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Hydronic Boiler/Combi & Indirect

kwoolfsm | Posted in General Questions on

HI Folks, continuing my theme of asking questions about HVAC improvements. here’s another. New build home, with roughed in hydronic in the basement slab and garage slab. No other emitters currently planned, although possible with a 4-season room next year.  I really want heat in the garage, nat-gas heater is posing to be a problem as venting is garbage in all directions, with slopes, property lines, front doors etc. And the cost is >$4000 CAD from all qoute.  So Trying to look at hydronic, as it’s roughted in, and seems to be a solid option.  We have 4 quotes, one of which was a full air-source to water heat pump install. It was well beyond $40k, so that is paused for the future.  The next  are focused hydronic installs. 

1 – First was using existing hot water tank, and and plate exchanger to heat the hydronic, by far the cheapest, but we’re a family of 5, so I’ve asked this contractor to look at a combi-boiler as I won’t want to decimate my domestic hot water. \
 – Can the combi be used to pre-heat the existing hot water tank and also effectively perform hydronic heating as well in this config?
2- Second contractor has simply started with combi and proposed using it to pre-heat the domestic watertank. Same as above.
3 – Last contractor proposed the boiler with an indirect tank. 

Cost estimate were from $8200- $14000, boiler proposal besides brand were all 150btu units.  

None of them seemed to propose a heat load estimate, as I provided a lot of info, but they did not reference any “calculations”.  

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  1. paul_wiedefeld | | #1

    It’s near impossible to get a quality heat loss from a contractor - they’re not interested and they’re not paid for it. If you have an indirect or a preheated tank, then 150kbtu is extremely, embarrassingly oversized for nearly all houses but if it’s a 10:1 turndown modulating boiler, it’s not the end of the world. 80,000 btu would be better though! You’ll have push them to size correctly though.

  2. Expert Member
    Akos | | #2

    If you are only heating a basement and garage slab, I would stick to a water tank with a plate HX. A decent power vented tank for space heat is about 80% efficient, spending $$$ on a modcon for the couple of extra percent better efficiency means an ROI of pretty much never.

    Even the smallest gas burner is around 40000BTU, so taking 10000BTU or so of heat off it won't effect your recovery much, plus you can lock out the floor heat during the morning rush if need. You can also always get a larger tank or one with a bigger burner for a piece of mind. Something with a 75000BTU burner that will easily heat your whole house.

  3. kwoolfsm | | #3

    Paul & Akos,
    Our current water heater is 62k btu- AO smith -pdv50-hin300-50gal. One contractor has come back with a lochinvar 80btu modulating 10:1 turndown with and indirect tank. Another proposal was 100k btu Bradford White Combi with 10:1 ratio. Pricing is within $1500cad of each other. The Combi proposal suggested using it to preheat my existing AO smith tank. The proposal with the indirect suggested remove the AO SMITH tank entirely.

    1. Expert Member
      Akos | | #4

      62k burner is plenty for a house. Even if you are heating the whole house off that tank, worst case for a reasonable sized house is using about 1/2 the burner capacity. You heating a basement and a garage should not be noticeable. Heating cold concrete up can take a fair bit, you can always adjust the flowrate through the plate HX to keep the heat load reasonable if you are doing night time setbacks in either place.

      If you are worried about hot water running out, add a 40 gal 120V plug in tank in series after the existing water tank. Set the thermostat on the resistance tank a bit bellow your gas tank. This way the only time the tank turns on is to make up for standby losses when you are not using hot water. Way cheaper than a combi or modcon+indirect.

      Modcon+indirect sounds like it would be efficient to heat water but in reality it is about the same as a power vented tank.

      To get efficiency out of a modcon, you need low return water temperatures. When you are heating reasonable amounts of hot water (ie shower) or running it to make up for standby losses, the return water temperature is pretty high for most of the burn. This means the unit won't be operating in condensing mode so about the same combustion efficiency as a power vented tank.

      So you are spending some big bucks to get about the same efficiency as you have with slightly better recovery. This is only really needed if you are doing something like snow melt which is a lot of load for a long time.

      If you want a lot of efficient hot water, a tankless unit is the way to go but again, not sure if the couple extra percent efficiency is worth the install cost unless your current tank needs to be replaced.

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