What is the maximum recommended temperature for domestic hot water?
A plumber recently turned down our cylinder thermostat to 60C, saying the
high temperature could generate bubbles and damage our shower pump. Since
then our showers are less powerful and the shower stats have to be set way
above the safety stop to get a reasonably hot shower. We probably hadn't
touched the cylinder stat for 15 years before that, so it was not generating
I am concerned more about the quality of the showers (thermostatic mixer
showers running off a pumped gravity supply) and having adequate hot water
in the cylinder than economy, though not wanting to burn money
unnecessarily. There are no children or vulnerable adults in the household
likely to scald themselves.
Normally 'hot' is considered to be 60C - even most immersion
thermostats are set to 55C.
We have an Aga so our water is very hot - typically around 68C -
so when we had a pump fitted the plumber fitted a fixed mixer
valve in the hot supply so that the pump never say more than 55C.
We have no problems with shower temp but if there are more than
three of us (i.e.visitors) we usually put the immersion on as
well. If - as does occassionally happen - one of the header tanks
runs out of water (think about it) the pump stops immediately.
Wait 2-3 mins and there is enough to finish your shower.
On Sun, 9 Oct 2011 19:27:53 +0100, ARWadsworth wrote:
Or he has only half an understanding of different and almost
Yes, air in a pump can damage it. Though this air is not the same as
cavitation, cavitation is when the pressure drops so low that the
liquid seperates from the surface of the impellor with a fairly hard
vacuum being formed it's this that causes the damage... Air being
drawn into a pump isn't quite the same, that may cause damage as
things are no longer lubricated by the water as intended.
Yes, as you heat water the disolved air in it comes out and if the
hot water take off and vent connection have not being plumbed
correctly this air can be drawn get into the hot water system.
Aye, but bear in mind that the plumber's (mis)understanding of the
subject is being reported through a third party who has little
understanding of it (otherwise they wouldn't have to call a plumber or
have the thermostat adjusted by said plumber). The plumber may have a
Phd in Fluid Mechanics, for all we know.
The thermostat numbers probably aren't accurate. If accurate, it may
not be firmly attached to the cylinder. Most shower pump installation
instructions have a maximum temperature specified which ISTR is
usually 60 degC.
The 'liquid separation' is caused by the liquid boiling as it passed
through the eye of the impeller (low pressure). Steam bubbles form on
the surface of the impeller and then implode as the liquid progresses
towards the discharge (higher pressure). The relevant bit is that
liquid temperature is very relevant to whether cavitation occurs, as
is the pressure losses due to friction in the pump inlet pipework.
Couple of perhaps obvious questions
(1) Why was the plumber in the house tinkering with the hot water? What
work, if any, did you have done? Just checking if anything else happened at
the same time as the hot water temperature being turned down which could
also explain a problem with the showers.
(2) Have you turned the temperature back up to check that the showers then
work properly? This would confirm that the temperature change was causing
If you have established that the water temperature is the problem then it
might be an idea to check the temperature of the hot water as it leaves the
tank heading for the shower and also where it enters the shower pump.
You can usually get cheap IR thermometers which can give a spot reading from
the copper pipe.
This was discussed over a year ago here.
I would post a link to the thread but Google Groups seems to have forgotten
about uk.d-i-y at the moment.
No plan survives contact with the enemy.
[Not even bunny]
Thank you Dave. The plumber was reconfiguring pipework to connect all the
showers to the existing pump, and changing the shower cartridges; previously
we had two low-pressure mixer showers that were purely gravity-fed. You're
right, I should try experimenting with the temperature.
There isn't a major problem: the showers do work and give an adequate flow,
but my impression was that the flow diminished when the temperature was
turned down. I may be wrong but I would expect that if the thermostatic
valve was cutting down the cold feed flow. Also the valves have a safety
stop on the temperature control, but they have to be turned way past it to
get an adequately hot shower.
Umm.. We have an Aqualisa shower unit and separate pump. There are
various features which lead me to believe that performance, owner
expectations and plumber expertise are not fully connected.
There appears to be a minimum flow rate, below which the temperature
control cartridge supplies mainly cold water. This initially led to the
plumber being called back to check that the hot/cold connections were
Following visits from daughters it is common for gravity hot water
supplies elsewhere to fail due to air locking. This can easily be cured
by turning on a particular bath tap so has not been permanently fixed.
(most likely cause is that a 1/2" ball valve in the already oversized
header tank cannot keep up with a pumped shower and the plumber did not
consider how long young ladies take to shave their legs). I am
considering doubling up the supply valve but the tuits haven't arrived.
Try turning up the flow rate at the shower control.
Check the plumber has fully opened the various service valves he must
have closed to do the work.
The system all seems to work as expected. I just suspect we could get a
marginal improvement in performance with hotter water, but I don't want to
damage the pump or waste money on excessive water heating for no good
reason. I'm pretty sure we haven't emptied the cylinder yet.
I really don't think there is anything wrong of this nature. We just have
cooler water resulting in less cold being mixed by the shower valves.
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