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external h/x vs combi waterheater?

layman33layman33 Member Posts: 4
My 16 year old radiant heat system consisting of basement in-slab wirsbo is powered by a Lochinvar 75 gal power vented combi style water heater. Last week the 30 psi relief valve began leaking and I isolated the problem to the internal h/x failing. From what I've read on this site the replacement combi style w/h are not to reliable. Since the w/h function of my Lochinvar still operates how feasible is it to replumb the system with an external H/X. That would probably include adding another circulating pump and a check valve? When the Lochinvar finally dies I would replace it another 75 gal PV W/H for a third of the price of a combi. Am I pipe dreaming? layman33


  • kcoppkcopp Member Posts: 3,513
    Personally I wouldn't waste my money of the external HX. I would get a reliable contractor to work w/ you on a design that fits your home.
    Get all you can out of what you have and make the switch to a condensing boiler that will work well w/ your slab before next heating season...
    When the time comes they add an indirect water heater off the new boiler.
    That's my 2 cents.
  • It certainly is feasible. Depending on how large the heat load is, I'd use a Taco X-Pump Block that has the heat exchanger and two pumps ready to go.

    Often wrong, never in doubt.

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  • layman33layman33 Member Posts: 4
    I should add that the system is only used 9-10 weeks in the fall and another 4-5 weeks in the spring. I shut it down and go south for the winter. Also, there is a 55 gal. whirlpool tub that occasionally gets filled.
  • hot_rodhot_rod Member Posts: 13,496
    I didn't know Lochinvar made a Combi tank style? Bradford White has the Combi 2 which has a dependable steel coil inside, same coil as the indirects I believe.

    At 16 years old out may not pay to add an external HX to that tank. But it is fairly easy to add a pump and plate HX on a tank. A flow switch on the supply triggers the pump.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • layman33layman33 Member Posts: 4
    I understand that higher priced items make more $ for contractors, but for the customer (like me) is the Bradford/White Combi2 worth 2X the $ than a W/H with an external H/X. Or, is a Taco X-pump worth 3X the $ than separate H/X and circ. pump?
  • hot_rodhot_rod Member Posts: 13,496
    I suppose it depends on the experience and comfort level of the plumber. There used to be some plug and play modules that made it easy. Commercial sized ones are still available.
    You have the radiant circ already, so you need a plate HX and a stainless circulator, some fittings. Easy to find inexpensive plate HX online. When the tank gives out, move the external HX to the new tank.

    Remember if it touched the potable water it needs to be low lead, not sure that Taco HX block is?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GroundUpGroundUp Member Posts: 991
    With that amount of usage, if you are handy I'd go with the external HX and just swap it to the new tank when that day arrives. Swapping to a high efficiency boiler as we'd like to see, you'd never see a payback. For just a few Benjamins you could set up the external HX with a SS or bronze circ and be done with it. I've done quite a few systems like this in vacation homes and homes with small additions where they wanted radiant, etc. It's not super efficient, but it gets the job done much more cost effectively than the alternative
  • layman33layman33 Member Posts: 4
    Thank you both for the suggestions. Fortunately the system won't be needed until late September.
  • GroundUpGroundUp Member Posts: 991

    Personally I wouldn't waste my money of the external HX.

    Even though it's 900% less money than a new unit which might also fail in short order?
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