Hot water starts running cold(-ish) after just a few minutes

[jondaniels]

Hi,

Let me start off by saying that I know very little about plumbing, so expect me to struggle a bit with some of the terminology and concepts

Now, to my question: I have an unvented hot water system, with an immersion heater, in a small flat. Recently the tenant in that flat reported that the hot water turned tepid after a few minutes. I have had a plumber, who I have used previously, look at it, and he suggested that I simply turn up the thermostat. However, I was not convinced that this was the problem and indeed the tenant reports that the problem persists. Please note that this hot water system has been working normally for a few years with minimal problems (replaced thermostats, etc)

To me it seems as if the hot water cylinder is simply not getting that much hot water put into it to provide more than a few minutes' worth of hot. The temperature is fine.

Further questions are:
- Is this analysis of the cause reasonable? Is it likely that the how water cylinder is simply not being given that much water to heat each time?
- If so, what can be done about it?
- If not, what might the cause of the problem be? I was told that there is some sort of pressurised sac in the hot water cylinder that helps force the hot water out.
- Might there be some sort of limiter on the hot water cylinder that can be set to allow some fraction of the total capacity of the cylinder of water to come in at a time?
- Might a trapped air pocket cause this? Do I need to bleed anything? There are no radiators of heating - this is just for hot water.

My concern is that I want to avoid having loads of people go to look at it and misdiagnose the fault and charge me for the privilege. I'm trying to build up a picture of what could be wrong so I can confidently get someone in to fix it.

A summary of my understanding of my hot water system is:

- Mains water comes in at 2 bars' worth of pressure
- That goes into a header tank, the bottom of which is probably about 50cm - maybe a meter max - above the top of the cylinder
- A pump draws water from the header tank into the hot water cylinder, also at 2 bars to match the mains pressure, forcing the hot water out into the taps (this was put in because the hot water pressure was initially very low).
- The hot water cylinder is immersion heated, and, I believe, vented. I don;t know the model name of it however.
- When I ran it for a few minutes the other day, the hot water coming into the bath started sputtering a bit. This coincided approximately with the time the hot water started to go tepid.

Many thanks in advance.

Jon

 

[spanner]

Sounds like you have sediment buit up in the tank feeding the cylinder and/or the pump which you explain supplies the cylinder at 2bar is not working quite frankly as a student, it sounds like you a have a direct hot water a cylinden drect mains with a small emmersion tank which is not providing sufficient amount of hot water.

don't forget that when you are drawing hot water it comes from the top of cylinder and that the emmersion has to replenish!!

sppanner...

 

[jondaniels]

Sounds like you have sediment buit up in the tank feeding the cylinder and/or the pump which you explain supplies the cylinder at 2bar is not working quite frankly as a student, it sounds like you a have a direct hot water a cylinden drect mains with a small emmersion tank which is not providing sufficient amount of hot water.

don't forget that when you are drawing hot water it comes from the top of cylinder and that the emmersion has to replenish!!

sppanner...

Hi,

Thanks for the reply. Thing is, there has never previously been a problem with tank size - in other words there has been enough hot water in the past. The pump still pumps out water at the same pressure as it always did. However I will check the sediment build up as suggested.

Thanks again
Jon

 

[justlead1]

Hi Jondaniels. It may be my reading but is it a typing error when you say vented. Para line 4 as this has an impact on diagnoses of the problem. (is the pump on the out let on cylinder or inlet?)

 

[tamz]

Does the cylinder have 2 immersion heaters or a dual one. If it has, the bottom (or longer) one is away.

If the water was spluttering at the bath tap it is drawing air ie the tank feeding the cylinder is filling slower than the pump is pulling the water out.

 

[jondaniels]

@ justlead1:
Umm I think I did actually mean "vented", thanks for that.

@ tamz:
The cylinder has 1 immersion heater. Regarding the pump, that sounds feasible, although I didn't think it would be that as the pressure seemed normal. On reflection though, the pressure actually seemed high, which woiuld explain that. I will look into it.

Thanks for the responses.