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Old 25-10-2014, 19:22   #1
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Pressure water pump overpowers hot water tank

I have a small Kumma 6gall hot water tank. Water gets hot, but as soon as the water pump kicks in it overwhelms the hot water in the tank. My theory is is that the smallness of the tank the force of the cold water is too high. the Pump is a parmax 1.9

Any advice?
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Old 25-10-2014, 19:38   #2
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Re: Pressure water pump overpowers hot water tank

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Originally Posted by NorthPacific View Post
...but as soon as the water pump kicks in it overwhelms the hot water in the tank.

I don't understand what this means. Overwhelms???
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Old 25-10-2014, 21:15   #3
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Re: Pressure water pump overpowers hot water tank

Do you mean that the pressure-relief valve is opening on the hot water tank and spilling water? If so, the valve is probably bad.

If you don't mean that, then I don't understand what you are describing.

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Old 25-10-2014, 22:19   #4
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Re: Pressure water pump overpowers hot water tank

It is very interesting that this question was asked today.

I just replaced a 14-year old 11-gallon Isotherm waterheater that was doing exactly the same thing the OP described.

The tank would sit quietly for hours with the AC power to it turned off and no other heat source applied to the tank. Suddenly the Shurflo 3.8 GPM freshwater pump would take off and start running and the overpressure valve on the tank would open and close very rapidly while discharging luke warm water.

Sometimes the pump would stop running and the valve would stay closed. Other times the only way to stop the over pressure valve from flowing water to the bilge was to shut off the Shurflo pump.

A few seconds after shutting off the Shurflo I would turn it back on and the overpressure valve would stay closed.

I lowered the pressure on the Shurflo and the problem continued. The hot water and cold water coming from the galley sink, closest to the Shurflo, showed no unusual pressure or pressure fluctuations.

Opening the galley hot water tap sometimes will cause the over pressure valve to close and stay closed, other times nothing will cause the over pressure valve to stop cycling open and closed, except for shutting off the Shurflo.

I installed a NEW 11-gallon SPA 40 Isotherm water heater earlier this week. Three times in the last two days the same problem occurred with the new heater: with no power or heat to the water heater - the overpressure valve starts surging open and closed and I have to turn off the Shurflo to stop the discharge.

Each time the over pressure valve starting cycling open/closed I checked the water temp coming from the hot water tap and found it to be only slightly warm.

It has now been 12-hours since the last time the over pressure valve opened.

I do have a filter in the freshwater line to the Shurflo and am pretty sure there is nothing but really clean city water is being pumped into the Isotherm.

SO - I have the same question as the OP!
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Old 25-10-2014, 23:03   #5
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Re: Pressure water pump overpowers hot water tank

Sorry for my confusing question.

The hot water tank heats the water no problem no leakage.
If I turn the water pump on until it primes and cuts out, then I turn the pump off, leave the water heater on for 20mins I have hot water at outlets. This only lasts until the built up pressure is used up.

As soon as I turn the water pump on the hot water gets cold very quickly.

My theory is that the pump is sending too high a volume of cold water into the small tank and mixing the hot with the cold water.

Do I try and find a smaller water pump or can I choke the fresh water feed? Or suggestions?
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Old 26-10-2014, 01:14   #6
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Re: Pressure water pump overpowers hot water tank

I think your idea of putting a restrictor in the line is a good one, but IMHO the best would be to call Kuuma directly and ask them:

Camco Manufacturing Inc.
121 Landmark Drive
Greensboro, NC 27409

1.800.334.2004
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Old 26-10-2014, 01:19   #7
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Re: Pressure water pump overpowers hot water tank

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Originally Posted by NorthPacific View Post
Sorry for my confusing question.

The hot water tank heats the water no problem no leakage.
If I turn the water pump on until it primes and cuts out, then I turn the pump off, leave the water heater on for 20mins I have hot water at outlets. This only lasts until the built up pressure is used up.

As soon as I turn the water pump on the hot water gets cold very quickly.

My theory is that the pump is sending too high a volume of cold water into the small tank and mixing the hot with the cold water.

Do I try and find a smaller water pump or can I choke the fresh water feed? Or suggestions?
Is it possible you have the inlet and outlets plumbed backwards? It makes a difference in most heater designs.

Also, make sure that you have all the air out of the heater. Look at the instructions for any hints at bleeding the air out.
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Old 26-10-2014, 05:19   #8
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Re: Pressure water pump overpowers hot water tank

The ParMax 1.9, pumping 1.5 gpm, will empty your 6 gallon tank in 4 minutes.
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Old 26-10-2014, 05:29   #9
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Re: Pressure water pump overpowers hot water tank

Sounds like you are plumbed backwards or have a broken dip tube in the tank. Both are easy to remedy.
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Old 26-10-2014, 06:44   #10
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Re: Pressure water pump overpowers hot water tank

Heater must have a non-return valve in the cold water supply line to prevent hot water flowing back out when cold water tap in opened.
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Old 26-10-2014, 07:50   #11
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Re: Pressure water pump overpowers hot water tank

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Heater must have a non-return valve in the cold water supply line to prevent hot water flowing back out when cold water tap in opened.

Blue Stocking,

Glad you survived Gonzalo and I hope all will be back to normal soon.

A non-return valve isn't mandatory. The hot water will back up into the cold pipe a little but if there are no cold taps close by it isn't a problem. When hot water is drawn from a tap then cold water flows into the heater. There is no tendency for hot water to flow backwards then.
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Old 26-10-2014, 09:59   #12
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Re: Pressure water pump overpowers hot water tank

I suspect your mixing valve is open to cold too much. When you lowered your pressure in the accumulating tank, your mixing valve spring adjusted to the lower pressure, but when the pump ramps up, your spring loaded mixer is allowing majority cold to come thru. Play with the mixer and see if it changes. Mixer is normally on the outlet of the tank.
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Old 26-10-2014, 13:47   #13
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Re: Pressure water pump overpowers hot water tank

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The ParMax 1.9, pumping 1.5 gpm, will empty your 6 gallon tank in 4 minutes.
Much quicker than that Gord. My taps will not allow that max flow rate anyway.
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Old 26-10-2014, 13:49   #14
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Re: Pressure water pump overpowers hot water tank

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Sounds like you are plumbed backwards or have a broken dip tube in the tank. Both are easy to remedy.
Cold goes into the bottom. hot out of the top. No idea what the dip tube is or how I would locate it Bletso?
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Old 26-10-2014, 13:53   #15
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Re: Pressure water pump overpowers hot water tank

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I suspect your mixing valve is open to cold too much. When you lowered your pressure in the accumulating tank, your mixing valve spring adjusted to the lower pressure, but when the pump ramps up, your spring loaded mixer is allowing majority cold to come thru. Play with the mixer and see if it changes. Mixer is normally on the outlet of the tank.
No mixing valve on the tank. Galley mixes at the sink same at the shower.

I can feel the heat from the hot water hose as it leaves the tank as soon as I turn on the pump it cools down in seconds?
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