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

[portuguesejohn]

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

I wouldn't have minded the old system if it weren't for a few things that I don't equate with modern living:
- The hot water cylinder was taking up too much space in the spare room, it also made the room unusable in the warmer days. This room is now my office and serves an actual purpose!
- Hot water on a timer is just not something I was ready for before coming to the UK (originally from Portugal and used to a perfectly fine 30 year old boiler that worked on demand, no central heating requirements though).
- Pressure in the shower was so weak, I can only assume past occupiers only had baths. Virtually unusable for me.

I understand combi boilers might not be a good fit for larger homes and families, but we live in a 3 bed property, just me and the mrs, we don't have high water needs and we don't mind the other person having to wait with tap usage whilst the other is having a shower . The flow rate was measured and deemed to be acceptable for our usage as well. Just need to get this shower sorted in the warmer days it seems!

 

[ianmcd]

Try as @denso13 suggests, remove the shower head, (no tools needed) what you are doing is allowing unrestricted water to flow through the boiler, if it runs fine with that off, it either needs de-scaled or replaced with a higher flow rate head

 

[portuguesejohn]

Try as @denso13 suggests, remove the shower head, (no tools needed) what you are doing is allowing unrestricted water to flow through the boiler, if it runs fine with that off, it either needs de-scaled or replaced with a higher flow rate head

Will do! Will also test the tap in the bath. I can't imagine the shower head being in that bad of a state, it's brand new (from October actually), I can't recall the brand and model, but it's a fairly big one that has plenty of power. Never felt at any point the pressure or flow was off, but I will definitely check.

 

[ianmcd]

Will do! Will also test the tap in the bath. I can't imagine the shower head being in that bad of a state, it's brand new (from October actually), I can't recall the brand and model, but it's a fairly big one that has plenty of power. Never felt at any point the pressure or flow was off, but I will definitely check.

No what he meant was open a basin tap a bit as you are having a shower, what you are doing is increasing the flow rate, you cant do that with a bath tap with your set up

 

[portuguesejohn]

No what he meant was open a basin tap a bit as you are having a shower, what you are doing is increasing the flow rate, you cant do that with a bath tap with your set up

Will try as well! Easy for me to do in me small bathroom - can reach the basin taps from the shower ...

 

[portuguesejohn]

So this has happened again twice in 2 days. Both times happened after the Mrs jumped out of the shower and I went straight in after.

Turning the basin hot tap on didn't make it better (only reduced the flow on the shower), removing the shower head didn't "fix" it either.

One thing that did stop the temperature fluctuations was completely shutting off the cold water tap and wait a few seconds before slowly turning it back on again - This seem to put a stop to the cycle and I could shower in peace again! I get the feeling the cycle is being triggered by myself opening the cold water tap too much at the start of the shower...

This is all baffling to me because I don't know how to address this... Come time to redoing my bathroom, how can I make sure the new shower won't be affected by this? Will a proper thermostatic mixer that is rated for combi boilers do the trick 100%?

 

[bernardgreen]

I get the feeling the cycle is being triggered by myself opening the cold water tap too much at the start of the shower...

Opening the cold tap will reduce the amount of water ( or water pressure ) available to the combi's hot water system and this will reduce the flow through the heat exchanger . Reducing the flow through the heat exchanger means the water will be hotter. If the boiler is already at maximum modulation ( least amount of heat being produced ) and the water is made hotter than the maximum temperature limit then the boiler will go into overheat and shut down.

 

[denso13]

Turning the basin hot tap on didn't make it better

So does the tap temperature fluctuate as well?

 

[portuguesejohn]

So does the tap temperature fluctuate as well?

Yes, I opened the basin tap and the water temp kept fluctuating, it also didn't take much of that tap being open to considerably reduce the flow on the shower.

 

[denso13]

We did test the mains water flow rate before having the boiler installed, which was more than adequate.

it also didn't take much of that tap being open to considerably reduce the flow on the shower.

I don't think those two statements are compatible. If the mains flow rate is more than adequate then I don't get how the shower flow is considerably reduced by slightly opening the basin tap.

I have a pressure reducing valve on my incoming mains set to 3 bar. There is no noticeable difference on any outlets if more than one is used.

As it is also affecting the taps then a new shower won't fix it.

 

[denso13]

Does the hot tap fluctuate if you use that on its own?

 

[portuguesejohn]

Opening the cold tap will reduce the amount of water ( or water pressure ) available to the combi's hot water system and this will reduce the flow through the heat exchanger . Reducing the flow through the heat exchanger means the water will be hotter. If the boiler is already at maximum modulation ( least amount of heat being produced ) and the water is made hotter than the maximum temperature limit then the boiler will go into overheat and shut down.

Sounds like I need to reduce the HW temperature on the boiler (if I want to have colder showers) during summer and increase the temperature during winter?

Does the hot tap fluctuate if you use that on its own?

As in the tap on the bath or the basin? Can't confirm 100% on the basin (although I didn't feel significant reduction in it for the couple of cycles), On the bath tap I could definitely feel a fluctuation.

It is not always easy to determine the fluctuation as the temperature swings aren't extreme, sometimes it's not even enough to be uncomfortable. One thing is for sure though: My Mrs has only noticed this once, ever. At this point she thinks I am being paranoid about it... The only difference between our showers is that I believe she likes it HOT, while I fiddle a lot more with the cold water tap...

 

[dal5band]

Perhaps the flow rate is being restricted somewhere on the cold input to the boiler ? Measure the max litres per minute you are out of hot tap - time how long ittakes to fill a 5 litre bucket or watering can.

 

[portuguesejohn]

Perhaps the flow rate is being restricted somewhere on the cold input to the boiler ? Measure the max litres per minute you are out of hot tap - time how long ittakes to fill a 5 litre bucket or watering can.

I'll give it a go when I get back.

Although I can perfectly understand concerns over my mains flow, I don't really feel like it is an issue here, I have lived in other places where that was definitely a thing, but my current shower works really great, if not the best shower I have ever lived under! There is plenty of flow and the pressure is just right, these issues that I am having did not come with an accompanying change in flow or pressure, the shower still feels as good as day 1. Everything was golden from October (date of installation) all the way to June.

 

[bernardgreen]

Sounds like I need to reduce the HW temperature on the boiler

The bottom line is that if a combi boiler is producing hot water then then amount of water flowing through the boiler has to be enough to remove the heat that the boiler is producing by burning gas. If too much heat is produced for the flow rate then the water will be hotter than the limit. Modern bilers in this situation will reduce the rate of burning gas to produce less heat. ( Modulation ). But there is a limit as to how far the boiler can modulate. If the rate of burning gas is too low the combustion may become unstable and thus unsafe.

If your wife likes hot showers and the water stays hot for her then she is taking enough hot water from the boiler to remove all the heat produced without the water getting hotter than the limit. Your cooler showers are not taking enough hot water from the boiler and thus the water is getting hotter than the limit and the boiler is shutting down.

Everything was golden from October (date of installation) all the way to June.

In June the incoming water was warmer than it was in October, thus less heat or more water flow is needed to remove the heat from the boiler without the water becoming too hot. ( above the temperature at which the boiler has to shut down because it cannot modulate it's heat output any lower )