pump, with a mixing valve and manifold for a very reasonable price. Supply House and others carry them. The manifolds can have numerous zones.
  • FriendlyFredFriendlyFred Posts: 27Member

    The Taco X-block is sure a neat new product. If it were me, and unless there's a specific reasons you're interested in using it, there are less expensive alternatives with other benefits.

    Two Taco 008's, a set of 'Freedom Flanges' and a 125k BTU heat exchanger will run you less than half of the cost of your XPB. That cost drops even further if you size a circ to your Rinnai that doesn't need the ΔT capability.

    Building a system with individual components, you have lowered the 'eggs in one basket' risk of a failure.

    As for your 'hydro coil' air idea, that's fine and dandy too, however, keep in mind even a small hydro coil and moderate CFM can suck up a ton of BTU, which could potentially cause DHW flow limiting issues if left uncontrolled.

    Give yourself options and buy a manifold with one or two more loops than you need currently.

    For scoping out your attic, buy or rent a thermal camera and go scope out the attic while it's very cold. Heat loss that's worth treating will show up on the surface of the blown in cellulose. It's also amusing to dig into the cellulose and see the temperature gradient on the camera. Get some rigid foam, cut to fit and expanding foam-in-place.

    For your loops, consider going with Pex-AL to minimize startup/shutdown expansion noises.

    For insulation under tubing, foam-in-place rigid foam boards and follow up with mineral wool for fire protection + sound dampening + insulation.

    Finally, the 'Modern Hydronic Heating' textbook will be your friend. You may be able to borrow from the library, or even find a copy online if you look hard enough.

  • mvoss15mvoss15 Posts: 4Member
    Thanks for the input! I was just realizing that the "hydro-coil" units all seem to be a minimum of 50K BTU - probably not necessary for the basement. What I liked about the Taco system, was the control unit that integrated the supply and outdoor temperature sensors. I've not had much luck locating a standalone unit with this capability - any recommendations of where to look?
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