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Tim Cooke
Tim Cooke,
Category: Plumbing
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Experience:  Owner at Self-employed
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The shower in our en suite gives off very warm (but not hot)

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The shower in our en suite gives off very warm (but not hot) water when we turn the thermostat up to full and open the shower head to the widest setting. However, when we turn the shower head to "massage" mode (narrower setting, less water coming out), it goes lukewarm. I recently replaced the head with a water-saving head that lets even less water out, but the water only comes out cold. When I take the shower head off the hose completely (so the water runs through the narrow hose), the water is warm!Separately, when we turn the basin tap on at the highest temperature setting, we get only stone-cold water. But if we turn the shower on at the same time, the water from the basin tap turns to boiling hot!It seems to me that the shower and basin taps run off the same pipe, and the hot water comes through only if there is a sufficient flow rate. Otherwise, we are getting cold water. Both the basin and shower user mixers.Do you know what could be causing this?
Hello, this is a fairly common occurrence but with many possible causes.

Please tell me about your heat source. Do you have stored hot water, or a combi boiler, or something else?
Customer: replied 9 days ago.
Hi Tim. Thanks for coming back to me so quickly. We have a Vaillant eco boiler and a Megaflo hot water tank (I believe unvented). It's also worth noting (I forgot) that we often get a bit pressure build-up in our hot water, such that, when we open a hot tap, it often rushes fiercely out, then the pressure calms back down...
The initial high flow rate is likely to be the natural result of the pressure in the Megaflo - this happens with any unvented vessel.

However, if the flow rate drops off markedly then it suggests that your incoming mains cold rate is poor. Are you certain that all stop cocks and isolating valves in the cold supply to the Megaflo are fully open?
Customer: replied 9 days ago.
Thanks, Tim. I think there are known issues with the cold water pressure on our road. The plumbing system dates back to the '30s and the neighbours often speak of issues. I'm not aware of any stop cocks or isolating valves in the cold supply being closed... But does this bear on the original problem, namely the hot water only coming through when there is a high flow rate?
This is going to sound slightly evasive of me, but I’m eliminating possible causes.

And this will sound pedantic, but I chose my wording carefully, so please indulge me - I have a reason for asking it this way: are you certain (or uncertain) that all of the relevant valves are fully open?
My next question:
Is there another tap in the house where the water runs hot quite readily, at a good flow rate?
Customer: replied 9 days ago.
No problem. I would have to say I'm uncertain that all of the relevant valves are fully open, because I have no idea where they are... There are other taps that run hot water readily. For example, the bath and shower in our main family bathroom both run nice hot water at a good rater, but I do know that the pipes to those faucets T off very early after the tank (they were installed when an extension was built), whereas the shower and basin in our en suite come off a longer circuit (they seem to be hooked up to the original piping).
Hmm. Intriguing. Logically, if you have poor flow to one room or area of the house, it suggestions a construction. It’s possible that there are two independent problems in the same room, i.e. a shower valve problem and a tap problem. While I’m thinking further, could you tell me the type of tap and type of shower valve? Make & model would be nice, otherwise a description.
Two more questions:
1. In your opening post, does “lukewarm” mean cooler than “very warm”?
2. If you keep the shower shut off, but turn its temperature setting to ‘cold’ (but still off) and then open the basin tap, do you get hot water from the basin tap?
Customer: replied 9 days ago.
I should say that the strength and flow of the water in the problem areas is the same whether it is hot or cold. It's just the temperature that changes. So, for example - if I have the basin tap on and turned to hot, it is stone cold. If I then turn the shower on and keep the basin tap on, the basin tap water goes scalding, but the flow rate doesn't change.

The basin tap and shower thermo mixer are "hansgrohe". The mixer also says "Axor" on it. There is no make or model on the shower faucet, but I assume it's also hansgrohe.

