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replacing natural gas fired hot water heater and boiler simultaneously

mnevans Member Posts: 5
Dear Wall forumites,

I am new here and inexpert in everything, so I'll apologize in advance if I am in the wrong place or am asking the wrong questions. Randy from and The Tank sent me here after consultation with him.

My 31+ year old, natural gas-fired, atmospheric/masonry-chimney-vented hot water heater suddenly failed, and needs replacement.

The basic questions are: With what to replace it? And: should I consider also replacing my 31+ year old natural gas fired boiler at the same time?

Here is the landscape: 1935 house, purchased in 2008, Maryland/DC area. Water heater and boiler (for radiator heat) are from 1989: AO Smith water heater, Burnham Series 2 boiler with three zones. Both live in a utility room in basement, vent into masonry chimney, but are within 15' of exterior wall; utility room currently stays warm in winter, as does the rest of the basement (65+F) without basement heat on. All three radiator zones are on programmable thermostats. Our water is hard: scale develops in the dishwater, must be cleaned seasonally to annually. Code requires lining the chimney at installation of a new efficient gas water heater or boiler.

I have a good local HVAC/plumber I trust to do the work. They service the boiler annually for about a decade now, no problems yet found with it. However, natural gas phaseout could be coming (it is here for new construction), and I would like to reduce our use of fossil fuels, if I can.

Domestic hot water load: 3-4 people, 2.5 baths, 1 dishwasher, 1 washing machine, 5-15 min showers, small tub rarely used. About 1-2 mins for hot water to reach baths. Old water heater size was 50 gal; this was sufficient with as many as 7 people living here. Currently we use about 12 therms per month of gas for hot water, based on nonheating months' data. Gas price: currently $0.55/therm. Electricity about $0.13/kWh with taxes and we use currently about 600kWh/month without A/C; up to 1000kWh/month with A/C. I am likely to sell this house in the next 5-10y once the kids leave for college.

Boiler load: up to 180 therms per month of gas (neglecting the gas range demand). It's usually running between Nov-Apr, otherwise idle.

My criteria for choosing equipment are: (1) energy efficiency/low carbon (2) dependability/ease of maintenance (3) operating cost (4) initial cost.

Based on my research so far, the leading choices seem to be (cost estimates pending with plumber):

1. gas direct or power vent storage hot water heater: uses existing gas infrastructure, requires new venting but not lining the chimney (est: $2000); relatively inexpensive to install and operate; small utility rebate; more maintenance with power direct venting required.

2. electric storage hybrid heat pump: requires new electrical circuit (capacity likely OK for this at electrical panel); no new venting needed; $500 rebate + $300 tax credit available brings down initial cost; probably more expensive to operate than gas, but at our low usage, not terribly so; as yet unclear whether cost is substantially different from option (1). Recommendation is larger capacity, maybe up to 80gal.

These solutions solve the immediate need for domestic hot water. But if I consider replacing the boiler before it fails, while doing the domestic hot water:

(3) Is there a combination domestic hot water and radiator heating system that would be better than options (1) or (2), in terms of carbon, dependability, efficiency, cost, retrofitting? I see there are natural gas powered combi-boilers now; and I also understand there are electric boilers if I want to go fully electric (I would expect that would need a heavy up at the electric panel). If I've avoided lining the chimney for the hot water heater, I would not want to then have do this when the boiler goes.

So: is there a good case to be made for (3)? If so, what options might make sense?

Thank you for your experience, opinions and ideas.



  • Ironman
    Ironman Member Posts: 6,592
    edited June 2020
    Look at using a modulating condensing boiler (mod/con) with an indirect water heat like the HTP UFT + a SuperStore indirect.

    The boiler will heat the house and your domestic via the indirect. It can vent through the sidewall using PVC or Polypropylene pipe. This is a better setup than a combi boiler, especially if you have hard water. It will have more initial cost, but a better value over its life span.

    I wouldn't even consider an electric boiler or water heater with the low cost of natural gas. Electric would cost three times as much to operate.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    EBEBRATT-Ed Member Posts: 10,356
    Forget about electric heat or hot water and scrap the heat pump water heater more problems than they are worth.

    If it was mine I would replace the water heater and run the boiler till it dies.
  • Jamie Hall
    Jamie Hall Member Posts: 17,134
    And a word on carbon. A good, properly setup mod/con boiler, such as @Ironman suggested, will have an operating efficiency approaching 95% much of the time, and never less than 85%. That's in terms of useful heat which gets into your hot water or house vs. total heat in the fuel you burn.

