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Community and Q&A

Design help for combined DHW and Space Heating

TorBu | Posted in Mechanicals on


I am finally at the point of my remodel where I can get the mechanical design completed and so I’m following up on my earlier post ‘Radiant heat design details – Bosch or Navien combi boiler?’.

To recap, I am trying to put together a heating system that can deliver domestic hot water and heat the radiant heating system put in place, which consists of a heated slab downstairs and joist heat trak upstairs and which was calculated and designed by Uponor.

I originally started out only considering a combination boiler, but as part of the before mentioned post the consensus seemed to have been that a water heater with heat exchanger is a more efficient approach. Sadly though, I need a long vent run, which requires a water heater with power vent. In addition, the county I live in (Marin County) requires Ultra LowNox heaters. That pushes me into the $2,000+ range for a water heater, when I had hoped to stay way below $1,000. (Since I am no longer looking at a combination boiler.)

The heater I have chosen is – primarily because of local stock availability and the additional side loops, – the GP6 50 YTVIT

I am also planning on using a Taco Genie 006-CT as part of the hotwater circulation.

With the boiler, I’d plan on using an external heat exchanger ( 10 PLATE 3/4 MIP SS HEAT EXCHANGE ),
I would use two pumps, one for the water heater loop, and one for the (radiant) heating loop. (Uponor calculated a toal flow of 5.31 for the 7 zones / 16 circuits.)


  • Any particular recommendations for the two pumps?
  • In the previous post, the Taco VT2218 ECM pump was recommended for the heating loop, which at the time was planned to be connected to a combination boiler, so I’m not sure whether that would still be the right choice? (It also appears to have been discontinued by the manufacturer.)
    I was quoted the following two pumps:

    • Water heater loop: Grundfos UPS 15-35SUC 3-Speed Stainless Steel Circulator Pump 115V, 1/15 HP
    • Radiant heating loop: Grundfos Alpha 15-55F FLGD
  • Is there any mechanical design plan or sketch available somewhere for my application or a similar application that I could use as guide and that could help me understand where to place temperatur gauges, valves, mixing valve, aqua stat etc.?
  • Any particular recommendations for the mixing valve?
  • Actually, I came across this pre-built Caleffi HydroMixer that seems to combine the valve and pump, which could help me during assembly.

    • Would that be worth it?
  • Any particular recommendations for an aqua-stat?
  • There is also a Watts LF 009QT RED PRESS BACKFLOW VLV on the quote. Does that make sense for my application?
  • Any other items to point out?

I’d appreciate any comments/feedback on the above!

Thank you,

GBA Prime

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  1. GBA Editor
    Martin Holladay | | #1

    I was hoping that a GBA reader with experience designing hydronic systems would step up to the plate to provide advice, but that hasn't happened yet.

    Two comments:

    1. Some building inspectors don't allow water heaters to be used for hydronic space heating systems. Before you go too far down this path, make sure that your local building department approves of your plan.

    2. One of your questions -- "Is there any mechanical design plan or sketch available somewhere for my application or a similar application that I could use as guide and that could help me understand where to place temperature gauges, valves, mixing valve, aquastat etc.?" -- makes me think that you are trying to design your own hydronic heating system, even though you don't have much experience in that department. If my hunch is correct, I advise you to hire a mechanical engineer or experienced hydronic system contractor to help you.

  2. Expert Member
    Dana Dorsett | | #2

    Most jurisdictions would allow using a hot water heater for space heating, as long as it's also heating the domestic hot water, and the potable water is isolated with a heat exchanger. Some may insist on particular materials and isolation specifications on the heat exchanger.

    A competent hydronic designer could make that pile of parts work for you, but they might re-spec the pumps, and maybe add a few other features based on their analysis. PAYING somebody to deliver a complete system design, including a bill of material, schematics etc would save you a lot on "fixin' & hackin' " money (and time), and you could then bid out the work competitively.

    Hydronic design by web-forum isn't a great way to get it done right, at the first go-around, and if it's screwed up there is no recourse. This is a professional discipline, with 1001 ways that folks like me (who have enough knowledge to be dangerous :-) ) can screw it up.

  3. TorBu | | #3

    I have a complete design for the radiant as well as a complete design for the domestic water, both from Uponor. (Radiant and domestic both will use Uponor's Pex types.)

    All that's left is indeed the proper calculation of pumps, heat exchanger, and the bill of material, schematic etc. I am ok with paying someone to do exactly that, although - call me naive - it appears that thanks to ECM pumps, there isn't much left to calculate.

    Would you be available to help and provide the calculations, design, bill of material, etc.? And how much would you charge for this?

  4. Expert Member
    Dana Dorsett | | #4

    As previously mentioned, I know enough about this to be dangerous, but I'm not a professional hydronic designer. You need somebody with the right tools & experience to take it to the next level. Richard McGrath (who posts here frequently) seems willing to provide that sort of service. Morgan Audetat of Badger Boiler out of Minneapolis does that sort of thing too and has done a number of hot water heater based radiant floor designs for California residents. (You can google the contact info.) I'm sure there are others.

  5. TorBu | | #5

    Got it. I believe Richard may have commented also on my original post. Perhaps he is reading this post and would share his thoughts and/or contact info?

  6. Expert Member
    Dana Dorsett | | #6
  7. Jon_R | | #7

    The Taco X-Pump Block would simply things somewhat.

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