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Hot water - pump/pressure/pulsing ??


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I have searched the forum but cannot find any answers, hopefully someone can assist.

For some reason i have acquired a couple of niggles and i would like to sort them out.


The first one...

When i turn on the hot water tap in the bathroom the water runs and slows almost to a stop before the pump kicks in. It didn't used to do this, the pump used to kick in pretty quickly.. Its also not quite as bad on any other tap, although it does slow a little in the kitchen. The cold water doesn't do this, the pump just kicks in instantly. Any ideas?


Second one...

This has been around a while but i have just lived with it. When in the shower, If i run just cold (or is it hot?) water, the pump pulses, if i mix hot and cold, or reduce the flow, the pulsing stops. None of the other taps in the motorhome pulse. Any ideas?


Many Thanks


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Without knowing the type of water system its difficult to give an answer. In some vans the system is permanantly pressurised and as soon as a tap is opened a pressure sensitive switch detects the drop and the pump comes on until the cut off pressure is reached.


If its that type perhaps the pressure sensitive switch is playing up


The other system is only presurised when the tap is opened and operates a micro switch in the tap.


if you have the second it may be there is bad bad connection or malfunctioning micro switch on the kitchen hot tap but the shower i have no explanation for.

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It is not unusual for the pressure to drop in the pipe when you open a tap as the drop in pressure activates a switch which turns the pump on.

Similarly when you close a tap the pump runs on for a second or two which creates the pressure to turn itself off.

You might have an adjustable pressure switch and reference to the maker or a handbook should be able to point you in the right direction for adjustment or replacement.

If the pump has a built in non return valve that could be sticking?

Another suspect could be some grot in the pipes somewhere.

Only way I know to check and to clear them is to disconnect each end of each section and blow back down against the flow to clear.

Suspect points for gunge build up are water tank pick up pipe, input to water pump and non return valve.

As it does it on both taps it probably is not a tap issue?

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It could be a if only the hot tap your water heater filling up and pressure going there first. I would try hot tap with heating not on to help resolve the issue especially if this is you first trips this year.
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Your motorhome has a Truma Combi 6E heater, so there's effectively some 10kg of water within the heater that acts as a 'damper' to the water being supplied to hot-water outlets. This explains why the water-pump reacts much more quickly when pressure drops in the cold-water hoses (which have very little volume of water in them) than to a pressure-drop on the hot-water side that contains 10 litres of water plus what's in the hot-water hoses. It also explains why, if you mix hot and cold water when showering, pulsing will be reduced.


It would be sensible to do as Tracker suggests and check that there's no obstruction to the water flow (eg. muck at the water pick-up in the fresh-water tank or muck in the water-pump's filter) but, if there's a good flow of water from the taps when the water-pump is running, it's fair to assume that the supply to and from the pump is unobstructed.


This document relates to SHURflo products




but the trouble-shooting guidance will apply generally to pressure-sensitive diaphragm pumps.


A motorhome water system that includes a diaphragm pump will tend to pulse. Depending on the installation and/or pump, some systems do it a bit and some systems do it a lot. As lennyhb advises, if pulsing at the taps is severe (or you find the pulsing irritating) fitting an accumulator should prevent this, or at least reduce the pulsing tendency significantly.

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Thanks for the replies.

It also occurred to me that it coincided with me emptying the hot water system and then refilling it. Prior to emptying the system the water pump kicked in much earlier, now i am wondering what i have done wrong re filling the system? or it could just be a coincidence??



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Strange you should mention about draining and refilling, I get exactly the same problem of flow slowing up before the pump kicks in if I have not bled all the air out of our system. As soon as it is correctly bled all is fine!

Try running water through each tap in turn for several minutes and listen for any gurgling or spluttering indicating air in the system.



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lennyhb - 2014-07-08 4:32 PM


If the pressure switch is set up correctly and it still does it probably worth fitting an accumulator, this will stop the pump pulsing.



I fitted one of these (accumulator tank) when my Autotrail water system seem to 'Pulse' and hesitate with delivery. I had already attempted to adjust the pressure switch, but it didn't fix it. The accumulator did.



the only question is ? has it always done this ? or only recently ?

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Truma's relevant Operating Instructions for filling a Combi-6E with water are as follows:




Check if the rotary switch for the drain valve (FrostControl) is set to “Operation”, meaning that it is parallel to the water connection and engaged.


Close the drain valve by pushing the push button until it engages.


When the temperature at FrostControl is below about 7°C, first switch on the heater to warm the installation compartment and FrostControl. After several minutes, when the temperature at FrostControl is above 7°C, the drain valve can be closed.


Switch on power for water pump (main or pump switch).


Open hot water taps in kitchen and bathroom, (set preselecting mixing taps or single-lever fittings to “hot”). Leave the fittings open for as long as it takes for the boiler to displace the air and fill up, and the water to flow without interruption.


If just the cold water system is being operated, without using the water heater, the heater tank also fills up with water. To avoid frost damage, the boiler must be drained through the drain valve, even if it was not operated.


When connecting to a central water supply (rural or city mains), a pressure reduction valve must always be installed to prevent pressures above 2.8 bar from developing in the water heater."


I use the following technique that I've found allows the water system to be 'bled' quickly and effectively.


Having filled the motorhome's freshwater-tank, I begin the process by placing the shower-head in a small bucket. I open the shower's HOT-WATER outlet and switch on the water-pump. This causes the heater's water-boiler to begin to fill (exhausting the air within it through the shower-head) and I keep the pump running (it will need to run for several minutes) until water flows from the shower-head smoothly and free of air. I then do the same with the shower's cold-water outlet, which takes much less time as there's only a small amount of air in the cold-water pipework leading to the cold-water shower-outlet. I then repeat the process (again using the bucket) for the wash-basin and kitchen-sink outlets. When this exercise has been completed the water-boiler and all the fresh-water pipework will be free of air and should remain so until the system is next drained down.


(I begin with the shower and use a bucket simply because plenty of air and water will inevitably be spat out from the water-outlets during the initial bleeding stage. Normally the bucket prevents any splashing: if it doesn't, then I'd rather the water splashed into the shower-tray than all over the living-area floor.)


If the pump's slower 'hot water' reaction has been noticed immediately after the water system has been refilled and bled, then (as in Keith's case) it's quite likely that the system has not been bled adequately. In such an instance it should be expected that there will be significant spitting and sputtering from hot-water outlets when hot water is subsequently run from them until the air left in the system's hot-water side has been expelled.


if there's no indication of air being expelled (ie. no spitting/sputtering) when water has been run through the hot-water taps in normal use for a while, it can be assumed that the system has been properly bled and, If the pump's reaction continues to be unusually slow when hot-water outlets are turned on, further bleeding won't improve matters.

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