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Thread: Hot water heater as a boiler?

  1. #1

    Default Hot water heater as a boiler?

    Hi

    Im new here and not a plumber by any imagination I am a real estate appraiser. I was doing a walk through on a multifamily house that was owned by a plumber. What he did was took your standard 30gallon hot water heaters and turned them into boilers for the radiators in the units (pumps on them and all). I asked him about it and he said that it was a lot cheaper than buying 6 new boilers. Basically he changed some valve, added the pump and some how got a thermostat on them.


    Since then Ive seen this in a few other random buildings.

    Questions
    Is this safe, stupid or ? and very importantly 'How is this done?'

    Thanks

    Matt

  2. #2
    Engineer Furd's Avatar
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    How it's done is quite easy. You remove the drain valve and use that tapping for the return water. The outlet tapping is used for the supply and the inlet tapping is plugged.

    Is it safe? Not in my opinion and not in the opinion of many building officials. Many jurisdictions prohibit using domestic water heaters for space heating systems. Most manufacturers void any warranty for domestic water heaters used for space heating purposes.

  3. #3
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by Furd
    Is it safe? Not in my opinion and not in the opinion of many building officials.
    Why not? What are the issues?

    I ask because I have been planning to add a recirculation line and pump to feed a small heater in the kickboard area of the kitchen cabinets. The heater and pump would be thermostatically controlled, and the heater's blower has a low-limit control so it will not run unless the water is hot.

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    Plumber jimbo's Avatar
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    The issues are that domestic water heaters are not designed for the kind of duty cycle which they would likely see if used for heating. They will yield whatever the spec'd gallons per hour is, but if tasked to deliver that basically 24/7, there is no way that they will last. The burner components and the tank itself will "cook".

    We do not see hot water heat out here, so I am quite uneducated in this area. But a question I would like to hear answered by some pros from back in "cold country" is this: when you put a WH in a closed system like a hot water heating loop, does that tank now require ASME cert, even though it is being operated at only 40,000 BTU, and fairly low pressure???

  5. #5
    Licensed Grump GrumpyPlumber's Avatar
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    30 gal water heater is about 30K btu's hour.
    Low end boiler for small residential application 80-100K.
    The BIG difference: recovery rate.
    32,000 BTU's will heat 30 gallons of water from 40 to 130 degree's in about 45minutes.
    That might possibly work in southern Texas, but not in the north.
    "The biggest regrets we have in life are the chances we never took."

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    Most radiators are spec'ed to produce their stated output with 180-degree water; radiant floor uses a lower temp (around 120 or so depending on load and design). Depending on what you have, you'd never get the comfort level required if you were using it for radiators. The other big thing is the duty factor...a WH is not designed to run continously, which could be required in the cold of winter.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7

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    I disagree to one point, if the water is used as closed system the water heater will last longer,
    this is according to the mfg's,
    MANY WATER HEATER MFG'S NOW MAKE UNITS THAT HAVE SIDE OUTLETS FOR THIS PURPOSE THIS ARE IN THE 50 GALLON ON UP RANGE!!
    these are combo units for potable water and space heating

    MACPLUMB. MASTERPLUMBER AND WATER HEATER SPECILIST

  8. #8
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by MACPLUMB
    ... if the water is used as closed system the water heater will last longer ...
    Does that mean a recirculation line can extend/enhance the life of a typical water heater?

    The thing that comes to mind here is that a water heater in a house system with a recirculation line connected at its drain port might not always cycle as deeply as otherwise.

