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

[lorraine]

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.

 

[Onetap]

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)

You should have a break tank, i.e., a storage cistern with a ball float valve and an air gap.

The reason is back-siphonage; the pump could suck water out of the mains. This obviously depends on the mains pressure, the mains pipe sizes and the pump sizes, but in theory the pump could cause a negative pressure in the mains. Any nearby defective fittings (garden hoses with no double check valve & left in ponds, shower handsets left immersed in baths, etc.) could then suck contaminated water back into the mains supply.

There isn't a well-known history of mains contamination in the UK, bacause the traditional system (loft tank + one kitchen tap connected to mains) was fairly foolproof re back-siphonage. There are many well-known instances in the US or in Europe, where mains connected fittings are normal. The most famous US case involved a football team (Holy Cross), who all contracted hepatitis after using drinking water fountains. Someone had urinated in a pit containing a submerged sprinkler head; there was a large fire in a near-by town and the large draw-off by the fire appliances caused a negative pressure in the water mains.

An internet search for similar back-siphonage incidents is a gut-churning experience. The pressure washer will have some approved safety devices to prevent this (pressure switch?).

Why didn't the lecturer know that?

 

[croydoncorgi]

Quite apart from the serious health risks, you have to ask yourself the question: what happens to the mains supply if several people on the same distribution leg install pumps? Obviously, at some point anyone WITHOUT a pump will get no water because the pressure will fall to zero!

I am also amazed that a 'lecturer' at ANY level did not know this! If what you say is correct, the individual does not know enough to be a plumber, let alone a lecturer.

 

[micky p]

it needs to have a double check valve on the cold mains before the pump to become legal. and i don't think a shower pump will have these built in as they are not meant for mains water and more for tank fed water at a lot lower pressure.

 

[chrishutt]

Why didn't the lecturer know that?

Well I didn't know that either. Hard to imagine a 5 bar pump causing negative pressure in the mains itself because the high resistance of the service pipe would cause the negative pressure to be concentrated on the pump inlet where it would cause cavitation, noise and rapid pump wear.

Apart from the damage to the pump, the problem is more likely to be mains pressure fluctuations in nearby properties. I had an example a while ago where the mains pressure pulsed on a cycle of 1 or 2 seconds. 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.

 

[lorraine]

hi guys,

thanksf or the replies, I too was stumped when my lecturer didn't know it!? but i'm in no position to question his authority yet.

I thought the main reason might be that the shower pump could not handle the mains pressure at the suction side or the unvented cylinder might implode.

thanks

lorraine.

 

[Onetap]

Why didn't the lecturer know that?

Well I didn't know that either. Hard to imagine a 5 bar pump causing negative pressure in the mains itself because the high resistance of the service pipe would cause the negative pressure to be concentrated on the pump inlet where it would cause cavitation, noise and rapid pump wear.

Depends on the mains, the flow rates, the demand from other properties, etc., etc., etc. A mains fault (broken pipe, partially open valve, defective pressure reducing valve) would make it more likely.

There are innumerable queries on here about low mains water pressures through combis or unvented systems, so it could happen. I'm fairly certain that ignorant people do connect pumps directly to the mains to solve their problems and there will one day be a major mains contamination incident.

 

[Onetap]

I thought the main reason might be that the shower pump could not handle the mains pressure at the suction side or the unvented cylinder might implode. thanks

lorraine.

The T&PRV on an unvented cylinder normally also acts as a vacuum relief valve, so the cylinder shouldn't implode.

The shower pump might not withstand the pressure, but that's not the Water Supplier's problem. Conecting a pump directly to the mains is prohibited in the Water Regulations to prevent contamination of the mains supply. It is not just shower pumps that it refers to, it is any pump, and there are some BIG pumps out there.

 

[chrishutt]

Oh dear, we're getting a bit muddled here, aren't we.

Firstly unvented cylinders are fed via a non-return valve so won't be affected by loss of mains pressure (why didn't onetap know that?).

Secondly, the issue of loss of pressure due to mains bursts, fire brigade draw-offs, etc. and the resulting back-syphonage risk is not directly relevant to why a pump shouldn't be fitted to the mains.

Let's not confuse the issue. Just consider that the pump is likely to be sucking on maybe 10 metres or more of 15mm pipe before the connection to a mains pipe of maybe 75mm. It's clear where the major resistance to flow is going to be.

 

[Spark123]

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?

 

[Agile]

I think that many of you should be taking theWater Regulations course!

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! )

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.

They have almost enough knowledge to teach the course material but often no more. Why would they spend house a week for about £15 per hour when they could be earning twice that doing a real job.

Ask them a question off subject and its unlikely that they will know the answer.

Another reason is that the shower INLET pressure maximum is usually less that the mains pressure so you could damage a pump seal.

Tony

 

[Onetap]

Oh dear, we're getting a bit muddled here, aren't we.

Firstly unvented cylinders are fed via a non-return valve so won't be affected by loss of mains pressure (why didn't onetap know that?).

Oh dear, oh dear.

I'd thought that maybe your previous post was possibly poorly worded, but it seems that you really are as arrogant as the post suggests.

The only person getting confused here is Chrishutt.

I do have an unvented ticket. I do know there is a non-return valve in unvented installations.

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I thought the main reason might be that the shower pump could not handle the mains pressure at the suction side or the unvented cylinder might implode. thanks

lorraine.

Lorraine had thought that the cylinder might implode, which would mean it would be subjected to negative pressure. So the unvented cylinder would be on the suction side of the pump.

It has nothing to do with the non-return valve; unless, of course you've got confused and thought she was referring to unvented cylinders being imploded by a negative pressure in the mains supply. It has a lot to do with a pump, downstream of the unvented cylinder, sucking water out of the cylinder faster than the mains can replenish it.

Secondly, the issue of loss of pressure due to mains bursts, fire brigade draw-offs, etc. and the resulting back-syphonage risk is not directly relevant to why a pump shouldn't be fitted to the mains.

Back-siphonage is directly relevant to why a pump should not be directly connected to the mains.

Let's not confuse the issue. Just consider that the pump is likely to be sucking on maybe 10 metres or more of 15mm pipe before the connection to a mains pipe of maybe 75mm. It's clear where the major resistance to flow is going to be.

You're assuming that the water supply system is installed and working correctly, a dangerous assumption. If the supply to your 75mm mains were restricted (e.g. a partially closed valve), then it is possible that the entire mains would be subjected to a negative pressure by a shower pump sucking water through 10m of a 15mm service pipe. A loss of mains pressure (supplier's pump failure, broken pipe, etc.,) would make it more likely

You seem to have started ridiculing everything I post here Chris. What's your problem?

 

[chrishutt]

You seem to have started ridiculing everything I post here Chris. What's your problem?

Nothing better to do, I suppose.

I misunderstood the point of the reference to imploding cylinders, so my apologies or that. 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.

My point was that there are good reasons for not allowing a pump to be connected directly to the mains even without considering what might happen in the event of the loss of mains pressure. As I described, it seems that it can cause pulsations in neighbouring supplies.

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.

 

[scatmanjohn]

Here we go again.

 

[chrishutt]

Here we go again.

And why not?