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Community and Q&A

Electric Tankless Water Heater without a pressure tank

stephenr | Posted in GBA Pro Help on


I am forgoing the traditional septic and well, due to the super high cost on my tricky building site, and putting in a “limited” grey water system instead. Here in Maine,  code allows for a small grey water system constructed of Elgins.  I will have a self contained central composting toilet and will be pulling water from a spring-fed pond on my property and into a 1000 gallon holding tank in the conditioned first floor of my 1000 square foot house.  The CEO says that I can’t have a pressure tank (no pressurized water without a septic) so I am DIYing a way to get hot water to my fixtures.  

The plan is to lift the water from the holding tank into a 100 gallon sub-tank suspended over the bathroom and kitchen.  I have gotten permission to use a pump to increase the psi from the sub-tank enough to operate a tankless electric water heater.  I am working out the pump requirement and heater specs to get the correct pump and heater and have a pretty good sense of filtration, etc.

I will use low flow fixtures with aerators and figure i will need  a heater that can handle about 5-6 gpm (tub/shower, kitchen sink, bathroom, washer, hose spigot).  Several starting point questions arise with this plan. 

1. Is it even possible or advisable to use a pump in this fashion?
2. I was hoping to use a whole house heater and a single pump leading away from the sub-tank..  Does it make more sense to use multiple pumps and multiple point of use heaters in this situation?
3. Will building the pressure on my system in this way stress out the fittings?  If so, is pex not an option?    

Thanks. Stephen

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  1. Expert Member
    Michael Maines | | #1

    I don't have an answer for you, but in case it's helpful, here are the rules for Maine septic systems. I have had several clients want graywater systems and I would like one myself but for reasons like the ones you list, we always end up with conventional systems. Except for one house, but I'm not sure what they ended up doing for mechanicals.

    For what it's worth, my in-laws live off-grid in China, ME and for a few decades had a gravity system, pumping water to a holding tank in the attic, which fed a gas-fired tank-style water heater on the second floor and provided hot water to the kitchen on the first floor and the bathroom in the basement. It worked, but they later added on and modernized the mechanicals to serve additional bathrooms with more water pressure.

  2. Expert Member
    BILL WICHERS | | #2

    I don't think you're likely to have a problem with the pump in this application. You could use a pressure switch (or overpressure relief valve) to ensure the pressure doesn't ever exceed allowable limits for your system. The pipe and fittings don't really care how you build pressure, they only care that you don't exceed their maximum rated pressure. A pressure switch to cut the pump out, or a relief valve, can both take care of that for you. A possible downside to a pressure switch is that it could cause rapid cycing of the pump, and that is a problem that will damage the pump.

    The pressure tank in regards to the hot water heater just provides somewhere for the water to go when it expands as it's heated. If you are building a free-flowing system, where the water is ALWAYS moving out through a fixture whenever the heater will be running, then you shouldn't have a problem with expanding hot water building excessive pressure in your system. If you use tank-type hot water heaters, then expansion can be a problem. You could try using a very small (1-2 gallon) pressure tank, which are usually actually called "expansion tanks", and see if that would satsify your local building people while still providing pressure relief for the hot water system. If that doesn't work, a pressure relief valve will bleed off water over a pressure setpoint, which is a somewhat messier way to accomplish the same basic function.

    Since you'll be drawing water from a pond, I would use a particulate filter and a UV sterilizer (in that order) to treat the incoming water. There will always be little microscopic beasties living in surface water, and the UV sterilizer will kill those.


  3. stephenr | | #3

    Thanks Michael. This is a tricky one. The attachment was useful. It states that fixtures would be limited to 3 in a limited system. So, if I can work out the pump and heater thing, I might have to choose between having a washing machine and a bathroom sink! Nevertheless, I appreciate the challenge and have a pretty cool CEO and septic designer to work with so I remain hopeful.

    1. Expert Member
      DCcontrarian | | #5

      Note that in plumbing "fixtures" doesn't mean what it means in ordinary English.

      Every plumbing device is assigned a "fixture count" and that is used to size the plumbing. A bathroom or bar sink is 1.0 fixtures, a kitchen sink is 1.5 fixtures, a shower is 2.0 fixtures (per showerhead), a tub is 4.0 fixtures, a toilet is 3.0 fixtures, a washing machine is 4.0 fixtures, a dishwasher is 1.5. And so on.

      If your limit is 3.0 fixtures you can't have both a toilet and a bathroom sink. But you're also not allowed to have a toilet without an accompanying sink. A 3-fixture limit basically limits you to a wet bar or shower.

  4. stephenr | | #4

    Thanks Bill,

    "If you are building a free-flowing system, where the water is ALWAYS moving out through a fixture whenever the heater will be running, then you shouldn't have a problem with expanding hot water building excessive pressure in your system. "

    Your above statement describes my situation nicely. I have crossed pressure building in the system off of my list of concerns. I was glad to see that you didn't see any other problems with my plan. Would you agree that a whole house water heater is the way to go? It seems that every time a fixture is engaged, the pump with turn on, the psi will raise , and the water will move through the heater and towards the fixture. I guess its just a matter of sizing the pump correctly so it kicks out the correct amount of psi that the heater requires. I feel there is a pretty wide range to work within here.

  5. stephenr | | #6

    Do you think a carbon filter at the entry point of the house and a RO system at the end (under the sink for drinking water) is a viable replacement for a UV system? I guess my concern is to keep my tanks free of pathogens so they dont get funky since cleaning them might not be an option once they are plumbed. With this concern in mind, maybe a UV light at point of entry would be the best bet.

  6. Expert Member
    BILL WICHERS | | #7

    Just work out the head for your system (“head” is the equivalent length of pipe in feet in this case), then get a pump that will run your desired flow rate (probably several gallons per minute) into your calculated head. You’re very unlikely to have any excessive pressure concerns with a simple system like that, and since it’s not a “closed” system — something will always be open when the pump runs — you won’t have expansion issues from hot water in the water heater tank. Your only pressure will be whatever it takes to push your desired flow rate through the piping here, and that’s probably not much, maybe a few tens of PSI at most.

    The particulate filter will get sediment (sand, etc), and larger gunk and should be the first stage of your filtration system since it will protect everything downstream in the system. The UV filter will kill pathogens — nasty wee beasties — that live in the water. You could use a carbon filter downstream of the UV filter to help with taste/smell, but don’t use it ahead of the UV filter. Filters can become infested with bacteria, so you want to use the UV filter ahead of as much of your system as you can. The only reason to put the particulate filter before the UV filter is because the UV filter should have some protection from sediment.

    Since you'll be using surface water here, I would have it tested before you put your system together. E. coli is a big concern, but not the only potential biological contaminant. You may need to do other things to make the water safe.


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