1. Lukewarm = cooler than very warm.

2. This is slightly complicated. I will explain in next message.
Customer: replied 9 days ago.
2 (cont). I started to run the basin tap at maximum hot with the shower mixed at maximum hot. The water from the basin tap was cold. I turned the shower mixer to maximum cold (I left the basin mixer as it was), and the basin tap water turned lukewarm (definitely not hot). I then turned the shower mixer back to maximum hot, and the basin tap water went back to stone cold. Finally, I turned the shower mixer back to maximum cold, but the basin tap water remained stone cold. (My hands are now shivering! :-) )
That’s all very detailed and therefore helpful - thank you. This will be an interesting one to solve!

I’m just re-reading your answers to understand the permutation that results in hot water at the basin. In the meantime, two more questions:

1. Is the basin tap a mixer, or a pillar tap?

2. What is the “shower faucet”? Is that the flow control for the shower?
Customer: replied 8 days ago.
I like to be thorough :-) To answer your latest questions:

1. The basin tap is a pillar tap.

2. By "faucet", I was referring to the tap that turns on and controls the water flow (rather than the "tap" that controls the water temperature), although the flow rate remains fairly constant after turning the tap only a fraction of its total axis.
Something I forgot to ask earlier:

1. When did this problem start?
2. Did the symptoms start suddenly, or gradually?
Customer: replied 8 days ago.
1. To be honest, I can't be sure. I don't recall a time when the basin tap didn't have problems. We have different problems with the hot water that come and go. For example, the shower hot water has fluctuated over time. Only a week or so ago it was tepid and not warm. Now it's warm again, albeit not as hot as it should be on the very highest setting. As I say, curiously, the more water I allow to pass through the shower flex and into the shower head, the warmer the water that comes out. So, for example, if I open the shower head to a wider setting, the water gets warmer. I even disconnected the shower head and remove a rubber washer, which means that water is now coming out of the shower head and flowing out through the connector and down the outside of the flex, and this had made the water warmer still!

2. Then come and ago, but when they do it seems pretty sudden.
Could I also confirm that when you did my suggested experiment with the shower faucet off, and running the basin hot tap, and turned the shower temperature control to fully cold, the shower faucet was still off?
Customer: replied 8 days ago.
Yes - it was definitely off. There was no water running through the shower.
I’m still thinking, and I’m leaning towards the the shower valve having a fault. I’ll explain more in a moment. Was the shower installed before you had the Megaflo, by any chance?
Customer: replied 8 days ago.
I'm afraid I don't know that. Both the shower and Megaflo were installed when we bought the house.
I just typed a long reply and the phone App crashed. Bear with me and I’ll live to the PC...
Customer: replied 8 days ago.
Ok. I'm sorry! I hate it when that happens.

I've come across a rare situation with showers, but many times, where the supplies are unbalanced - the cold is from mains cold but the hot from an vented cylinder. This is wrong, but sometimes works. And sometimes it works but only for a while. The failure mode is that an interval seal in the shower gives way, and even with the faucet off it will leach cold water from the cold inlet to the hot inlet, and backfeed the hot service. Water will take the path of less/least resistance, so if you open a nearby tap (like the one at your basin) you get cold water flowing via the shower valve. The exact behaviour can vary according to time of day as the incoming mains pressure various in your neighbourhood, and other demands in the house.

So far, this fault condition is consistent with the symptoms you're seeing. You might now ask "how to fix it".

What I usually do to prove the theory is install an isolating valve on the cold supply to the shower. This can be very easy to do, but can be very hard - it depends on your pipework layout and your access to it. If there's already a valve somewhere, your way ahead is already easy - you simply shut that valve and then run the basin hot tap, and see what happens.

Are you following so far?

The reason I asked about age of shower relative to Megaflo is that the shower might be older, and might have been running on unbalanced supplies early in its life, and been damaged a long time ago.

 

Customer: replied 8 days ago.
I'm following. This is the most sensible potential explanation a plumber has given me so far to this problem, so thank you. I was wondering whether that might be some kind of backfeed. The only thing I'll say in response is that, when I set either the basin or shower to run, then go to the tank and put my hand on the hot water outflow pipe, it is hot. I don't know whether it's as hot as it should be, but I can't hold on to it, so I wonder whether the backflow is going all the way to the tank or is simply routing down other pipes.

