12 volt water pump runs in bursts on hot side only

Sounds like a blockage somewhere
For a test drain the water heater
Open a hot water tap and turn on water pump
If it does the same thing blockage is on the input side to heater
Possibly a strainer ?
If the it happens after the tankless fills up
Blockage is on the output side of heater going to RV water system

I know that doesn't sound right

It just might be that you need to replace the water pump

Could be flow has decreased thru/from the pump

MrWizard wrote:
Sounds like a blockage somewhere
For a test drain the water heater
Open a hot water tap and turn on water pump
If it does the same thing blockage is on the input side to heater
Possibly a strainer ?
If the it happens after the tankless fills up
Blockage is on the output side of heater going to RV water system

I know that doesn't sound right

It just might be that you need to replace the water pump

Could be flow has decreased thru/from the pump

Remember this is a tankless water heater. Nothing to drain. I have been suspecting a failing pump.

If the pump runs fine on the cold water side, then there's nothing wrong with the pump.

Timbtb

just how many "quarts" of water are in the piping of that tankless

Yes there is water in there, not 10 gallons, but there is water

try connecting an open hose to the output side of the water heater

a weak pump would trickle water at a slow rate and NOT cycle on off, because it would not build enough pressure for the pressure switch to open and shut off the pump
not a high enough flow rate for the the tankless to fire up

There IS a Restriction/Blockage someplace

could even be mineral deposits inside the tankless

open all the hot taps including the outside shower and turn on the water pump. If it flows better look for blockage near the one that flows poorly.

Did you verify that your bypass valves if any are set correctly?

Is there an accumulator on the cold side of the water system?

Tim,

I have only seen this once before.

There is a flow measuring device in the inlet of the water heater. The heater only fires when there is flow enough. That flow device is a plunger with a spring and a switch on the plunger. When the plunger gets sticky, there is no flow. That poor little pump is trying to do its job and finally the plunger moves, but now the flow is higher than the continues capability from the pump. So after the flow device opens and relieves the pressure, there is now not enough flow to the inlet device shuts up again.

If it was that the pump had just lost some capacity, then the water is supposed to flow but at too low a rate for the heater to fire.

Good luck

Matt

Problem solved.....(drum roll).....It was the adjustable flow restrictor valve on the in-side. Manual said it was set by the factory to be full open. When I tried to move the valve, it was stuck and needed slight help from a plyers. Bottom line is was only 3/4 open. Fixed

Thanks to all who commented here.

:B Thank You Matt