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AO Smith HWH - can heating capacity be increased by changing orifice?

This is a AO Smith 50 gallon power vent unit rated at 40K BTUH operating on LP gas. It's in a very northern climate where the incoming water is bone-chillingly cold. So it takes a very long time for tank recovery - just washing a few dishes by hand kicks off a 30 minute cycle.

Seeing that the unit is rated at 46K BTUH on NG, I assumed that the power vent and PVC could handle a slight increase in flue gas temp.

I was wondering if changing the orifice is an acceptable method of increasing heating capacity? I know that efficiency will drop somewhat. The stock orifice is #48 and I was considering drilling out to a #46 - which should take the capacity up to 45K BTUH.

I'd probably buy a new orifice and drill it out, rather than sacrifice one that is working - just in case. It looks like it's a brass hex 3/8" type - is this a standard orifice or will I need an actual AO Smith part ?

Advice welcome!


  • Jamie HallJamie Hall Posts: 10,548Member
    You'd be doing your own experimenting. It's quite normal for the ratings to be different for LP and natural gas, since they are not the same thing at all and are certainly not interchangeable.

    If you have the equipment to measure the air/fuel raito, and the means to change it as required, and the equipment to measure stack gas chemistry and temperature...

    I'm not going to sit here and say "don't change it" -- it's your unit -- but I will sit here and say that it's not just a matter of orifice size -- and if you change that, you own it. AO Smith is not going to help you with it.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • neilcneilc Posts: 661Member
    how old is this?
    does it make any percolation / rumbling noises when firing?
    Have you drained sediments from the tank?

    it was converted from NG to LP, correct?
  • ratioratio Posts: 2,036Member
    Perhaps a series preheat tank? I believe there's been some pics here of a grey-water HX to preheat incoming water as well.
  • weekendguyweekendguy Posts: 181Member
    Thanks for the comments. I have a digital combustion analyzer which I could use but I'm not sure what adjustments I would have to work with other than the gas valve...? This was installed as an LP unit (may have been factory) but the manuals show both options and the parts that vary so I think it's safe to regard it as having been converted from NG to LP. It's 18 years old, is used in a seasonal home and therefore has probably the equivalent of a year or two on the ignition/burner section. It's always been slow so I had not pursued other causes like sediment, although I could look into that. I thought about a series pre-heat but space is really tight. Would a very small preheat tank (20-25g) be sufficient typically? I'd have to find a way to vent it so might even consider an electric unit if you guys think a smaller capacity series unit is worth the effort. I had not thought of using a grey water HX but that's a pretty great and low energy use idea - I'll check into it. General conclusion is to avoid up-firing the burner 10%?
  • Larry WeingartenLarry Weingarten Posts: 1,513Member
    Hello, If you make ANY modifications to a heater, the manufacturer will assume no liability for anything that might go wrong. As it's an older tank and you didn't say you've been replacing the anode periodically, I'd not expect it to live much longer. So, how about a modified preheat arrangement? Can you put together some two inch (or bigger) PVC pipe and run it around high on the walls? Two inch pipe holds about 16 gallons per 100 feet. B)

    Yours, Larry
  • weekendguyweekendguy Posts: 181Member
    @Larry Weingarten - that's good info, thanks. I'm moving away from modifying the HWH while I consider options for preheat. The grey water HX would be logistically difficult but your idea of using ambient heat to warm up the inflow is something I could consider, with or without a storage tank of some kind.
  • ratioratio Posts: 2,036Member
    I'm not sure, but I think a preheat tank that's not substantially the same size as the main tank will gain you only a little, I'm guessing like an extra 5 minutes or so. As soon as the preheat tank is cold, you're back to where you were before.

    What kind of demand do you have? Long showers with a high-flow shower head? Filling a Jacuzzi? What is your incoming water temperature?

  • Larry WeingartenLarry Weingarten Posts: 1,513Member
    Hello, Regarding drain water heat recovery, two basic options exist, vertical and horizontal. Have a look at these: and ;)

    Yours, Larry
  • HVACNUTHVACNUT Posts: 2,266Member
    edited December 2018
    What type of heating system do you have?
    I'll assume an LP warm air furnace since no mention was made to the option of a different DHW source altogether from a boiler.
    As @Larry Weingarten said, its probably on its last legs anyway. Seasonal use or not, age is age.
    I'm not crazy about them but maybe think about a wall hung on demand heater.
    Are we talking well or municipal water?
    What's the incoming water temp?
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,862Member
    Don't know what your hot water load is but that is a good size heater. I would check the stack temperature, combustion efficiency and gas pressure. It may point you to a problem that is other than cold inlet temperature
  • GBartGBart Posts: 753Member

  • mikeg2015mikeg2015 Posts: 806Member
    With the cost of LP. Might as well just add a small electric tank in series if you have a spare 20A circuit. Even a small buffer tank will make a difference.

    Can also store at a higher temp in the buffer tank and gas wh and use a mixing valve.
  • GroundUpGroundUp Posts: 545Member
    I'd turn up the temp 10 degrees and add a direct connect mixing valve if it were mine. Then you'll have the mixer for the new tank when this one inevitably fails sooner than later
  • weekendguyweekendguy Posts: 181Member
    Thank you all for your comments. It's a septic and private well system. I happen to have a Honeywell mixing valve just sitting here in a box so I'm going to start with the suggestion from @GroundUp when I am back on the property in a few months.
  • GroundUpGroundUp Posts: 545Member
    Is it an AMX valve or at least something lead-free for domestic water? Just want you to be aware that leaded fittings are outlawed in domestic systems now
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