On the point about an isolating valve, I had read something about this online. I'm not sure where it would be fitted. The pipework layout is not obvious, and the shower mixer is mounted on a tiled wall. So I'm guessing we'd need to go behind that wall or go under the floorboards.

It's not impossible the shower is older. The tank was moved at some point when the previous owners but one did their extension. I just can't confirm, I'm afraid. But it is a possibility.

Regarding the backfeeding, I wouldn't expect the cold water to reach the Megaflo - that would be very strange. The path of lesser resistance occurs only when you open the basin tap. Well, at enough of a pressure differential for any more than a negligible flow.

This usually foxes almost all plumbers. I've even had online arguments with other plumbers who say that it's impossible. It's very possible, and I've fixed it many times.

You could play the hunch and replace the Grohe cartridge, but they're not cheap. I would attempt to second-guess the pipework route from cylinder to the problem bathroom, and if it means lifting floorboards then so be it. It might help you to imagine where the branch to shower/basin is placed.

Ultimately, if this theory is proven, you won't have any choice but to replace the faulty Grohe cartridge.

(not interval seal - *internal seal)

 

Customer: replied 8 days ago.
Thanks, Tim. That makes sense. I guess we'll be lifting some floorboards! Ultimately, we're planning on re-fitting this bathroom completely, so replacing the cartridge would probably be disproportionate cost when we may well do away with it soon anyway. So we'll go with investigating the pipework and, did you say, installing an isolating valve? Then at least we know what the problem is and can decide whether to replace the valve or accelerate the renovation.

Out of interest, do you know roughly how much a Grohe cartridge replacement costs?

I've just thought of another way of [nearly] proving the theory...

1. Wait until everyone is out, or ask them to refrain from running ANY hot water for the duration of this short test.

2. Shut off the mains cold supply.

3. Open the problem basin hot tap, until the Megaflo pressure depletes and the water stops, then shut that tap.

4. Open the mains cold supply and allow the Megaflo to 'recharge'.

5. Repeat steps 2 & 3, and see if you get VERY hot water from the hot tap. You might have to repeat a few times until you get the hot water, but if I've got this right then it can't fail to run hot, so have patience.

 

Another way to test, but with a less certain outcome...

1. As above.
2. Run the kitchen sink cold tap at full flow, or a garden tap if you have one.
3. Open the problem basin hot tap and wait for it to run hot.

With that second type of test, if you don't get water it doesn't disprove my theory, but if you get nice hot water then it tends to support the theory.

Customer: replied 8 days ago.
The second test sounds less drastic, so I might try that now. Then I'll try the first test if I get no luck.

Regarding the cost of the cartridge, I'd have to look it up. You can get an idea of it if you just Google Hans Grohe Axor.

Yes the 2nd test is easier, and you can do it tonight. The problem is the lack of any conclusion if you don't get hot water. With the 1st test I expect you to get hot water without any doubt, therefore it will reveal something interesting if my forecast turns out to be wrong.

OK. That's me for tonight. I have an early start in the morning. I'll be grateful if you have time to update me after your test(s). If you want to talk on the phone then I could do that sometime tomorrow afternoon. If you happen to live close to me (Stevenage) and want me to visit then I'd be prepared to.

Customer: replied 8 days ago.
Tim, I just ran the second test. I left the kitchen tap on cold (it has a pretty high flow rate) and then opened the en suite basin tap on hot. It started out cold, then got lukewarm. Then it progressed to hot until it was so hot I had to take my hand away.
Customer: replied 8 days ago.
Thanks, Tim. Very much appreciated. I'm actually in St Albans, so, if you do get some time in the neat future, it would be great if you could swing by and do some more investigating.

Aha! Well that's very promising :-)

I'll be in St Albans sometime next week - not sure which day yet.
My number is 07753 637786 if you want to text me your contact details.

Tim Cooke and other Plumbing Specialists are ready to help you
Customer: replied 8 days ago.
Perfect. Thanks, Tim. I'll text you my details now. Have a great evening and speak soon.