Shower water pressure problem

[Penners]

My old dad's house has an electric shower unit - a 9kW. If I turn off the main domestic water stopcock, the shower won't operate, so I deduce that it's fed from the cold water supply at mains pressure (as it should be).

However, if you're having a shower, and any demand for water is made elsewhere in the house, the water supply to the shower reduces sufficiently to make the overheat cutout operate. So the water runs cold. Needless to say, this always happens when you're covered with soap.

I therefore assume that the mains water pressure in the house is insufficient to supply both the shower and any other service simultaneously.

How would you solve this problem? I am thinking of fitting a single-impeller electric pump into the feed pipe to the shower. Is that what you would do? If not, do you have other recommendations?

Many thanks for any views or advice.

 

[NT]

The last thing you want is to put the mains water pipe under negative pressure, thereby sucking god knows what into your drinking water supply.

NT

 

[Gervase]

I'd be tempted to feed the shower from a tank in the attic, where it would be independent of any other demands on the mains water system. It would also be a cheaper solution than fitting a pump.

 

[Lime]

I'd install a tank with or without a pump depending on the pressure required and head available.

 

[Pford75]

Cheap option - tank in the attic feeding the shower only

Ideal option - switch to unvented system with an accumulator of sufficient size

 

[Flyfisher]

I've never liked those heater/pump shower units; they seem far too susceptible to pressure changes and I've also never known one able to produce a really good 'power shower' (admittedly my comments are based on experiece of only a few models many years ago, so they might be better now).

Feeding such a unit from the cold water tank should resolve the pressure variation problems, but you'll probably find that the performance will vary considerably between summer and winter. In the summer you might be able to get a decent flow, but in winter the incoming water will be much colder and you'll have to reduce the shower flow in order to allow the units time to heat up the water. I've experienced quite miserly flows just to keep the water lukewarm - more like a 'misting' than a proper shower.

Personally, I now use a pumped shower fed directly from the hot tank. This gives a powerful flow and is unaffected by the rest of the plumbing.

 

[Aidan]

My old dad's house has an electric shower unit - a 9kW. If I turn off the main domestic water stopcock, the shower won't operate, so I deduce that it's fed from the cold water supply at mains pressure (as it should be).

However, if you're having a shower, and any demand for water is made elsewhere in the house, the water supply to the shower reduces sufficiently to make the overheat cutout operate. So the water runs cold. Needless to say, this always happens when you're covered with soap.

I therefore assume that the mains water pressure in the house is insufficient to supply both the shower and any other service simultaneously.

How would you solve this problem? I am thinking of fitting a single-impeller electric pump into the feed pipe to the shower. Is that what you would do? If not, do you have other recommendations?

Many thanks for any views or advice.

You can't feed it directly from a tank, the pressure will be inadequate (unless you're in a 8+ storey building).

You might be able to supply it from a single impeller shower pump, operated by a flow switch, supplied from the tank, as you've stated. You'd need to check the minimum pressure requirements for the shower and the pressure you'd get from a pump. If this is an approved/recommended method, you'll find diagrams on the manufacturer's web-site.

You can't/shouldn't fit a pump directly to the mains water supply, there should be a break tank.

Shower heaters scale-up in use (in hard water areas), reducing the flow rate. It may be approaching the end of its life. I don't know if they can be descaled.

They're all fairly pathetic. If there's a hot water storage cylinder, I'd fit a twin-impeller shower pump every time; lots more plumbing work though. Or a single impeller pump on the outlet from a shower mixing valve.

 

[Penners]

You can't feed it directly from a tank, the pressure will be inadequate (unless you're in a 8+ storey building).

Thanks very much for your replies, everyone.

I tend to agree with Aidan, that a header tank alone is not going to provide sufficient water pressure for an electric shower. So it looks as though I may need to fit a dedicated header tank plus a pump.

However, I wonder whether anyone has - or can point me towards - a table showing the head needed for, say, 1 bar of pressure. I've tried googling it, so far without success.

 

[Aidan]

However, I wonder whether anyone has - or can point me towards - a table showing the head needed for, say, 1 bar of pressure. I've tried googling it, so far without success.

I Bar is equivalent to 10.2 m (approx 33' 6" ) head of water.

P = fluid density x 9.81 x head.

P = rho x g x h.
No symbol for rho.

