I'm learning. So the 3.7m refers to how high the valve can push water up a pipe when opened fully? I guess that includes the length of pipe between the valve and cylinder in addition to overflow pipe. How does the capacity of the cylinder affect it?
marte,
Aug 9, 2:40pm
Once water starts comming out the overflow pipe. Undo the screw untill it stops happening. Run a few litres of hot water out of the tap ( well cold water as it would not have warmed up yet ). Check the overflow pipe to make sure there's no water overflowing any more, if there is, undo it a bit again & run some water & check it again, untill it's not overflowing. Try the taps water pressure to see what you think.
As the water heats up, it will expand, so if there's no water run out of a hot tap during the heating up stage, it may expand enough to overflow the pipe a little. Don't worry about it, just drain it a bit & check it.
Once the hot waters hot & not overflowing, you can adjust the pressure up a bit untill it overflows a little & then back the pressure off a bit to get the most pressure possible. You don't want to get too close to maximum pressure, but it helps to get the most out of it.
Maximum head? I guess how high the cylinder is makes a difference.
trade4us2,
Aug 9, 4:08pm
The cylinder should be marked to show the maximum pressure allowed. All the cylinders I have had are marked with 7.6 metres. If the top of the overflow pipe exceeds that height above the BOTTOM of the cylinder, the cylinder could burst. If your roof is only 3 or 4 metres high you may be better off with a pressure release valve, but most of those are horrible and leak hot water all day. Perhaps you can find one that merely leaks cold water.
tegretol,
Aug 9, 5:57pm
Correct. Capacity of cylinder irrelevant.
tegretol,
Aug 9, 6:01pm
That's the whole point of the adjustable valve - set it so that water does NOT come out when the tank is sitting there full of hot water. Then ignore the fact that some water may come out when the tank is heating due to expansion - this is uncommon as most of the expansion is allowed for when hot taps are briefly opened. But the overflow is there for two purposes: 1. To allow for expansion of water as it heats and 2. to provide a safety point should the ajax fail and try to create a bomb.
Pressure release valves are intended to help raise the release pressure (increase the head) when adequate overflow pipe height isn't possible but quite frankly, I wouldn't add complication coz if that fails, your tank may well blow up. An open overflow pipe is a lot safer. I believe there are a pile of extra regs around fitting those valves.
msigg,
Aug 10, 5:49am
So you can fit a pressure relief valve to the top of you roof pipe, you can cut it off and put it anywhere along the vent pipe, the valve must be for your 3.7m head reducing valve. Easy . Done deal.
trade4us2,
Aug 10, 11:01am
I think it is a bad idea to have a pressure release valve as well as a roof pipe. The cylinder is likely to burst. However they make regular work for plumbers, most of who know nothing about physics.
tegretol,
Aug 10, 11:43am
True. I seem to recall that aa PRV has to be fitted at the tank and that the outflow from it goes into a drain - that's the whole point of them - they don't require a roof pipe. Used extensively in hi-rise flats back in the old days.
marte,
Aug 10, 3:24pm
What's the situation with putting a combined 'two taps- one spout ' setup in?
I'm wondering if water can get from the cold tap & flow back up thru the hot water pipe into the cylinder & make it leak?
I know that in theory any leaking water should run out of the tap. But them showerheads sometimes have a seperate valve on them & sometimes there's additions like water filters that have taps as well.
Or uneven water pressure.
trade4us2,
Aug 10, 4:56pm
Yes the cold water is usually at a higher pressure than the hot water, so It is possible for the cold water to go into the cylinder. However the relief valve or the overflow pipe will allow that water to flow out on the the roof, so no big problem. I think all the systems that allow water to overflow are crude and wasteful and the plumbing industry should change the code. And they should stop the incidence of Legionnaires' disease by having the temperature reduction only at the tap, NOT at the cylinder. But plumbers don't understand how to do a proper hot water system. They are just hopeless.
tegretol,
Aug 11, 6:44am
Just add a one-way valve ($45) into the hot side of the mixer. End of problem.
In fact, I see that some up-market mixers have that built in.
marte,
Aug 11, 1:30pm
So uneven water pressure with the cold wster back flowing into the hot water cylinder, or a single spout tap setup could be the cause of the OP's problem.
And a extra one way valve after the hot water cylinder would fix it.
bill1451,
Aug 11, 2:30pm
so you want to make a bomb out of your HW cylinder when the releif valve fails. the releif valve is fitted to the cold water inlet, the stand pipe is to give max head ie pressure.
bill1451,
Aug 11, 2:37pm
long term fix for this is go back to the good old header tank in the ceiling, with a metal or plastic tray with a drain to the outside. Reducing valves are for lazy, too tired plumbers who have little or no knowledge of how water systems work we have a log burner with a wet back and the switch is turned off most of the winter.Header tanks have the advantage that when the power goes off and the water stops you will still have a shower, especially in rural, where we pump our own water.
tegretol,
Aug 11, 3:39pm
Spot on. 100%. KISS.
budgel,
Oct 8, 12:01pm
The relief valve is NOT fitted to the cold water inlet, that one is the pressure reducing valve. A relief valve is fitted off the hot water outlet from the cylinder so that if pressure becomes excessive it opens and relieves that pressure!
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