Tank less water heater in low flow house

Discussion in 'Tankless Water Heater Forum' started by George M., Dec 8, 2018.

  1. George M.

    George M. New Member

    Dec 8, 2018
    Boston, MA

    We are thinking of replacing our current natural gas tank water heater by a high efficiency tank less heater.

    However we have a water main issue, the house is from 1930 and the main is partially clogged. We had people from the town water service and a local contractor who does water main replacement come to our house and long story short we have no short term solution to solve the main clogging issue. Anyway, because of that we have good water pressure but low flow. Meaning as long as we only one faucet opened water pressure is great but it drops as soon as we open a second one.

    I read several posts online about people complaining of pressure issues with tank less heaters but it seems to usually be because of undersized units for their house. In our case we have only one bathroom and are already use to not using 2 faucets at the same time so I don't think this would be an issue. We are looking at a 9 GPM rated unit.

    Now my question (sorry for the long introduction): Not considering the sizing question, do people observe a water pressure drop when installing a tank-less unit compared to a similar flow rated tank unit? I'm not sure how a tank unit works. Does it refill at the same time as you are using the hot water or is the refilling process asynchronous? I'm afraid that the tank less unit would actually behave like if we had 2 faucets opened at the same time...sorry if the question is stupid or doesn't make sense.

    I will obviously also ask his opinion to our plumber but I like to get different opinions.

  2. Reach4

    Reach4 Well-Known Member

    Sep 25, 2013
    Yes, that one.
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    The whole thing with a tankless system is that it has literally, no tank...it makes hot water as you call for it. The cold comes in, goes past a heat exchanger, and comes out warmer. How warm depends on a few things, biggest is probably the volume you are trying to pass, second is how much you are asking it to raise the temperature. In Boston, after a cold spell, your incoming water could be close to freezing, making it a bigger task to make hot water.

    Most of them can adjust the burner to accommodate your request, up to its capacity. FWIW, the volumes listed on those things is assuming your incoming water is (typically) 50-degrees, lower it around Boston (I've measured 33 in Nashua, just north of Boston), and it will either output less so it can dwell longer in the heat exchanger (if it has a flow restrictor), or not achieve as hot an output. Think passing your hand through a flame. That's the water. To get things hot, it either uses a huge flame or it has to slow down. You may or may not like the end result. There's also a slight delay after you turn the hot water tap on for the flame to come up and then to purge the ambient temp water out of the lines. And, there's a minimum flow rate required to turn it on, so trying to get warm may or may not be possible except at higher flows. The other side of the coin is that it doesn't have standby losses. They require significant gas supply, so you will need to probably run a new, larger gas line to it, and that assumes your existing meter and system can handle the increased demand. Be prepared for annual servicing, or see a degradation in performance. Probably at least a couple of hours service charge unless you're willing and capable of doing it yourself. This is to remove the mineral deposits in the heat exchanger...think about deposits in your tea kettle...but worse since you're heating LOTS more water. That has to be cleaned out (circulating acid through there to dissolve it).
  5. DIY Lemon

    DIY Lemon New Member

    Apr 24, 2015
    I went from a tanked water heater to a tankless. I did not notice any pressure drop. I did notice the flow rate drop on the hot water side though.
  6. jacobsond

    jacobsond DIY Junior Member

    Apr 30, 2010
    Fairmount, North Dakota
    If you got the room put in a tank.With your issues why mess with something that may or may not work to your satisfaction. We already know a tank works just fine.
  7. Dana

    Dana In the trades

    Jan 14, 2009
    There is no advantage to going tankless here. Your flow problem is bad enough that you probably won't see a noticeable difference with a tankless, but there is always at least some pressure drop across the tankless with flow.

    At Boston's ~50F (annual average) incoming water temperatures you'll never see 9gpm out of any residential tankless at a tub-filling temperature of ~110F. At a 60F rise 9gpm would be 270,000 BTU/hr. The biggest residential tankless out there is "only" about 195,000 BTU/hr of output which would be good for about 5.6 gpm at 110F out, 50F in. But that's all moot if the most you can get out of your water mains is 3 gpm, eh?

    One possible fly in the ointment is that tankless water heaters have a minimum pressure at which they will operate properly. If another tap opens when the water heater is firing and the pressure drops below that minimum the controls in many tankless water heaters would register that as an operational fault and shut down, spitting out an error code. This is probably the biggest problem with installing at tankless before the water mains issues are fixed.

    If you're heating the house with a hydronic boiler (pumped hot water) it may be more efficient and make more sense overall to heat water with an "indirect" fired water heater running as a zone off the boiler. The installed cost would probably come in a bit lower too- the bigger burner tankless units usually require upgrading the gas plumbing inside the house with a dedicated 1-1/4" gas line to the tankless (no branches or tees) back to the regulator/meter, and sometimes requires upgrading the gas meter too.
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