Combi boiler with 2 showers at once - enough power?

Madrab · 9 Jan 2022

Ah, ok, you have a communal cold roof space cistern, quite high above you is it? Any idea roughly in M? To give a guide measure the height of your room times how many are above till getting to the cistern in the roof.

A combi will always struggle, it just in their nature. You will certainly get one good shower, that's for sure but even then it will drop if you use another HW outlet, it's their one major drawback.

You can certainly lose the mexico and put a wall mounted boiler in, how that's to be used is another question. It really needs a close look at what you want, what you think will be acceptable and from there what would be the best model to achieve that. 10L/min is all fine and well but if your cold mains can't delivery that dynamically then you'll struggle, pressure is a key factor to that and that is all centered around what your mains can deliver.

floob · 9 Jan 2022

oldbutnotdead said:
Do you know where your drinking water comes from (kitchen cold tap may be the only one fed from rising main rather than roof tank). And have you checked the flow rate on the rising main- don't be surprised if it isn't all that impressive. Having your own vented hot water cylinder does insulate you from work being done on the pipework- someone turns supply off to combi, no hot water as well as no cold.
Modern control systems can be grafted on to older boilers easily enough.
Space saving- fair enough. Though I'd rather have 2 decent showers than space for more Stuff (I do have a lot of space though, end of the day it's up to you).

The cold water/drinking water comes from the mains, not the tank. All our cold taps appear to be on that feed. Cold water comes out of the taps at a fair old rate, the plumbers who have quoted all checked it so I guess thats relevant. They said it was a 'decent' pressure, I'm not sure how to measure it unless I see how much is chucked out in min?

floob · 9 Jan 2022

Madrab said:
Ah, ok, you have a communal cold roof space cistern, quite high above you is it? Any idea roughly in M? To give a guide measure the height of your room times how many are above till getting to the cistern in the roof.

A combi will always struggle, it just in their nature. You will certainly get one good shower, that's for sure but even then it will drop if you use another HW outlet, it's their one major drawback.

You can certainly lose the mexico and put a wall mounted boiler in, how that's to be used is another question. It really needs a close look at what you want, what you think will be acceptable and from there what would be the best model to achieve that. 10L/min is all fine and well but if your cold mains can't delivery that dynamically then you'll struggle, pressure is a key factor to that and that is all centered around what your mains can deliver.

I would estimate its about a good 8 meters above me. The cold water from the mains in the flat seems to have decent pressure, if I try a hot tap then the cold tap, the cold tap runs faster - at a guess 30%.
The mexico was installed in 1990 in the middle of the flat and the plumbers dont seem keen on putting anything there, so the new combi is destined for the kitchen wall.

I think you commented on this last year actually

https://www.diynot.com/diy/threads/looking-for-recomendation-gas-vs-electric-heating.563281/

I appreciate the existing boiler is still working, but I'm not sure how much longer I should rely on it for given its timers on the way out and no-one seems to have parts.

I'm hoping as it looks like I need about 20l/m to get 2 showers and this says its capable of 17l/m it might be pretty close to what I need.
https://www.plumbnation.co.uk/site/...35kw-b1kf-combination-boiler-natural-gas-erp/

Madrab · 10 Jan 2022

Looks like I was suggesting the same then too

To understand if you would get anywhere near the combi's output, first thing you will really need to do a pressure and flow test to understand what your mains is delivering, not difficult to obtain with a cheap pressure gauge on the washing machine valve and a bucket and timer. You will probably need to upgrade your gas supply to allow a combi that size to be installed so budget for that too.

The combi's output is a design target from the manu in perfect conditions, there are other factors that can impact on that, output temps selected, cold water input temp, pressure and flow from the mains during peak times etc, I wouldn't plan things on a theoretical figure. A combi will be lucky to deliver what it says it can do on paper, it really is just the nature of combi's unfortunately.

floob · 10 Jan 2022

Madrab said:
You will probably need to upgrade your gas supply to allow a combi that size to be installed so budget for that too.

Does that mean most 35Kw combis would need more than a 'standard' gas supply?
The plumbers I've had quote so far havent brought this up so I'll see if they are expecting me to do that separately.

denso13 · 10 Jan 2022

cdbe said:
I had two reasonable showers with my old 28kw combi.

Same here. One is good pressure, two at a time not as good but acceptable.

Madrab · 10 Jan 2022

floob said:
Does that mean most 35Kw combis would need more than a 'standard' gas supply?
The plumbers I've had quote so far havent brought this up so I'll see if they are expecting me to do that separately.

Depends on what is supplying your current boiler. Some supplies were run in 1/2" 15mm for boilers, if yours is 22mm then it may be suitable, depends on how long a run it is from your gas meter. I would suggest it should be at least tested.

floob · 10 Jan 2022

denso13 said:
Same here. One is good pressure, two at a time not as good but acceptable.

Ooh - that sounds good, thanks for the feedback. Hopefully mine will be similar.

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Combi boiler with 2 showers at once - enough power?

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