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On demand water heater in hydronic slab?

Mark_25 Member Posts: 67
I'm looking at a Toyotomi OM-148 for a hydronic application. I didn't find any info on their website about hydronic applications. It's an oil fired water heater with a 5.6 gallon capacity, 148,000 BTU, I'm heating a pole barn workshop with Manual-J losses of 35,000 BTU/hr at design temp. Blue board under slab, R10, R19 walls, R38 ceiling, etc. I need to keep it cheaper than a $4K boiler, so I'm looking at water heaters. Design inputs to the floor are 90F water, with 70F return. 250Ft loops, 1/2" on 12" centers at outside walls, small area in center of shop has 18" spacing. Water heater will heat 6 gal/min at 40F rise. Does anyone have any experience in on demand heaters in slab heating apps? I know I could throw a gas water heater in there, but I would rather do oil, and the Toyotomi looked attractive. Could I use a line level thermostat and just switch the pump on and off? Not elegant, and I'd want to put in a Honeywell AQ675 later, but I am getting cold and just want heat pretty quick for what I can afford!


  • bigugh_4
    bigugh_4 Member Posts: 406

    is the culprit here 90* out water temp will be too cool. the manufacture must have a say about this condition. Most low mass heaters dislike constant low water temp outputs in that they condensate far too much at that end of the curve.
  • Mark_25
    Mark_25 Member Posts: 67

    So what I am hearing is that 90F is too cold, but that's the range the floor needs. Looks like I might have to bite the bullet for a injection valve and the AQ675. Would a high mass system work better? Say a 32 gallon BOCK oil fired?
  • Dale
    Dale Member Posts: 1,317

    Got involved with a Bock discussion on one of their water heaters for swimming pool heater use and the factory person said no problem just to put in a little fixed bypass to minimize condensation. If you call the Bock distributor they draw it out for you.
  • Mark_25
    Mark_25 Member Posts: 67

    I guess this discussion is moot now, as I just scored a Weil-McLain oil boiler 5 years old for $500. Thanks for the ideas.
This discussion has been closed.