Electric Shower running with Low Mains Pressure

John.G · 13-12-2025 05:07PM #1

Triton T80Z

Mains fed shower.

For years my mains pressure was 3.5bar (static)so never a problem while showering if someone opened a tap somewhere as that pressure ensured that the flow stabilization valve which in effect is a mini pressure reducing valve could maintain the required showering flow/temperature. However the new now “normal” mains pressure is 2.02bar (static), at the present 10C mains temp it means there is very little pressure available for stabilization as can be seen below and if the pressure drops any lower if a tap is opend anywhere then the pressure switch which on this shower seems to operate at ~ 0.7bar will open the microswitch(s) resulting in just cold water, its quite obvious looking at the same flow demand with the original 3.5bar pressure that there is ample pressure differential to allow for another tap opening somewhere else, the shower flowed 8.0LPM with this (3.5bar) pressure (and temp control to fully cold) . The shower is located upstairs and is 4.6M above the incoming mains and is fed with ~ 8M of 1/2ins copper pipe. Its just about acceptable at the moment as there are now only two of us in the house but if the pressure permanentl falls any lower then will have to install a pumped shower fed from the CWS cistern

I had a 9.0kW Mira sport for nearly 19 years before this one and the pressure switch would close at around 0.25bar (2.5M), I know this because a good few years ago there was a lot of water pipe repairs going on with mains interruptions for hours/day so I installed a bypass from the CWS cistern to the main supply with a double non return valve to give some cold water for temporary usage and I was amazed that the Mira would run with this very low head of ~ 2.5M to give a shower flowrate of 3.5LPM, we were able to have a (better than nothing) one element (4.5kW) shower regularly for a few days until the mains was restored. I tried the T80Z on this CWS cistern supply when I installed it but it didn’t or won’t pass a single drop of water under these conditions, while I’m not suggesting for a moment that a “mains” shower should be run off the CWS cistern, I often wonder why this shower will not flow a drop of water under this low head, if it flows 5.9LPM at 0.75bar then it should flow, 5.9*sqroot(0.25/0.75), 3.4LPM with a 2.5M (0.25bar) head?.

The first column below was done with the temp control to fully cold.

Flow LPM	5.944	4.41	5.944	4.41
Mains Press Static	2.02	2.02	3.5	3.5
Mains Press Dynamic	1.78	1.8	3.26	3.28
Pipe Losses	0.572	0.315	0.572	0.315
elevation	0.46	0.46	0.46	0.46
Internal+Shower Head Loss	0.748	0.412	0.748	0.412
Press before Stabilizing V/V	0.748	1.025	2.228	2.505
Press after Stabilizing V/V	0.748	0.412	0.748	0.412
Residual Press	0	0.613	1.48	2.093

youtheman · 13-12-2025 07:49PM

First shower in my house was main fed. Advantage : no need for a pump so no noise. Disadvantage : 1. was constant power, so during the winter when the incoming water was colder you had to wind back the flow to get the same outlet temperature. I used to joke that you had to run around the shower to get wet!. 2. if someone flushed a toilet the pressure dropped, so the flow dropped, and the temperature went up. One day my son came in and asked where his older brother was. I said "he's having a shower". So he went into the downstairs loo and flushed the toilet. I then realised I was living with a monster!. Changed the shower shortly thereafter to a tank fed pumped unit.

Escapees · 13-12-2025 10:06PM

https://www.boards.ie/discussion/comment/124067779#Comment_124067779

Do pumped showers not suffer from the same temperature control issues as mains fed ones, in that they also just restrict flow in order to increase the temperature. Plus the tank in the attic will tend to be very cold in the wintertime and very hot in the summertime, often to an even bigger extreme than water in the mains, thus requiring a different setting of the flow to achieve the same outlet temperature. The only advantage of mains fed showers in my experience is that they're quieter and there's less to go wrong as there's no pump, but at the drawback of being sensitive to water pressure fluctuations below a particular threshold. Sorry OP for going OT.

