Water pump running for about 7 seconds after dispensing hot water from Truma Combi

[JonMN]

Greetings! I was dewinterizing today, and running hot/cold through my Class B. I have a Coachmen Beyond with a Truma Combi Eco water/cabin heat system.

After setting the valves for use, I ran hot water until the air cleared and I assume the tank filled. My Truma and most of the plumbing system is completely visible under one of the bench seats. Everything seems to be working, but I have the following question/problem.

If water is dispensed from the Cold tap the pump stops instantly when shut off.
If water is dispensed from the Hot tap the pump runs for about 7-10 seconds and then shuts off.

When taps are off there is no pump action, so I don't think I have a leak anywhere. When I dispense Hot it comes immediately, so I don't think I have air trapped somewhere.

Any thoughts? Or is it typical and I just did not notice it before as I rarely use hot water anyway . . .

 

[Mein Sprinter]

Try purging the water tank of air trapped within. Also, it would help to add an accumulator where the pump cycles frequently or has poor flow. It helps to provide a consistent water pressure, reduce noise, and extend the pump's lifespan.

 

[wmgeorge]

Greetings! I was dewinterizing today, and running hot/cold through my Class B. I have a Coachmen Beyond with a Truma Combi Eco water/cabin heat system.

After setting the valves for use, I ran hot water until the air cleared and I assume the tank filled. My Truma and most of the plumbing system is completely visible under one of the bench seats. Everything seems to be working, but I have the following question/problem.

If water is dispensed from the Cold tap the pump stops instantly when shut off.
If water is dispensed from the Hot tap the pump runs for about 7-10 seconds and then shuts off.

When taps are off there is no pump action, so I don't think I have a leak anywhere. When I dispense Hot it comes immediately, so I don't think I have air trapped somewhere.

Any thoughts? Or is it typical and I just did not notice it before as I rarely use hot water anyway . . .

Might have air in your HW tank if its worked before no mods to water system needed.

 

[Mein Sprinter]

Might have air in your HW tank if its worked before no mods to water system needed.

Correct wmgeorge... he also need to open the water tanks purge valve...run the pump and wait till water comes out of the purge valve then shut it; which then has expelled all air.

 

[Jhv2]

On my Coachmen with Truma. I find it best to use City water to fully purge the hot water tank of air. Let it run for a while. AFAIK the Truma combi does not have a purge valve. Also do you have a shower mizer? Make sure you run the water through both setting on it to get the air out.

 

[MN1-ClassB]

Probably refilling the Truma tank

 

[wmgeorge]

Correct wmgeorge... he also need to open the water tanks purge valve...run the pump and wait till water comes out of the purge valve then shut it; which then has expelled all air.

JH has it correctly there is no purge valve on the HW tank and filling with a pressure hose might help. Don't have a clue as to MN1 is posting about?

 

[MN1-ClassB]

JH has it correctly there is no purge valve on the HW tank and filling with a pressure hose might help. Don't have a clue as to MN1 is posting about?

Since you asked so nicely. What I was thinking is that when he runs HW from the tap it is coming from the Truma tank. So the few seconds of the pump running afterward could be pump refilling into the Truma heater.

 

[booster]

I can't speak to Truma specifically because we don't have one and I have not tested one, but what you see is a characteristic of nearly all pump driven water system. Even our home water system that is from a well does the same thing.

The biggest thing that causes this is an intentional setting of the pump on and off pressure points which is a quite wide range. Our home system comes on at about 30psi and goes off at about 52psi for instance.

When the faucet is turned out the pressure relieves in the system until it gets to the on setpoint. Any air in the system works as an air spring to slow this coming on as the air decompresses and there is really never no air. None of the plumbing is ever 100% rigid, especially in plastic systems like an RV so pipe expansion also moderates the change rate. Many/most larger systems use an air over water tank to add air to system to keep the pump from cycling too often, even.

Also remember that the system needs to be able to accommodate expanding and contracting water volume from temperature changes, so air in the system is a good thing as long as it is not causing an air lock somewhere. The top of side filled water heaters in RVs like ours always have air at the top of them, for instance, and pump probably has some on the suction side somewhere, etc.

So, I would say don't sweat it. I would worry more if it turned on immediately and off immediately as it can cause pump cycling with minor temp changes or tiny faucet leaks, at that can be very irritating to have to listen to.

An exception to this is if the system has a variable speed water pump as it will be more likely to hold higher pressure when it is being used and slow down quickly when use stops so you may never even here it.

 

[wmgeorge]

Since you asked so nicely. What I was thinking is that when he runs HW from the tap it is coming from the Truma tank. So the few seconds of the pump running afterward could be pump refilling into the Truma heater.

Of course when you turn on the HW it comes from the tank? The pressure in the system is pushing it out as it refills the tank when you turn on the tap.

 

[MN1-ClassB]

The pump provides the pressure.

 

[wmgeorge]

The pump provides the pressure.

Really?

 

[booster]

The pump provides the pressure.

Sort of but indirectly. The pump creates a flow volume and resistance of the system sets the actual flow and pressure. If there is no resistance, you get no pressure. When the resistance is too big, the flow stops or slows a lot and pump hits the max pressure setting and shuts off. The pressure can stay in the system if it remains closed because of checkvalve at the pump to keep it from bleeding off. Positive displacement pumps like the diaphram pumps in RVs will slow down when flow stops or is lowered because the just don't have the power to move any more flow at the increased resistance.

 

[MN1-ClassB]

Oh, Ok, Well, I must have misunderstood b/c this guy was concerned w his pump RUNNING after he used then closed the hot water tap...I didn't see where he had a problem w the pump shutting down on him. I read just the opposite.... Maybe I missed that part of his inquiry?.....If the Truma was being emptied out of the HW tap, I would think the resistance would be lower b/c the Truma tank was then no longer full.