Immersion hot water

[Ian Cowley]

In message [e-mail], Pete C burbled:

We tried that a few weeks ago - the shower temperature doesn't change all the way down to shutting off the cold water entirely - at which point the hot water also stops. My understanding of the physics is that if you have less cold water flow it'll 'suck' less hot water from the tank - the result is that the mixing ratio is the same.

By the way, we're in a first-floor flat. The hot water tank is in our flat, so the head of water is really small - the shower head is at about the same level as the top of the water tank. I think that's why the shower doesn't get hotter with lessened cold water pressure - there's no flow of hot water without the venturi effect.

[Pete C]

I see, fair enough.

The hot water pressure will be governed by the level in the cold water tank the hot tank is fed from, which should be a bit higher up in the attic.

Something else to check is to take the thermostat out, put it in a pan of water, heat it up and see what temperature it opens. It's possible the thermostat pocket has been bent towards the heating element or the immersion heater orientated in such a way that the thermostat opens prematurely.

Another potential problem is there may be strainers on the inlet to the venturi shower that have been blocked with dislodged scale. If the shower hose itself is quite narrow, or there is any scale blocking the shower head that won't help either.

Something to try would be compare the hot flow just before the shower mixer with a 1/2" hose siphoning from the same cold water tank that feeds the hot tank, it should be at least as good.

If all else fails and the thermostat itself is faulty there are some thermostats on this page that should do:

However I'd set it to a sensible temperature using an decent thermometer in a pan of water, rather than a dangerous temperature to compensate for any problems. Otherwise there's a risk of people getting injured later on for reasons you may not appreciate right now.

cheers, Pete.

[Ian Cowley]

In message [e-mail], Pete C burbled:

Unfortunately, the cold water doesn't come from the attic but a small header tank on top of the hot water tank itself, which is fed from the mains. Thus the head of water for the hot water is near ceiling level on our single-level flat. Cold water for all taps seems to come straight from the mains :)

I'll check to see if there's any scale near the shower, but I think the plumber did that the last time he spent ages looking for a problem.

I've installed an old-style thermostat my (electrician) father gave me at the weekend. It has a temperature scale on the dial from 50 to 85 degrees (rather than the lame + and - on the new ones). I set that last night and this morning's water is 62°C versus 56°C the other day with the +/- thermostat. As such the shower was decently hot with the dial on 3/4 of the way round. So we're in a more sensible place for when the incoming cold water starts to cool down in the autumn (i.e. there's adjustment on the shower temperature now) and I might even up the stat a little more. the hot water from the taps feels much more like it used to - unpleasantly hot rather than the recent bearably-hot stuff we've had. I know there are issues with having scalding water, but we've had that for the last 8 montsh and we're happy with it :)

So it looks like it was a simple case of the new stats being colder than old style ones. i now have a small stash of old style ones in case they ever fail :)