Hot water temperature

[Chris R]

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 obvious problems.

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.

Chris R

[Woody]

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.

[ARWadsworth]

Another one X-posted to uk.d-i-y in order to get a better rsponse.

IMHO the plumber is talking rubbish.

[ARWadsworth]

Another one X-posted to uk.d-i-y in order to get a better response.

IMHO the plumber is talking rubbish.

[ARWadsworth]

harry, I am not 6 years old. If ever do you use a word that I do not understand I will look it up for myself.

Thanks

[The Natural Philosopher]

I agree.

[The Natural Philosopher]

You have to remember than only 6 years old take harry seriously.

[Dave Liquorice]

saying

Or he has only half an understanding of different and almost unrelated facts.

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.

[Onetap]

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.

[ARWadsworth]

Why?

[Harry Bloomfield]

Dave Liquorice presented the following explanation :

Its, the minor explosion when the vacuum collapses which does the damage and can quickly erode the impeller. Ships propellers can suffer and can large pump impellers.

[jgharston]

My reason for keeping water temperatures high is to ensure it comes out of the tap hot.

JGH

[David WE Roberts]

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 the problem.

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.

Cheers

Dave R

[Chris R]

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.

[The Natural Philosopher]

Murray Wanker: How important are the Tyres in Formula One? Alan Jones: They are really good at stopping the bottom of the car dragging along the road, Murray.

[Tim Lamb]

In message , Chris R writes

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 correct.

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.

regards

[Chris R]

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.

Fully open.

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.

[ARWadsworth]

I did not ask the question. I crossposted it from another group so that OP may get some help.

Do you want me to give a link to the wikipedia article on crossposting?

[The Natural Philosopher]

Or a link to the correct spelling of cavitation?

Harry reminds me of that line in a song;

'and you can relate in full detail what we already know'

[David WE Roberts]

O.K. so you have new and different pipework and new and different shower cartridges so you can expect different performace from the system. If your pump is under specified or your cartridges are for a lower flow rate then you might not get the same flow from your power showers as you did from gravity. However normally I would expect a better flow from a pumped shower. Does make me wonder - if you had a shower pump AND two gravity fed showers then did you have a third shower with a pump? If not, what was the pump for?

Probably not the problem you seem to think it is.

Firstly, I would expect the flow to increase if the temperature was turned down (within reason). The limiting factor for the flow (up to the maximum flow available from the pump) is normally the amount of hot water available to mix with cold to get the desired temperature. Lack of hot water (volume ot temperature) tends to slow the flow. I think maximum flow is at whatever temperature you get when 50% of the water is hot and 50% is cold so both sides of the pump are delivering maximum throughput. If the hot water is way too hot then the flow slows down again as you are getting very little hot water through and so are maxing out on the capacity of the cold side of the pump. Been there and got very confused. Tinker with the hot water temperature until the flow seems good. Accept that a high pressure cartridge may not give the same results as a gravity fed cartridge if you had a good head of water for the gravity feed.

Secondly, the safety lock is normally set at a very conservative temperature. In my last house I had two power showers run off one pump and they both worked O.K. at the same time. To get what I would consider a reasonable temperature I had to go well past the red button. Same with my shower in my other house in Berkshire - combi boiler so mains pressure to hot and cold shower inputs. Still had to go past the red stop to get a decent temperature.

HTH

Dave R