Plumbing Questions? Ask a Plumber for Answers ASAP
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.
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)
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.
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.
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.