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    In a potable water system, you are constantly inserting new water, with it's disolved contaminants, minerals, oxygen, etc. In a closed system, you've only got a finite amount, so things reach equilibrium quicker. You end up with water that isn't as reactive, thus it should last longer.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Master Plumber master plumber mark's Avatar
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    [quote=MACPLUMB]

    I disagree to one point, if the water is used as closed system the water heater will last longer,
    this is according to the mfg's,
    MANY WATER HEATER MFG'S NOW MAKE UNITS THAT HAVE SIDE OUTLETS FOR THIS PURPOSE THIS ARE IN THE 50 GALLON ON UP RANGE!!
    these are combo units for potable water and space heating

    I have an outlet in my own home on my 75 gallon gas

    but it is meant for going through a heat exchanger in my
    forced air system on the furnace...


    that 30 gallon heater will burn out and leak or will not keep up with the demand in a bitter frigid temp

  11. #11

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    Hi thanks for the reply

    The heaters were used in a 2500 Sf building. So there were 3 "boilers" total 1 for each unit. Does it get cold here yea im in Philadelphia.

    Were they 30 gallon units, I don't know. I'm sure they no more than 40 mabey 50 gallon.

    So what would you do to set for the thermometer? I'm curious really on the mechanics on the whole thing.

    He did say that they would last quite a long time since they were in a closed loop. (My gas hot water boiler for my house is pushing near 50 years old)

  12. #12
    Licensed Grump GrumpyPlumber's Avatar
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    Water heaters just aren't designed to return enough BTU's that a heating system requires, they're also awfully inefficient compared to a boiler.
    A relatively cheap boiler runs at 80% efficient, a water heater runs at 60%, you'll see it on the gas bill.

    Think of it this way, it's February, 24 degree's outside, the zone kicks on with a full 50 gallon tank maxed @ 160 degree's (boilers run at 180).
    As the zone runs it draws the heat from the tank, the tank will lose it's heat very quickly, depending on size of loop.
    Once that tank has gone to room temp it's just heating with the circ running overtime and you now have a zone thats trying to compete with 24 degree's but takes 45 minutes to recover @ 130, nevermind 160.
    Your tennants might not notice the higher gas bill, but they will notice it's cold.
    "The biggest regrets we have in life are the chances we never took."

  13. #13

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    I came across this discussion by chance and cant help commenting.

    I live on the shore of Lake Ontario and have used a 30 gal 30000 btu water heater for all my heating and domestic needs in an 1100 sq ft home with minimal insulation, for well over 10 years.

    The notions of inefficiency are false if the heater is located in heated space and the vent passes thru the building to the roof.

    The notions of faster "wear out" or excessive condensation are false since the number of on off cycles is much less over the life of the heater and wear from expansion and contraction is thus minimized.

    Scalding is no more of a problem than owning a dishwasher but can easily be solved with an old kitchen faucet as a mixing valve.

    Recovery problems, although present in very fast outside temperature drops, are not what might be expected, since the pump runs continually. The thermostat is simply the delta t formula with a little help by manually adjusting the temperature at the heater.

    Most recovery difficulty would be eliminated by correcting the very wide temperature differential (30 degrees) in the gas valve. If anyone knows of a valve with maybe a 10 degree differential that would fit a standard heater, I would sure appreciate knowing about it

  14. #14
    Computer Programmer Bill Arden's Avatar
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    1. The normal element in a hot water heater will get to hot for the antifreeze solution and this will cause bubbles. This means that you have to replace the elements with a "folded" element.

    2. These bubbles can sit at the top of the tank depending on how the tank is plumbed. (The top should be the exit)

    3. The antifreeze tends to turn to goo if there is a leak. This can cause the T&P valve to stick.

    4. Most people don't put a big enough expansion tank on the system and this causes the T&P valve to leak a tiny amount. (See above for the result)

    They are not allowed here, but I've found that they make small electric wall mount heaters that are designed to be used as boilers.

    So Physics says that they can be used safely, but idiots have used them incorrectly and the results were bad. (See myth busters TV episode)

    I've even thought about starting with a piece of steel pipe and making my own heater since I only need about 5Kw to heat up the concrete floor.
    Important note Ė I donít know man made laws, just laws of physics
    Disclaimer: I'm a big fan of Darwin awards.

  15. #15

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    My system is not closed. Anti freeze is not necessary and air leaves via the domestic hot water taps.

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