Shower alternates between hot and cold... Boiler is fine.

[portuguesejohn]

Earlier in the summer started noticing sporadic hot/cold intermittent supply. Hot supply for a couple of minutes and then endless hot/cold alternates, the cold cycle seems to last around 10 seconds, with the hot cycle a bit more. This is not an everyday occurrence, although it seems to happen more in times when the shower is used more often (say for example, the mrs uses the shower and I jump in straight after - I'll get the hot/cold cycle!)

Boiler is a brand new Worcester 30i combi installed last October (which we had installed immediately after moving in, so I don't have much of a previous comparison). Bosch was happy to send an engineer around who run a whole lot of tests and concluded that the boiler was fine. He mentioned our hot water temperature setting was set maybe too low (it was set to 48c) and that the boiler could potentially be reaching that temperature very easily, which could cause the hot/cold cycle. He jacked the temperature up to 60c.

He also said the issue could be with the shower itself, but he offered no further details on this.

It had been a few weeks since we last noticed the issue, so I thought he had been spot on about the boiler temp - but this weekend it started happening again!

Shower/tap picture attached - what could be causing this?

 

[muggles]

Could well be that. It's recommended to have thermostatic mixers with combi boilers to overcome temperature fluctuations

 

[portuguesejohn]

Could well be that. It's recommended to have thermostatic mixers with combi boilers to overcome temperature fluctuations

Hi Andrew, thanks for the input.

We plan on renovating the bathroom in the following year, but until then, I'd like to understand what could potentially be happening here? I am completely clueless when it comes to plumbing, but how could these fittings impact the temperature? I have always felt it was very hard for me to get the temperature I want out of the shower (I am constantly fiddling with the cold tap to get it "just right"), but where could things be going wrong when it comes to the temperature fluctuation? And how did this only start happening recently?

Thanks.

 

[bernardgreen]

The problem is that the boiler is producing too much heat for the amount of water flowing through it. Hence the temperature of the water leaving the boiler is above the set level. The boiler cannot modulate ( reduce ) its heat output any lower so it has to shut down to prevent the water being too hot ( higher than the set maximum ). The boiler then has to operate in cyclic mode turning ON and OFF to produce an average heat output that is low enough to not over heat the water.

And how did this only start happening recently?

In summer the incoming water is warmer than it is in winter so in summer less heat is required from the boiler to bring the water up to the set temperature. Hence in summer the boiler is more likely to go into cyclic mode than in winter.

The "solution" is to increase the flow of water from the boiler so that there is enough water flowing through the boiler to remove the heat from the boiler without the water becoming too hot.

 

[portuguesejohn]

The problem is that the boiler is producing too much heat for the amount of water flowing through it. Hence the temperature of the water leaving the boiler is above the set level. The boiler cannot modulate ( reduce ) its heat output any lower so it has to shut down to prevent the water being too hot ( higher than the set maximum ). The boiler then has to operate in cyclic mode turning ON and OFF to produce an average heat output that is low enough to not over heat the water.



In summer the incoming water is warmer than it is in winter so in summer less heat is required from the boiler to bring the water up to the set temperature. Hence in summer the boiler is more likely to go into cyclic mode than in winter.

The "solution" is to increase the flow of water from the boiler so that there is enough water flowing through the boiler to remove the heat from the boiler without the water becoming too hot.

Hi Bernard,

That's very informative, thanks. I assume this is why this is more noticeable for the second person using the shower in a short period of time? Also, how do I go about the "solution" though? Is this only about upgrading my bathroom hardware? How does a thermostatic mixer addresses these issues?

 

[bernardgreen]

How does a thermostatic mixer addresses these issues?

Unless it increases the flow rate of hot water from the boiler it cannot address the issue. Fitting a larger shower with a higher flow rate might solve the problem in summer but then in winter with colder incoming water the boiler may not be able to provide enough heat to heat the water to a temperature suitable for the shower.

 

[portuguesejohn]

Unless it increases the flow rate of hot water from the boiler it cannot address the issue. Fitting a larger shower with a higher flow rate might solve the problem in summer but then in winter with colder incoming water the boiler may not be able to provide enough heat to heat the water to a temperature suitable for the shower.

What do you mean by "larger" shower? And if fitting one could have adverse effects, what is the actual proper solution to all of this then?

 

[bernardgreen]

What do you mean by "larger" shower?

One with a higher flow rate.

what is the actual proper solution to all of this then?

Use a hot water cylinder to supply hot water to the shower and thus avoid the need to continually match the boiler's heat output to the shower's heat requirement.

Installers often install combi boilers into houses where a combi is not the best option.

The pros and cons and history of combination boilers can be found HERE

 

[portuguesejohn]

One with a higher flow rate.



Use a hot water cylinder to supply hot water to the shower and thus avoid the need to continually match the boiler's heat output to the shower's heat requirement.

Installers often install combi boilers into houses where a combi is not the best option.

The pros and cons and history of combination boilers can be found HERE

Thanks, you've forced me go and read a bit more on the subject. We did test the mains water flow rate before having the boiler installed, which was more than adequate. I also noticed much better pressure and flow coming through the shower when the system was changed to combi (to the point some of the pipework in the house had to be tightened and checked over for the increased pressure...).

I did find the following topic where the author was having similar issues: https://community.screwfix.com/threads/new-thermostatic-shower-hot-then-lukewarm.158481/

The solution for them:

We had Mira send out a number of flow limiters and tried them on the showers. Reducing the flow down to 8l/min results in hot water from the shower.

This is what I found in terms of flow limiter within my own system (attached picture), would it hurt me trying a few other limiters? I suppose that in my case, I'd be looking into increasing? The current one seems fairly standard in shape to others I find online, although there is a spring like attachment which I am not soo sure about...

 

[denso13]

Shower/tap picture attached - what could be causing this?

Try taking the shower head off and running the water through the hose only. See if the temperature becomes more stable.

You could also try running the basin hot tap at the same time as the shower. Just to see if it helps.

 

[portuguesejohn]

Try taking the shower head off and running the water through the hose only. See if the temperature becomes more stable.

You could also try running the basin hot tap at the same time as the shower. Just to see if it helps.

Thanks, will give it a go next time it happens, which seems to be quite random at times! Went through a couple of weeks without the issue!

 

[denso13]

Installers often install combi boilers into houses where a combi is not the best option.

Installers often install combi boilers into houses where a combi is the best option.

 

[portuguesejohn]

I am wondering if the flow could be an issue on the hotter days also? We are down in Kent and the heat has been murder in the last few days, we keep getting emails from the water company asking customers to mind their water usage and how some in the community might be affected by supply issues. I am now wondering if the our supply is being affected in the warmest days. Can't say I notice it from the shower though, I feel like pressure and flow is just about right since we have had the combi boiler installed....

 

[bernardgreen]

Installers often install combi boilers into houses where a combi is the best option.

I agree, sometimes the installer gets it right.....flats and apartments where there is no space for a hot water cylinder are right places for combi boilers

 

[denso13]

flats and apartments where there is no space for a hot water cylinder are right places for combi boilers

I have a fairly large detached property and my fairly small combi is just fine.

Each property needs a proper survey to determine what is best. Generalisations are just guesses.