My father-in-law has a five-bedroomed house with three bathrooms, two of which are in use. The household is gravity-fed with a cold water tank on the roof, and a gas-fired tankless water heater on the uppermost of three floors.

It is only possible for one shower or tap (faucet) to be used at a time without reducing pressure. If the kitchen sink is used (hot) then the shower can reduce to a trickle.

I have read that a second heater, connected in parallel, would permit a higher outlet pressure at the temperature of a single heater, but my father-in-law is suggesting the installation of a pump at some point in the system to permit more than one hot water fixture to be used at the same time.

Is this possible, and can anyone with experience offer some advice on the general setup?

N.B. the house is in Mexico, so specific product recommendations may or may not be helpful.

  • Since the piping is gravity fed from a tank on the house, it will have very low pressure (say 15 psi) compared to a house on a city pressurized water system at 50 to 70 psi . You could put another tankless heater in parallel to feed the sink, but I would think it would be cheaper to use a pump to pressurize the entire water system. This would give increased flow rate at both the cold and the hot water delivery points. Commented Jan 1, 2018 at 20:54

2 Answers 2


sounds like you want more volume (gpm) of hot water ( thus,two tankless in parallel) because it takes energy (Btu) to make hot water. The heater will also limit its gpm output into the water pipe by the temperature you set. Look up the volume to hot water temperature curves for your rated Btu tankless heater. I think a big 200,000 Btu units can provide 6gpm of 115 F hot water and a smaller 60,000 Btu heater may only do 3gpm of 115 F hot water. If the kitchen is asking for all 6gpm of hot 115 F water, then all the other valves located after the kitchen valve will have no hot water from the water line. The 200 MBtu will make 7 gpm of warm water if the temperature is turned down (check the 90 F curve). Now, the kitchen gets 6gpm of 90 F water, and 1 gpm of 90 F water can go down the pipe for the next fixture. System with multiple loops of heaters installed closer to the fixtures, will fire on independently with no pressure lost. Parallel heaters will use more Btu and water if the the last fixture and heaters are far apart. These heater also have a minimal in coming water pressure to operate correctly, so a pump may help if placed before the heater.

  • Hello, and welcome to Stack Exchange. It's a little hard to read this long paragraph of text, but it mostly focuses on heater capacity, when it sounds like the current limitation is the flow of water through the heating system due to pressure loss. Commented Mar 2, 2018 at 2:26
  • By my read the answer gets right at the problem. Could be improved with some judicious formatting. This is the key sentence: "The heater will [limit it's output pressure] by the temperature you set.". So to improve pressure: install more heaters, install them closer to where you need hot water, and/or lower the temperature on the heater.
    – Stanwood
    Commented Mar 5, 2018 at 14:26

Most tankless water heaters have a lot (3–5 metres/10-16 ft) of relatively thin pipe in them for the heat exchanger. That adds significant flow resistance. The specification for most tankless water heaters specifies/suggests/presumes minimum water pressure of municipal systems, typically 40 psi (2.7 bar), though I know of one water-scarce area that offers up to 25 psi (1.7 bar).

In Rheem's specifications, they appear to assume a minimum supply pressure but assure adequate flow by specifying a maximum length of input and output pipe (400 feet for 3/4 inch and 100 feet for 1/2 inch).

However, even though my municipal supply is about 100 psi (6.8 bar), the water heater cannot reach its theoretical 7.5 gallons per minute flow rate because it is too restricting. But it certainly can provide reasonable flow for two sinks, a shower, and a (low flow) bathtub running simultaneously. It is nowhere as abundant as the cold supply. (Yes, the pipes work fine; bypassing the water heater provides abundant water to all points.)

I expect that a water pump to bring pressure up to a reasonable amount would do wonders. A suitable selection is among Home Depot's shallow well pump offerings

  • Thank you for your answer. Could you clarify at what point in the system you are proposing the pump would sit?
    – Sam_Butler
    Commented Jan 4, 2018 at 16:32
  • @Sam_Butler: Any convenient location between the roof tank and the pipes in the house. Convenient would include good access to the pipes for easy installation and maintenance, availability of power, and sufficient environmental protection. Maybe an add-on box (which you install) on the roof next to the tank. Probably build it out of bricks, masonry, or wood frame, though there might be pre-made solutions in your area. See what the neighbors might have done to improve water pressure.
    – wallyk
    Commented Jan 4, 2018 at 21:45
  • That makes sense, though I was referring to the position wrt other elements in the system. So immediately after the cold water tank, but considering the environmental and access criteria you’ve mentioned above. Thanks for your help.
    – Sam_Butler
    Commented Jan 5, 2018 at 23:01

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