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On Demand Hot Water Tank

I have an On Demand Hot water tank, and love it. My Issue is I am using a manifold to distrubit both the hot water and cold water, through a pex system. I'm having issues with the hotwater pressure. The unit I've installed is designed for multi uses at one two to three appliances at once. When runing any hotwater sourse and you turn on the second, the pressure immdeiatly drops by 1/2. I have the heating unit set to 115 degrees f. I have a 1 supply line runing from the heater to the manifold. and all lines out of the manifold are 1/2" pex. I have no issues with the cold water pressure out of the manifold. Any one have any ideas.

Re: On Demand Hot Water Tank

redapa, I have never owned one and have never worked on one so I may just be talking through my hat. As I understand it the on demand heaters have a flow control that will only allow so many GPM through the unit the cold line has no such restriction.


Re: On Demand Hot Water Tank

It sounds like you have a electric tank less heater. Electric heaters have flow restricter to slow the water going through the heater to enable it to heat the water. When you turn on the second faucet you are putting more of a demand on the on the supply then the restricter will allow. We have removed as many of these as we have installed,for the same problem you are having.


Re: On Demand Hot Water Tank

Sorry the unit is a Natural gas Unit It is a Takgi T-K3 max output is 199000 btu,

There are shut offs on both the inlet and out let, with a "drain hook up and pressure releif valve on the output. I can fill a 5 gallon bucket in 53 sececonds, so I think the out put pressure is ok, I'm starting to wounder if it's the manifold not the heater.

Re: On Demand Hot Water Tank

call a plumber and have them hook it up to an acid pump. tankless heaters need flushed and acid cleaned periodiclly. your symptoms all point to the heater being all scaled up.

Flow vs Volume

Depending on where your located this unit will do:

199,000 divided by 500 divided by (temp rise) = gpm

Temperature rise is the difference between your incoming cold water temp to the unit and your setpoint. If I was making 115 degree water and my incoming water temp is 50 then my temp rise would be 65 degrees.

199,000 divided by 500 divied by 65 = 6gpm

The unit can deliver 6gpm and we can run 2 showers with basic shower heads that are delivering up to 3gpm each at the setpoint your looking for. Change the setpoint you change the temp rise thus the gpm.

I believe your problem is that the unit regulates flow in the flow switch. Pipe size of 1" requires a higher volume of water but the heaters supply outlet is only 3/4". It can't deliver the volume of water so you get a pressure drop when multiple fixtures call. The drop happens in your hot water feed to the manifold not at the 1/2" pex lines. The velocity that water moves through pex is much faster than copper due to less friction loss in pex.

Change the manifold supply pipe from 1" to 3/4" and it should solve your problem.

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