P is pressure in Pascals.
1Pa = 1 N/ m^2.
100,000 Pa = 1 bar

density = 1000 kg/m^3 for water

9.81 N/kg is the standard force due to gravity

 

[Penners]

Thanks, Aidan. As I don't have the individual spec. for my father's shower, I've just been doing a bit of research on various models available from internet vendors.

It seems that a typical pressure requirement for an electric shower is around 0.7 to 1.2 bar. So a header tank (as you suggested) would need to be one side or the other of 10 metres above the shower head.

I think dad's bungalow might need a bit of a modification! :shock:

I'm interested in your earlier comment about scaling up. The shower in question is perhaps 10 years old, and very rarely used. Do you think that could result in scaling? Could the symptoms I've described by due to a scaled up shower?

 

[Aidan]

I'm interested in your earlier comment about scaling up. The shower in question is perhaps 10 years old, and very rarely used. Do you think that could result in scaling? Could the symptoms I've described by due to a scaled up shower?

Could be, but I can't really say. The scale coats the heater element and acts as an insulator and it also restricts the water flow. The result is the element disspiates the heat to the water less efficiently and there is less water to carry the heat away. The shower hose and head also exert some resistance to flow. I've seen some shower hoses with a tiny bore.

The heater element is much the same as an immersion heater or kettle element; have a look in the kettle and see what state that's in. Then consider what it would be like after 10 years.

You might be able to descale it by pumping descaler solution through it, but there's flow switches, solenoid valves and some interlocks, which may be plastic and which might not tolerate descaling. You might have to connect it to the mains to get water to flow, which would also heat it up. I think they're consumables.

 

[Penners]

Many thanks for your thoughts on that, Aidan.

 

[chuckey]

Stupid question Penners, just before your shower water runs cold, does it get very hot? It could be that the over temperature trip has gone bad and tripping out too early. My other thought is that are you certain that all the stop cocks down the line are fully open (including the one under the pavement)
For all PPB lurkers, always let your electric showers run cold before turning it to off. My first shower heater boiler lasted about 20 months due to hardness build up. Then my Dad told me the above, my second boiler has now lasted about 23 years.
Frank

 

[NT]

So it looks as though I may need to fit a dedicated header tank plus a pump.

It depends what you want. If you run cold to your electric shower you're limited to a low flow rate because the 9kW is not a great deal of power for water heating. If you want a good shower then you'd be better off fitting a heated tank plus thermostatic mixer shower.

Scale does not reduce heat transfer with electric heating, whereas it does with gas & solar panels. With electric the only thing it affects is element temperature, not water temp or heat delivery.


NT

 

[Penners]

Stupid question Penners, just before your shower water runs cold, does it get very hot? It could be that the over temperature trip has gone bad and tripping out too early. My other thought is that are you certain that all the stop cocks down the line are fully open (including the one under the pavement).

Why on earth would that be stupid, Frank?

Yes - that's exactly what happens. And yes, it's the overheat trip that is activating and making the water run cold. It happens when someone/thing in the house draws water elsewhere, thus lowering the pressure available for the shower.

Your point about the stopcocks is a very good one - I haven't checked that, but I'll certainly do so.

Scale does not reduce heat transfer with electric heating

No - but it could reduce water flow, couldn't it? Couldn't scale in the shower pipes be be the reason why it's so easy to reduce the pressure to the shower and activate the overheat cutout (as Aidan suggests)?

 

[NT]

Scale does not reduce heat transfer with electric heating

No - but it could reduce water flow, couldn't it? Couldn't scale in the shower pipes be be the reason why it's so easy to reduce the pressure to the shower

It can reduce flow by narrowing the already narrow pipework wtihin the shower, but it wont reduce the pressure to the shower. Reducing flow will actually increase pressure to the shower fwiw.

and activate the overheat cutout (as Aidan suggests)?

sounds like whats happening.


NT

 

[Gervase]

On the subject of flow, it's a good idea to use 22mm pipe to feed the shower, only necking down to 15mm where it actually enters the mixer - it makes quite a difference to the amount of water it can chuck out.

 

[Penners]

Reducing flow will actually increase pressure to the shower

Ah - another theory knocked on the head! Thanks NT.

 

[Nemesis]

I'm just so pleased we have the adjacent burn, and think will ablute in that from now on - so much easier than all this technical stuff.

 

[Penners]

I bet it's cold!

Hang on - how can a burn be cold? :?