Shower temperature fluctuating after bathroom renovation

[zamandguth]

Hello!
I recently had my bathroom renovated by a company who did a pretty good job, however I am facing a strange temperature fluctuation issue.
Everything in the bathroom was replaced, all shower and sink tapware etc, but the water heater (downstairs, under-the-kitchen-sink Stielbel Ultron DHCE) was not replaced.
Before the renovation, the water temperature in the shower was nice and warm and consistent.
After the bathroom renovation, I will set the temperate to a nice point, then after about 30 seconds, it heats up to scalding, 30 seconds later, back down to lukewarm etc. This all happens without me touching the mixer at all.
During winter, the water fluctuates between freezing and scalding.
Where do you think the root cause of the issue is?
Could it be the new shower head/mixer, with a higher flow rate than old one, meaning my little under-sink water heater can't keep up? Causing the fluctuating temperature.
Or could it be my (admittedly quite old) water heater breathing its last breath?

Also, if you think the heater is on its way out, and have any suggestions of good, under-sink style water heaters that are suitable for a single shower, bathroom and kitchen sink, please let me know!

Cheers!

 

[Geofd]

If before your renovation all was fine I would call the contractor back if your hot water heater is older that could also be it but you would notice hot water issues everywhere if you added a higher flow head that could also be an issue, try changing the head to what it was before and see what that does

 

[zamandguth]

If before your renovation all was fine I would call the contractor back if your hot water heater is older that could also be it but you would notice hot water issues everywhere if you added a higher flow head that could also be an issue, try changing the head to what it was before and see what that does

Thanks for the reply Geofd
Everything else in the house (bathroom sink and kitchen sink) is fine, you can only notice the issue in the shower.
Also, unfortunately the old shower head is long gone and probably wouldn't fit the new setup (traditional shower head changed to a rainfall style)
Water heater (instantaneous style) is probably over 10 years old.
Maybe I should try to find a lower flow rate shower head that fits the current setup?
Or would this be a good excuse to upgrade to a different style of water heater?
Cheers!

 

[Geofd]

Just to get your answer try a conventional shower head that will give you your answer
Then someone can calculate flow rates and what size heater you need IF that's the case

 

[Rickyman]

What type and brand of shower valve/mixer? Possibly post a picture of it

 

[zamandguth]

What type and brand of shower valve/mixer? Possibly post a picture of it

Thanks Ricky, I have attached some pics, no brand name in sight so I will go hunting for the invoice. Does it shed any light on my situation?

Just to get your answer try a conventional shower head that will give you your answer
Then someone can calculate flow rates and what size heater you need IF that's the case

Thanks Geofd, having a look at the pictures attached, do these look like particularly high flow rate parts, or quite standard? I'm also not sure how I would attach a conventional shower head to this arm?

Cheers all!

 

[Geofd]

If the renovation was recent can't you call the plumber back,????? You maybe able to remove the rain head it would not have passed inspection unless it maintains a certain temp

 

[zamandguth]

If the renovation was recent can't you call the plumber back,????? You maybe able to remove the rain head it would not have passed inspection unless it maintains a certain temp

Indeed, but he won't be available until late January due to the xmas/new years holidays, so I wanted to get on the front foot.
That being said, I'm sure he will suggest to replace/upgrade the water heater, whereas I find it incredibly unlikely that the heater has suddenly died the second a new bathroom goes in, more likely that the rain head flow rate isn't a good match for the little heater.
So, I was hoping to find an alternate solution before dropping a few grand on a water heater.
Cheers!

 

[Aggie83]

I had a similar problem last year on a new shower setup. go from hot to freezing to warm in the space of two minutes. Water everywhere else was fine. Replacing the valve cartridge fixed the problem.

 

[GReynolds929]

Does the water heater service the whole house where temperature is stable, or just the bathroom that is having the temperature issues?

 

[FossilPhill]

Indeed, but he won't be available until late January due to the xmas/new years holidays, so I wanted to get on the front foot.
That being said, I'm sure he will suggest to replace/upgrade the water heater, whereas I find it incredibly unlikely that the heater has suddenly died the second a new bathroom goes in, more likely that the rain head flow rate isn't a good match for the little heater.
So, I was hoping to find an alternate solution before dropping a few grand on a water heater.
Cheers!

Assuming that this cold to hot cycle repeats during your shower time, and assuming the water flow remains constant, I would suggest that your heater is quite capable of heating that flow, but something is either turning the heater off (in cycles) or, cold water is being introduces via a tempering valve that is malfunctioning, or plumbed incorrectly possibly.

 

[zamandguth]

Thanks for all the replies guys, really appreciate it!

I had a similar problem last year on a new shower setup. go from hot to freezing to warm in the space of two minutes. Water everywhere else was fine. Replacing the valve cartridge fixed the problem.

Awesome Aggie thats great to hear, I might attempt to do the same thing! The cartridge was faulty in a new mixer tap, or did you change it to a different model/style?

Does the water heater service the whole house where temperature is stable, or just the bathroom that is having the temperature issues?

Yep correct it services the whole house. Its only the bathroom shower that has the temperature issues, the bathroom sink tap is totally fine. Same with the kitchen.

Assuming that this cold to hot cycle repeats during your shower time, and assuming the water flow remains constant, I would suggest that your heater is quite capable of heating that flow, but something is either turning the heater off (in cycles) or, cold water is being introduces via a tempering valve that is malfunctioning, or plumbed incorrectly possibly.

I am not aware that I have a tempering valve, at least it is not visible anywhere near the instant water heater. Are these only used for tank-style water heaters, or would it be hidden behind a wall somewhere?

Thanks again everyone!

 

[MicEd69]

The valve in the wall likely has an antiscald feature that "controls" the mix of cold and hot water supplies. That is what failed in Aggie83's situation. It can very well be that yours has a defect that is causing your issue as well. As a test for your water heater, turn on your shower and the nearest sink hot water faucet and monitor the sink's temperature. If the issue is the water heater and the high flow rate to the shower, the hot water to the sink will likely vary like the shower varies.

I'm betting on the shower cartridge.

 

[zamandguth]

The valve in the wall likely has an antiscald feature that "controls" the mix of cold and hot water supplies. That is what failed in Aggie83's situation. It can very well be that yours has a defect that is causing your issue as well. As a test for your water heater, turn on your shower and the nearest sink hot water faucet and monitor the sink's temperature. If the issue is the water heater and the high flow rate to the shower, the hot water to the sink will likely vary like the shower varies.

I'm betting on the shower cartridge.

Hi MicEd, awesome I agree that sounds like a good test, thank you!
I'm hoping its the shower cartridge, I'll let you know how the test goes!

Cheers