Technical question (my lecturer couldn't answer it!)

[Agile]

As far as college lecturers are concerned, they are often second rate plumbers who have to teach because they cannot hold their own doing practical work.
Tony

And there I was expecting a backlash of lecturers who think they are competent plumbers!

Instead poor Chris is getting all the flack!

Tony

 

[Onetap]

However I can't see how the T&P relief valve would act as an anti-vacuum valve, nor why that should be necessary. I can't imagine that a centrifugal pump would be able to create a negative pressure sufficient to implode an unvented cylinder.

The T&P relief valve is built so that it has a secondary (or tertiary) role as a vacuum relief valve.

My CITB Unvented Hot Water Storage sytems manual (see, I really did do the course!) says ;

" an implosion or cylinder collapse could occur for example;

a) when water is drawn off faster than the inlet can replace it
b) after a heat-up period when the water has expanded, both the heat source and water supply are isolated. The water would start to cool, contracting in volume until a partial vacuum was created."

A pump on the outlet could cause a) above; good one, Lorraine!

I once had an involvement with a 3 (or 4?) storey house, with a vented cylinder on the top floor, tank in loft, bath on ground floor. The draw off from the hot bath tap caused air to be sucked in the open vent pipe. The pressure at the OV tee was negative when the hot bath tap was run at full belt. An unvented cylinder would be sucking in air through the vacuum relief valve; or else collapsing!. This was a normal, correctly installed vented system.

If it was purely a matter of guarding against loss of mains pressure it might be sufficient to link the pump to a pressure switch to prevent operation if the inlet pressure was inadequate.

Maybe it would work, but I think the Water Regulations say "Thou shalt not..." in very specific terms. I'd look it up, but can't find my guide book.

 

[chrishutt]

Instead poor Chris is getting all the flack!

Well I deserve it on this occasion. It's good to be shot at from time to time.

 

[wilhelm]

if its any help tony. when i saw your reply .i did think. "what a f £kkin ijeet"

and decided not to say 'owt but then....

remember mate not everyone on this planet is as good as you

horses for courses. and all that.

 

[micky p]

Is it illegal to have a pressure washer connected directly to the mains, or is it OK as it only has a low flow rate?

it is fine to have a pressure washer connected to the mains as long as it is connected via a double check valve bib tap.

 

[Onetap]

As far as college lecturers are concerned, they are often second rate plumbers who have to teach because they cannot hold their own doing practical work.
Tony

Those who can, do.
Those who can't, teach.
Those who can't teach, become heads or OFSTED Inspectors.

(My wife is a teacher.)

 

[scatmanjohn]

Here we go again.

And why not?

Go on then, tell him what your problem is and why you keep ridiculing him

 

[chrishutt]

My CITB Unvented Hot Water Storage systems manual says ;" an implosion or cylinder collapse could occur for example; a) when water is drawn off faster than the inlet can replace it
b) after a heat-up period when the water has expanded, both the heat source and water supply are isolated. The water would start to cool, contracting in volume until a partial vacuum was created."

Maybe the CITB are as useless as Lorraine's lecturer.

Firstly, how could you draw water off faster than the inlet flow, assuming a closed system (unvented) and a fully expanded expansion vessel? It just won't happen!

Secondly, just as the expansion vessel absorbs expansion, it will compensate for contraction on cooling. (If the expansion vessel had failed or somehow been isolated I suppose you could get a partial vacuum on cooling, but that's not mentioned in the extract quoted.)

 

[Onetap]

Maybe the CITB are as useless as Lorraine's lecturer.

Firstly, how could you draw water off faster than the inlet flow, assuming a closed system (unvented) and a fully expanded expansion vessel? It just won't happen!

With a pump on the outlet, or say 3 storeys of suction on the outlet. If the volumetric flow rate out of a sealed container/system is greater than the flow rate in, the volume of the container will decrease. It does happen.

Or, of course, the classic case where the idiot plumber turns off the mains supply and drains the system through the lowest hot water taps. Do check there is a vacuum relief valve, or quickly open one of the pressure relief valves to let air in. How do I know that?
All the dents popped out again!

Secondly, just as the expansion vessel absorbs expansion, it will compensate for contraction on cooling. (If the expansion vessel had failed or somehow been isolated I suppose you could get a partial vacuum on cooling, but that's not mentioned in the extract quoted.)