    Now... consider electricity. In most parts of the US -- and your area is no exception -- a sizable chunk of the electricity you use (like 80%+ or more) is produced by a carbon fuel burning powerplant -- natural gas, increasingly, but also oil or coal. These systems, by the time the power gets to your house, have an efficiency on the order of 40% in terms of electricity produced vs. fuel burned.

    It takes no mathematics at all to deduce that you will produce less carbon. Like about half as much. If you heat your water and house directly with the mod/con than if you resort to resistance electricity.

    Now a heat pump system is considerably better, and in terms of carbon production you would be close to the same, whether you burn the fuel at home or burn the fuel at a power plant.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mnevans
    mnevans Member Posts: 5
    Thanks very much everyone for their advice!

    @Ironman, thanks, this was also suggested by Randy at Waterheaterrescue - I am looking into it. Makes good sense. IIUC, the modulation would allow the boiler to only provide firing to give me hot water during the non heating months, and use more capacity for the radiator heating during the winter?

    @EBEBRATT-Ed , I see your point, but the problem is the venting. That is into the chimney, requiring relining, is an expensive proposition; or else direct or power vent, which should be possible. Also, I'd rather not drill two sets of direct vent intakes and exhausts (one for water heater and one for the boiler eventually). This is one reason I am considering replacing the boiler before it dies with a unified system like @Ironman suggested. If the boiler died in December, it would get cold and miserable in the house really quick.

    @Jamie Hall , I see your point. We currently buy 100% wind electricity, but that of course really is just offsetting local coal and NG, as the wind power is offshore Chesapeake and in PA. There are still the transmission losses, so you have a good point. But then I think NG prices are artificially low: Fracking is unprofitable at recent NG prices; and there are the externalities of climate change and groundwater pollution, localized effects on human health of fracking activity. Wind and solar keep getting cheaper and more efficient but are also forecast to get more expensive over time. My town is also banning NG in new construction. Who knows, they might do this for new installations at some point.

    @Ironman, I also calculated a similar 3x operating cost increase to go electric, but this would send me the right signal about energy which is to conserve and insulate more. That is offset by efficiency gains of the new system and possible the mod-con with indirect heating. For the hot water, this is trivial because we don't use much and the indirect or heat pump is UEF 3.4 vs UEF 0.66 for NG direct vent. But for radiator heating, this is likely $2k additional operating cost per year.

    I need to price the install options and also find out if there are NG or gas mod-con incentives. So far I only found incentives for heat pump water heaters, and small ones for AFUE 95% boilers. Any pointers where to find that info - much appreciated.

    Again - Thanks very much everyone for their advice!
  • Ironman
    Ironman Member Posts: 6,592
    The boiler would go into Warm Weather Shut Down when the outdoor temp is about 70* or above. It would heat the indirect regardless of outdoor temp. It would actually use full modulation until it neared the set point of the indirect to provide as quick recovery as possible for domestic.

    The modulation is mainly for space heating. That, coupled with Out Door Reset provides higher efficiency and comfort for heating the house.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • hot_rod
    hot_rod Member Posts: 15,058
    here is the site to search for incentives. Combi boilers are becoming very popular, a boiler with a heat exchanger that performs like a tankless water heater.

    A heatload calculation should be your first step, determine how much heat the home requires.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • mnevans
    mnevans Member Posts: 5
    @hot_rod, thanks! This is really helpful!!
  • mnevans
    mnevans Member Posts: 5
    Following up - now the two plumbing/HVAC people I know in the area have said the same thing as @EBEBRATT-Ed: just replace the hot water heater with something similar, line the chimney, and keep running the boiler. This is tempting because (1) these people know a lot more than I ever will; (2) the efficiency difference is perhaps large, but the cost of operation differences might be $50/year (, but the possibility for equipment failure or expensive maintenance is larger. (3) boiler at 30 years is still apparently fine; (4) I've been without domestic hot water for about a month and cold showers and hand dishwashing is getting old.

  • mnevans
    mnevans Member Posts: 5
    And in the end I went with a basic atmospheric venting gas storage water heater. As it happened, the chimney was already lined - so the job was done the next day. Thanks again to everyone for their advice!