General plumbing query

[ldpooley]

I've recently done a mock test for a plumbing exam and wasnt happy with some of the answers given by the tutors, can any of you more knowledgeable plumbers give me your opinions on the following:

1.what is required soundness test pressure for hot water pipework which is normally subject to static head pressure of 6 metres?
a.1.2 bar b.0.9 bar c.0.6 bar

which of the following can be fitted to a hot water storage cylinder to provide protection against type 1 pitting corrosion?
a. pressure reducing valve b. magnetic conditioner c.sacrificial anode

what backflow protection device should be fitted to mixer tap fed with from cold mains that permits mixing of hot and cold water in valve body
a.double check valve b.single check valve

during soundness testing on an open vented hot water system ?
a. cold feed gate valve should be opened b.open vent pipe should be capped c.two taps should be opened

i went with b. c. b. and a. but was given other answers,so confused

 

[jonnyswamp]

Long time since I was in college, but I would have said

B 1 1/2 times working pressure
C anode will corrode before zinc in brass etc
A regs state double I believe (no doubt if wrong, I'll be pulled up)
B if your testing the hot water system, then the vent pipe has to be capped

 

[chris watkins]

I've recently done a mock test for a plumbing exam and wasnt happy with some of the answers given by the tutors, can any of you more knowledgeable plumbers give me your opinions on the following:

1.what is required soundness test pressure for hot water pipework which is normally subject to static head pressure of 6 metres?
a.1.2 bar b.0.9 bar c.0.6 bar
1.5 x max working pressure x 1Metre = 0.1bar =

which of the following can be fitted to a hot water storage cylinder to provide protection against type 1 pitting corrosion?
a. pressure reducing valve b. magnetic conditioner c.sacrificial anode
Process of elimination on a very poor question. not B as these are for scale reduction -50/50 but as type 1 is nothing to do with water velocities (if anything it occurs in stagnant water) I would go with C.

what backflow protection device should be fitted to mixer tap fed with from cold mains that permits mixing of hot and cold water in valve body
a.double check valve b.single check valve
Water Regs R15.13.2: = single on each

during soundness testing on an open vented hot water system ?
a. cold feed gate valve should be opened b.open vent pipe should be capped c.two taps should be opened
Again not the best question, soundness / pressure testing would mean that the cold feed would have to be closed to prevent loss of pressure up the Cold Feed to cistern - so not A. To pressurise open system the SOVP would be required to be temporally caped. so B.
i went with b. c. b. and a. but was given other answers,so confused

Hope that helps

 

[Ric2013]

what backflow protection device should be fitted to mixer tap fed with from cold mains that permits mixing of hot and cold water in valve body
a.double check valve b.single check valve

I'd agree with B if mains pressure cold and tank pressure hot, but note that R15.13.2 paragraph 3 states '[t]he water supply industry deems that the particular requirement of the Regulation will be met if there are no check valves on either 'side' of the supplies to 'mixer' taps where water mixes within the body and the supplies are on balanced pressures'. Which I would read to mean that if the both cold and hot water is at mains pressure, or if the cold water and hot water are both tank-fed, then no check valves are required. Please shoot me down if you think I'm wrong in my reading of the regs.

 

[chris watkins]

I'd agree with B if mains pressure cold and tank pressure hot, but note that R15.13.2 paragraph 3 states '[t]he water supply industry deems that the particular requirement of the Regulation will be met if there are no check valves on either 'side' of the supplies to 'mixer' taps where water mixes within the body and the supplies are on balanced pressures'. Which I would read to mean that if the both cold and hot water is at mains pressure, or if the cold water and hot water are both tank-fed, then no check valves are required. Please shoot me down if you think I'm wrong in my reading of the regs.

You are correct Ric & I would go along with you regarding the single checks on mixers fed from a cistern (open vented),

However experience shows us that it is not the same for unvented mains fed mixers (& often combi's systems) the reality is that the hot water side is often at a high pressure due to expansion when the water is heated or indeed as one system is in use without the other.

The amount of problems that are cause by water back flow from showers or taps fitted without single checks it is just not worth the risk I would always fit them.

 

[Ric2013]

However experience shows us that it is not the same for unvented mains fed mixers (& often combi's systems) the reality is that the hot water side is often at a high pressure due to expansion when the water is heated or indeed as one system is in use without the other.

The amount of problems that are cause by water back flow from showers or taps fitted without single checks it is just not worth the risk I would always fit them.

Hi Chris,

I took your comments in my mind when I was at college the other day and was asking what sort of problems you might have encountered. Sadly, no one (including the staff) could think of any. Any chance of an anecdote or two of times when lack of check valve on nominally balanced pressures caused you grief?

I can see that if a combi had an expansion vessel and check valve on the incoming cold supply, the cold pressure would drop but the hot pressure would remain high, and I can see that an unvented system might have a similar setup with same outcome... but that's as far as I can get.

All I got from others at college was an anecdote of how mains cold water forced its way up PAST the single check valve on a hot pipe to a mixer (still trying to get my head around that one).

Thanks, if you've the time...

 

[chris watkins]

As I say this is so common nowadays I am sure a search on hear would come up with a good number "why have I hot water coming out of all my cold taps"

But hear is a good example from unvented hot water installations. The call is "Can you help I have hot water coming out of my WC when I flush it first thing in the morning but it seems to be OK for the rest of the day". How can this be as both hot & cold are mains fed?

Not sure if you have covered unvented systems as yet Ric but as you probably know they are mains fed cylinders & have a pressure reducing valve to control the incoming water pressure. It is common practice to take what is called a balanced cold water supply off the cold feed to the cylinder to supply cold water to the bathroom fittings thus in theory both hot & cold being fed from the same pipe should alway be at the same pressure yes ?

Well no, here is what can happen, last thing in the evening or overnight the cold water in the cylinder is heated and expands (even if we have bubble tops or expansion vessels to control it) the pressure still rises & as no hot water is used the pressure in the hot water system is higher than the cold main.

As there is a connection between cold water (Category 1) & hot water (Category 2) the Water Regulations require us to install a single check valve to stop back flow & prevent the hot from contaminating the cold but this often gets missed off or wrongly positioned.

So here is what happens-
The master of the house gets up first to have his constitutional movement (dump) & flushes. Now the hot remember is at a higher pressure than the cold so instead of water flowing from the mains he gets hot water from the base of the cylinder flowing back up the cold feed & along the balanced cold supply to the bathroom WC which has just been flushed (so pressure in this pipe has dropped) hot water will flow so long as the cold water fittings are open & the pressure is greater than than the cold supply.

Once the pressure equalises due to fittings being used & no more expansion, all goes back to normal until tomorrow.
Hope you can remember all that but might be wise to copy & paste it to them. That one has made me a few bob over the years. It might do you a good turn in a couple of years time.
Knowledge is power my young friend so learn as much as you can while the learning is good. Best of luck Ric.

 

[Ric2013]

Thanks for that one Chris. While I haven't got to the hot/cold modules yet, your answer was vaguely along the lines I'd imagined, but it is very good to have that confirmed and an actual example given.