It won't compensate for contraction because the expansion vessel air charge ( usually about 1 bar) is set at 0 bar water pressure (gauge), i.e., with the water side drained. If you have a connected vessel with a 1 bar air charge, then at 1 bar water pressure, there is no water in the expansion vessel.

At more than 1 bar, the water compresses the air and the expansion vessel contains some water.

At less than 1 bar water pressure , the air charge is contained within the expansion vessel by the bladder, it can't expand into the water system. The air charge is still 1.0 bar.

 

[Doctor Drivel]

Hi all,

in my latest night class we were covering pumped shower systems and in the lecture one of my colleagues asked the lecturer why it was against the water regs to be able to hook up a shower pump directly onto the mains (ie not have a cold cistern and HW cylinder)

Lecturer came back and said because its the law. At which point I pointed out that my pressure washer is effectively hooked up to mains (i'm on an unvented ssytem) and it doesn't appear to be a problem.

So does anyone actually know the reason? One of my colleagues even asked if he could hook his 5bar pump up to mains coming into his house at 1.5 bar to give him a net 6.5 bar?!!! which got a bit of a cagey response. I know this is not good for the pump but if the flowrate is good enough and provided you don't pull a vacuum i can't see his megaflo cylinder imploding?!?

thanks in advance

Lorraine.

The idea is not to cause negative pressure in the mains. This can cause back siphionage of contaminated water from a house into the mains. Hose taps require double check valves inside them. If the hose pipe is in a dirty pond and there is back siphonage and no check valve contaminated water will enter the mains. The same with flexible shower hose too. Pretty simple.

A pressure washer has a very low flow unlike a shower pump. It has intergral check valves and is never used for long periods.

 

[ChrisR]

Onetap has explained some of the health reasons for pumping from the mains to be prohibited.

The simple answer is that it is not permitted. ( Thats a full stop! )

Oh yes it is permitted. Like diy gas work: under certain circumstances
If your flow rate is under a minimum figure you can use a pump as long as it doesn't give a flow over another figure (7l/min iirc). But you have to submit your plans to the water authority who can say no. For example if you're in a low pressure area in a block of flats, your pump might deprive those above you of water, so they would not give permission. This was Thames Water a couple of years ago - other regions may be different.

 

[Doctor Drivel]

I spent a long time (unpaid) trying to diagnose the cause (I'd just installed a combi and suspected a lump of debris) before the Water Company engineer told us it was caused by a pump on the mains in a neighbouring property.

Was he made to take it off?

 

[Onetap]

The idea is not to cause negative pressure in the mains. This can cause back siphionage of contaminated water from a house into the mains. Hose taps require double check valves inside them. If the hose pipe is in a dirty pond and there is back siphonage and no check valve contaminated water will enter the mains. The same with flexible shower hose too. Pretty simple.

See? Exactly what I'd said.
Dr. Drivel knew you wouldn't believe me, but now that HE has deigned to confirm it, surely no-one will dare to doubt it.

Welcome, Drivel. Why have you chosen to discard your alter ego identity, Water Systems?

 

[geoff@cetltd]

"Welcome, Drivel. Why have you chosen to discard your alter ego identity, Water Systems?"

same reason he no longer posts as "Adam", "MiniMe" and all the other nom de plumes

blown out I expect

 

[chrishutt]

1. Anti-Vacuum Valves. T&PRVs do not in themselves function as AVVs, but may have an AVV incorporated into them. It seems that some manufacturers do not fit AVVs!

2. As onetap says, greater outlet flow than inlet flow on a closed system unvented cylinder would mean contraction (collapse) of cylinder volume. However the negative pressure required to collapse an unvented cylinder would be unlikely to be achievable in practice. The expansion vessel membrane would surely explode first, releasing air into the system.

3. I accept that expansion of the air charge in the expansion vessel could be limited in its capacity to compensate for cooling contraction resulting in a partial vacuum. However I would expect membrane explosion would occur well before any possibility of cylinder collapse.

4. My Vaillant unvented manual says much the same as the CITB extract quoted above, but it refers only to a partial vacuum being created, not to the likelihood of a cylinder collapse, which is only likely on thin walled vented cylinders.

5. I accept the point that pumps might well exacerbate the dangers of back-siphonage in the event of a loss of mains pressure.