Replacing 9kw shower with 10.8kw - been told 6mm cable is fine

[ban-all-sheds]

As a general note to anybody reading this in the future (it really belongs in Plumbing and CH, but the people there have problems in the reading dept) - if you are putting a shower on a combi, make sure you buy a mixer which can cope with a small Δt. The temperature of DHW from a combi is lower than that from a hot water store, and some thermostatic mixers work fine with the latter, where there is a big difference between the incoming hot and the outgoing mixed, but cannot cope with the much smaller difference when the incoming hot is from a combi.

 

[JohnW2]

... if you are putting a shower on a combi, make sure you buy a mixer which can cope with a small Δt. The temperature of DHW from a combi is lower than that from a hot water store, and some thermostatic mixers work fine with the latter, where there is a big difference between the incoming hot and the outgoing mixed, but cannot cope with the much smaller difference when the incoming hot is from a combi.

Interesting. Most mixer showers seem to indicate whether or not they are suitable for use with a combi, but I confess that I have always thought that was related to pressure, rather than temperature.

I do not have a combi myself, but I know plenty of people who do, and I must say that I have never really noticed their DHW temperature being any lower than I am used to with stored hot water.

Kind Regards, John

 

[EFLImpudence]

Surely, with a combi the DHW is at mains pressure.

Stored in vented cylinder, it can be very low.

 

[JohnW2]

Surely, with a combi the DHW is at mains pressure.

Exactly. Stored in vented cylinder, it can be very low.[/QUOTE]Exactly. Traditionally, therefore (in the 'pre-combi' days), thermostatic mixers did not have to be able to cope with high pressures, but when fed from combis they do (both hot and cold water). That's why I assumed, maybe wrongly, that those said not to be suitable for use with combi boilers were not able to 'cope' (or, at least, function satisfactorily) with high pressure feeds.

Kind Regards, John

 

[EFLImpudence]

I thought you were implying that it would/should be the other way round.

I do not have a combi myself, but I know plenty of people who do, and I must say that I have never really noticed their DHW temperature being any lower than I am used to with stored hot water.

Just had a quick look and the first random one recommends a pressure of 1 to 5 bar so ...

 

[JohnW2]

I thought you were implying that it would/should be the other way round.

No

Just had a quick look and the first random one recommends a pressure of 1 to 5 bar so ...

A quick look at what - combis or mixer showers? What is your point?

Kind Regards, John

 

[EFLImpudence]

A quick look at what - combis or mixer showers? What is your point?

Mixer showers. If they can handle 5 bar then they must be ok for combis, surely.

Am I missing something?
I do seem to be getting confused the last couple of days but I was putting that down to others.

 

[JohnW2]

Mixer showers. If they can handle 5 bar then they must be ok for combis, surely.

Of course. As I said, a lot of mixers around do say that they are OK with combis (although, since BAS has raised the issue, I'm no longer sure it is because of pressure, rather than DHW temp!).

However, your 'random' one seems to have the problem 'the other way around'. You said that it needed 1 to 5 bar. Even in my three storey house, I probably would not be able to quite get 1 bar (about 33 feet of water) from my vented DHW system, even for a shower on the ground floor (and no hope for one any higher up the house) - so you seem to have found one which is only suitable for use with a combi!

Kind Regards, John