DIY Doctor

Fix or replace immersion and heating system?

Postby steamrolla » Thu Jul 16, 2020 9:17 pm

I am no where near capable of being a plumber and anything written below is taken from my research only.

Bought a 1995 one bed single story flat in London. The flat has electric wall mounted heaters and a vented direct cylinder for hot water. The cylinder is heated via a single immersion heater. There is a header tank directly above the cylinder at a height of 170cm, while the cylinder is an inch off the ground. All of the cold water taps in the flat are running off mains.

On initial inspection the cold water flow is excellent (~24 l/m) while the hot water flow is terrible (~4 l/m in shower/bath and ~3 l/m in kitchen). Note, the taps in the bath / kitchen are roughly 92cm off the ground, therefore about 78cm below the header tank.
The flat has recently been refurbished and all the faucets look to be brand new, I do not believe any consideration was taken into the faucets specs as I managed to get hold of the manual for the kitchen faucets which says a min head of 3m (min pressure 0.3 bar) is required.

Below are images of the setup;
https://ibb.co/WkJydKb
https://ibb.co/n1B5bsK
https://ibb.co/JtnxWwk
https://ibb.co/wMDkHGT
https://ibb.co/88S2qj8
https://ibb.co/0Ytt8S2

Note, the electricity to the immersion heater has been turned off for all the tests performed below (reason: we haven't moved in yet), therefore the water in the cylinder is cold.

Problem 1: The low hot water pressure. I understand this is to be expected in a vented system with such low difference in height between the header tank and taps. Can someone please confirm if these flows (3 - 4 litres per minute is about right for this height? The pipes I believe are 15mm).

Problem 2: When turning on the shower mixer with hot and cold water, the vent pipe started flowing into the header tank and subsequently overflowed the tank as the overflow pipe didn't seem to take any water.
From my research it seems like the unbalance in pressure between the cold, main water supply and the really low hot water supply is what is causing the hot water to be pushed back up the venting pipe into the header tank.
I believe the correct solution to this problem is to increase the hot water pressure however with the current system I believe this would mean installing a pump of sorts, however there seem to be mixed results and potentially more issues going this route? Is this the right way to be going or would our money be better spent in converting to a combi boiler or a unvented system that would supply the hot water directly from the mains which would mean our water pressure would be the same for both hot and cold.

Problem 3: After this test I turned on the immersion heater and immediately noticed a whistling noise coming from the cylinder. Since the water would be stone cold at this point my thoughts are that this noise is being caused by the immersion heater itself? It may be caked in limescale as well as there being lots of limescale inside the aging cylinder. Is this thought correct or could this potentially be the cylinder taking in air which may be an explanation for the poor hot water pressure / flow?
Again, in order to fix this we may need to replace the immersion heater and potentially clean out any debris from the tank. This also seems like it might be like flogging a dead horse. I am not sure on the age of the cylinder but it might be as old as the flat... 25 years?
Im not convinced it is a air leak as the whistling start immediately after switching on the immersion heater in a stone cold tank...

My thoughts are that while we could look to revamp the current vented system, we will never achieve very good hot water pressure without investing in a pump or replacing the entire system with a combi / unvented.

Given that the flat is a 1 bed, what would be your recommendations?

1. Clean out / service existing system and invest in a pump.
2. Replace with Combi. - I wasn't actually sure if we needed a combi as our heating is electic, but basically what system could we use that would heat the water on demand directly from mains?
3. Replace with unvented.

Finally, any recommended plumbers in South East London, SE7?
steamrolla
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Postby stoneyboy » Thu Jul 16, 2020 10:00 pm

Hi steamrolla
Suggest you go for a mains fed unvented hot water cylinder.
Regards S
stoneyboy
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