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Subject: Pressure Relief Valve - position /leaking

I recently re-routed the PRV discharge from a bottle to the shower tray, since then it seems to discharge for about 30 secs or more immediately following use of a hot tap - tap closed PRV discharges.

I am puzzled why this seems to have suddenly occurred, I've tried turning the valve knob to de-scale the seating but on improvement. The PRV is sited at the base of the (vertical) calorifier is this OK?

Diagrams seem to have it at the top.


Dear Sir

I suspect this is the combined result of having the pump cut out pressure set too close to the PRV opening pressure and pressure pulses in the pipes caused by the tap being closed against a pipe full of strongly flowing water. Inertia in the water flow causes the water to "pile" up on the back of the tap as a pressure pulse. This then "bounces" back the way it came and then "piles up" against the non-return valve. In fact its something like inaudible water hammer.

As a test I would first try turning the hot tap of very, very slowly, so the water flow in the pipes is slowed down gradually before it is stopped completely. If this does not produce the problem you will have confirmed my diagnosis.

I can not say why it has suddenly started happening, but if the old outlet was sited in a typical place it may have been doing it all along, but you never took much notice of it. If you have introduced an air pocket into the hot water pipes it might have made the symptoms worse (or it could have made them better by providing a cushion of compressible air to prevent a sudden pressure rise).

If you have an accumulator in the water system (a round or oval "tank" hidden at the back of a cupboard or under a bed) and its internal pressure is incorrect, it may cause this.

The easiest way to solve it, I am sure, is to reduce the cut out pressure on the water pump. This may have the added benefit of helping reduce the amount of water that splashes out of any small washbasins.

More about accumulators and the sighting of PRVs can be found in the Maintenance notes on

You will see my PRV is close to the front of the boat, by the water pump. The notes discuss the water system and how it works in more detail than I can give here. They also shows how to use a "remote" pressure switch if you do not want to damage the "removing this label invalidates the warrantee" label to get at the pumps own pressure switch.

I hope this helps.

Tony Brooks


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Subject: Water Pump Running


Since having my Narrowboat from new I have noticed that at night after a days cruising, the water pump activates for a second or two during the night probably three or four times. I am not aware of any leak, although jus treading a response you gave to another boater I will check this out.

My assumption is that this maybe due to the water in the immersion tank cooling down reducing pressure in the system and hence requiring a top up. I notice during the day when running that there is a slight dribble of water from the overflow from the expansion valve located on the tank.

I would appreciate your views.

Phil R




Dear Phil

If the dribble is the result of the odd "spurt" especially shortly after hot water has been drawn off when the engine or central heating is running or a while after first starting, then that is normal. It is the cold water expanding in the calorifier and forcing its way out (rather then split the calorifier).

If the dribble is continuous then either your pump pressure is too high or the PRV on the calorifier is faulty.

Locate the PRV (one pipe goes to that dribbling overflow) and find the knob on the top - it should click up and down when you twist it. As you twist it with the pump switched on the pump will run and the overflow pulse as you open and close the valve. If the problem was scale on the seating there is a good chance that this will cure it.

If your system does not use an accumulator at a suitable place, then it mat well just be the water cooling and contracting as you say.

Look at the maintenance notes on the website, especially in the water system part of them.

Tony Brooks


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