John.G · 13-12-2025 11:00PM

Pumped showers give almost guaranteed constant flowrates of whatever you set the temperature control to (which is just a flow contoller), it doesn't matter how many mains taps you open, the flowrate will not change unless the CWS cistern runs dry. You will have to make very small adjustments as the mains temperature changes gradually during the seasons. A mains shower as you said is sensitive to water pressure fluctuations, the greater the mains pressure the less effect opening a tap or switching on a washing machine will have, If I had thought for a instant that my mains pressure was to be permanently reduced to 2bar as above I would certainly have (re) installed a pumped shower which are now also reasonably silent, the SR in the Triton T90SR stands for silent running. Overall, IMO, it's far better, especially if a new installation, to opt for the pumped shower.

youtheman · 13-12-2025 11:12PM

https://www.boards.ie/discussion/comment/124068096#Comment_124068096

The inlet temperature to a mains fed shower is essentially ground temperature. It winter this can be close to zero. In a tank fed shower the the inlet temperature is your attic temperature, which should be above ground temperature. And if your tank is insulated (not underneath) then it could be close to room temperature. Big difference. When I changed from mains to tank fed I never had that issue in winter when the flow was so low that I could hardly get wet!.

Escapees · 14-12-2025 08:08AM

https://www.boards.ie/discussion/comment/124068277#Comment_124068277

Hold your horses... Ground temperature is actually quite stable compared to air temperature, a bit like sea temperature. Therefore, in wintertime air temperature is usually colder than ground temperature and in summertime air temperature is usually warmer than ground temperature.

An attic of a house is typically the same temperature as outside, although some can get quite warm in the summertime if not properly ventilated. The point is that a tank of water sitting in an attic will often cool down in wintertime and likewise heat up in the summertime. A perfectly insulated tank of water will keep the mains water that filled it at the same temperature it entered. The point is that a pumped electric shower fed from an attic tank is actually typically subject to larger variations in inlet water temperature over the different seasons, unless the tank refills regularly due to frequent water usage.

EDIT: Sorry, I see now the comment you included in brackets below. This obviously makes a huge difference. I've come across attic tanks in the past where the side insulation doesn't join to the main attic floor insulation, so air just flows freely under them!

And if your tank is insulated (not underneath) then it could be close to room temperature.

John.G · 14-12-2025 10:43AM

There's probably not a whole lot of difference between the cistern temperature and the mains except with very little usage, you can work out the effect of any temperature changes/differences below.

LPM = 14.333*kW/dT. where LPM=shower flowrate. kW=shower power. dT=(required showering temperature-coldwater temperature).

With a 9.0kW shower and a required showering temp of (my preferance) 40C and a coldwater temp of a now 10C, the flowrate is, 14.333*9.0/(40-10), 4.3LPM. I have seen the coldwater temp as high as 20C in the summer so flowrate then is 6.45LPM and the lowest winter coldwater temp at ~ 6.0C, = flowrate of 3.8LPM, still sufficient to wet oneself and at a heavenly 40C!!.

John.G · 30-04-2026 08:29AM

Unfortunately, my mains pressure, for the past few months, is reduced still further at night from 2.1/2.2bar (static), to 1.7/1.8bar until around 0700 every morning, I get up well before 0700 each morning so mostly have to do with a one element shower (on reduced flow), no good to me. I stuck a PG in the supply right at the shower and this shower requires 1.1/1.15bar to close the micro switches, I'm probably going to replace the T80Z with a Triton T900PSR (P stands for portrait) to fit the available space. Its a pity that I didn't foresee this pressure reduction before I replaced my Mira Sport a few years ago with the Triton as the Sport (see post #1) probably closes the microswitches at 0.5/0.75bar and possibly lower since mine was closing them with a gravity head of ~ 2/2.5M.

ImTiredOfItAll · 01-05-2026 01:47PM

https://www.boards.ie/discussion/comment/124431577#Comment_124431577

You are so lucky John. In parts of Dublin you are lucky to get 1 Bar. A leak on the road or even very high demand the pressure drops enough that a mains fed electric shower can't heat the water at all. In some cases there might be barely enouge pressure to engage the pressure switch, maybe from midnight & 7am. The rest of the day it's cold showers

The only place I can think of that has so much pressure that a PRD is needed would be Howth. The only reason they have great pressure is because it's a tall hill. They have to pump the mails water to get it to the top. This gives great pressure

Electric Shower running with Low Mains Pressure

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