Hot/cold water.

[Marky 1971]

Hi.
I have a problem with the water tap in my 2016 Lunar van. The water heats up fine, but when I move the mixer over to cold, it runs cold for a few seconds, then hot for a few seconds, and keeps alternating like this. I’ve no idea why it would do this. Any help is appreciated.

 

[RogerL]

The hot/cold pipes are routed side-by-side from the heater to the taps, so the cold water absorbs heat from the hot where they're very close - I've insulated my hot pipe everywhere I can reach it, using domestic 15mm foam pipe insulation.

 

[Beehpee]

It wouldn’t alternate from hot to cold if it was just being affected by the closeness of the pipes, you’d get a bit of warm and while the cold was running it would get warm , this has got to be something else.

BP

 

[Marky 1971]

I also thought at first that it might be heat transfer from pipe to pipe, but the temperature never seems to steady. The alternation of temperatures is persistent and rhythmic; maybe 2-3 seconds each.

 

[RogerL]

It wouldn’t alternate from hot to cold if it was just being affected by the closeness of the pipes, you’d get a bit of warm and while the cold was running it would get warm , this has got to be something else.

BP

Trust me it does, well mine does anyway. The hot/cold only touch intermittently which causes it.

 

[Jcloughie]

Easy to check if it’s heat transfer, just run the taps for a short while and it will should sort itself out.

Otherwise, knowing what sort of heating does the van have Would inform the answers.

John

 

[Marky 1971]

I also thought at first that it might be heat transfer from pipe to pipe, but the temperature never seems to steady. The alternation of temperatures is persistent and rhythmic; maybe 2-3 seconds each

Easy to check if it’s heat transfer, just run the taps for a short while and it will should sort itself out.

Otherwise, knowing what sort of heating does the van have Would inform the answers.

John

John, it is a truma combined blown air/water heater currently running on gas. The water is a Whale off board pump with adjustable pressure switch operation.

 

[Gralay]

Hi
I think there is some heat transfer both between hot and cold pipes and also between heating ducts to cold pipe and also inlet pipe as it is off board, try running cold tap for about a minute should then be OK

 

[Marky 1971]

Hi
I think there is some heat transfer both between hot and cold pipes and also between heating ducts to cold pipe and also inlet pipe as it is off board, try running cold tap for about a minute should then be OK

Hi Gralay, I ran it for probably 1/3 of an aquaroll and this didn’t diminish the effect.

 

[Beehpee]

If you run the cold tap constantly, and the hot water was at it hottest setting the cold water going past at the speed it passes, it would never be in contact with the hot pipe for long enough to warm it up, not a chance it could be heat transfer from the hot pipe in my opinion.

 

[Thingy]

I suspect reading the initial post, its a problem with the mixer tap itself. I am assuming your shower is also of the mixer type, does the effect replicate itslef at the shower head?

The other thing that springs to mind is whether or not you have an onboard tank, and if so, is the selector fully turned rather than half turned?

 

[Gralay]

Hi
After 1/3 of an aquaroll it cant be heat transfer, so I would agree with Thingy a tap problem, are other taps running cold

 

[Marky 1971]

The bathroom sink tap shows the same problem. There’s no onboard water tank. I have just noticed something; there is an adjustable pressure switch and so the pump operates only as the pressure drops. It seems to be the case that the burst of hot water is triggered when the pump operates. The pump stops and as the pressure drops slightly, it goes cold. The pump kicks in again which gives the burst of hot again...

 

[Marky 1971]

When I said there’s no onboard water, I meant cold water storage. There is a hot water tank built into the Truma unit.

 

[Jcloughie]

Marky, try this, open a tap (any), to a slow pore, tighten the knob on the top of the pressure switch until the pump runs continually, now turn the knob back until the pump switches on and off.

The switch should now be calibrated.

See if you still get the hot and cold issue.

The switch will need calibrating whenever there is a voltage change.

John

 

[Marky 1971]

Marky, try this, open a tap (any), to a slow pore, tighten the knob on the top of the pressure switch until the pump runs continually, now turn the knob back until the pump switches on and off.

The switch should now be calibrated.

See if you still get the hot and cold issue.

The switch will need calibrating whenever there is a voltage change.

John

Thanks John, I’ll try that in the morning.

 

[ProfJohnL]

Your problem is not unique. I'm guessing that you will have noticed how the changes in temperature are synchronised with the behaviour of the water pump, warm when the pump runs and hot when the pump stops. This is more common with caravan's with inboard diagram pumps but it has also happed to some high flow submersible pumps also .

This happens becasue inside teh water heater is an air cap - its designed to be there and its job is to give cold water space to expand when its heats up. The down side to this is the pressure in the air cap also raised becasue its compressed by the expanding water, and its hotter.
The pressure is released when a hot tap is opened, as the water is expelled teh pressure drops and eventually the cold water pump will kick in a few seconds after the tap is opened.

If the pump can deliver more water than is being used, it will begin to replenish the hot tank until the pressure switch is satisfied, That stops the cold flow, but the air cap in the hot tank will again deliver hot water, so you can get an alternating hot/warm cycle.

This is usually worse with showers becasue the mixer tap effectively connects hot and cold together and the rose provide more back pressure which causes the hot pressure to feed back to the pressure switch via the mixer tap exacerbating the problem.

There is a solution and that is to fit a surge damper to the cold water pipe work. This mirrors the action of the air cap in the hot tank. Whale sell such a device, and Truma sell a combined damper and pressure switch. There may be other manufactures - your caravan dealer should be able to supply the parts .

 

[Jcloughie]

The prof may well be right, but check to see if there is already one fitted.

John

 

[Marky 1971]

Thanks for all the advise everyone; the pressure switch adjustment was the necessary fix. The effect is still there slightly at a very low flow rate when the pump is on and off, but otherwise it’s all good. Thanks again.