# Combi boiler with 2 showers at once - enough power?

#### floob

I am currently using a system boiler (Ideal Mexico Super 2) and water tank, which is about 30 years old. I'm looking to replace this with a Viessmann 100-W (B1KF) 32Kw combi.

I was wondering would I be likely to see a significant difference if 2 showers run at once?
I'd like to avoid needing to go for a Viessman 111-W if at all possible. If relevant, the cold water pressure appears to be pretty strong in the property.

We have 2 bathrooms (1 of which is en-suite), both have showers over bath.

Yes ,there will be a difference with two showers running simultaneously ,compared to one running on its own.
No matter what make or model of combi you choose.

Yes ,there will be a difference with two showers running simultaneously ,compared to one running on its own.
No matter what make or model of combi you choose.

Is there anyway to get an idea of the difference? I mean if both run, would neither of them get hot or reach a 'reasonable' pressure?
I presume there would be a difference in pressure using a 32Kw vs the 19kW 100-W variant?

A combi boiler will deliver X amount of litres of hot water at a given temperature. Let's say for instance 10 litres per minute at 40degreesC, for arguments sake. If more than one outlet is open ,say One shower and one tap ,that 10 litres would be shared between the two outlets ,possibly equally ,and probably disproportionately. The combi can't double the flow because two outlets are in use .

Bit of homework for you- run your shower, set the temperature and flow so it is to your liking for the hottest maximum flow shower you will ever want then measure (bucket and stopwatch) the volume delivered per minute and the temperature.
This'll give you some numbers.
Let's say you find 12 litres/min at 45 degrees is your maximum shower.
Double the volume (2 showers) so you now need 24 litres/min, you might want to check that your water supply can deliver this at adequate pressure (at the moment you have a tank and gravity on your side).
Assume your cold water supply is at 10 degrees, so you need to warm 24l/min by 35 Deg C.
Now some sums.
Short version- for every litre of water you want to heat by 35 Deg C you need about 2.5kw of energy delivery capacity.
So 24l/min at ∆35°C is going to need near enough 60kw.
See where you get to with your measuring.

Bit of homework for you- run your shower, set the temperature and flow so it is to your liking for the hottest maximum flow shower you will ever want then measure (bucket and stopwatch) the volume delivered per minute and the temperature.
This'll give you some numbers.
Let's say you find 12 litres/min at 45 degrees is your maximum shower.
Double the volume (2 showers) so you now need 24 litres/min, you might want to check that your water supply can deliver this at adequate pressure (at the moment you have a tank and gravity on your side).
Assume your cold water supply is at 10 degrees, so you need to warm 24l/min by 35 Deg C.
Now some sums.
Short version- for every litre of water you want to heat by 35 Deg C you need about 2.5kw of energy delivery capacity.
So 24l/min at ∆35°C is going to need near enough 60kw.
See where you get to with your measuring.

So the flow is coming out at 10 l/min (on one shower) and I'm running with 10 degree start and finishing at 40 which is the max I have it.
Based on this calc: https://myboiler.com/calculators/hot-water-flow-rate-to-kw-input/ I would need 41Kw - so if I ran with the 32Kw it might not get me to exactly where I want to, but not massively far off? I guess I need to bear in mind this is only during those times we run 2 showers at the same time.
(If relevant, I'm in an apartment, so the water tank is same level as showers).

Presumably you mix some cold with hot. I had two reasonable showers with my old 28kw combi.

Is your hot water from an unvented cylinder ?

So the flow is coming out at 10 l/min (on one shower) and I'm running with 10 degree start and finishing at 40 which is the max I have it.
Based on this calc: https://myboiler.com/calculators/hot-water-flow-rate-to-kw-input/ I would need 41Kw - so if I ran with the 32Kw it might not get me to exactly where I want to, but not massively far off? I guess I need to bear in mind this is only during those times we run 2 showers at the same time.
(If relevant, I'm in an apartment, so the water tank is same level as showers).
Only 30% off, yeah it's close.....Is that 10l/min the most it'll do or the most that you want? Any particular reason you want rid of the cylinder and header?- since you already have them, you could give yourself a luxury shower setup using the combi on 1 shower and the tank on the other shower/bathroom.
EDIT Yes being an apartment can make a difference if you're in a tower block...

Presumably you mix some cold with hot. I had two reasonable showers with my old 28kw combi.

I have no idea, I just set the heat dial thing to the temp I want - It has 38 on the safety notch and I have it a bit below that at around 35.

Is your hot water from an unvented cylinder ?

No idea - the water tank in the flat is fed from the roof water tank (i think) then the gas boiler or immersion heats that.
But all the hot water comes from that tank.

Only 30% off, yeah it's close.....Is that 10l/min the most it'll do or the most that you want? Any particular reason you want rid of the cylinder and header?- since you already have them, you could give yourself a luxury shower setup using the combi on 1 shower and the tank on the other shower/bathroom.
EDIT Yes being an apartment can make a difference if you're in a tower block...

10l is the most it does at the temp I want. I want rid of the old system as its over 30 years old now and has an incredibly limited scheduling ability on the boiler. I want to remove my reliance on the tank and the pipes that feed from the roof as there often seem to be repairs going on to various pipes in the building. I just want to bite the bullet and modernise. And it'll give me 2 large cupboard spaces back (new boiler is going on kitchen wall).

It all depends on what you want from your hot water system.

If you are running 2 'normal' mains pressure fed bar showers I can just about guarantee you wont see 2 good showers from most combi's. If you are running 2 water saving heads with low output showers then you may be able to get there but I can assure you they won't be very good. Again though I refer you to the 1st point, it all depends on what you want

If you mains is good enough then an unvented cylinder is you best best as it will happily feed 2 good strong showers with unfettered 5 position heads or rain showers and still have enough to run other outlets, it is mains dependents though. You can do away with the cistern in the attic but you will need the cylinder space.

It all depends on what you want from your hot water system.

If you are running 2 'normal' mains pressure fed bar showers I can just about guarantee you wont see 2 good showers from most combi's. If you are running 2 water saving heads with low output showers then you may be able to get there but I can assure you they won't be very good. Again though I refer you to the 1st point, it all depends on what you want

If you mains is good enough then an unvented cylinder is you best best as it will happily feed 2 good strong showers with unfettered 5 position heads or rain showers and still have enough to run other outlets, it is mains dependents though. You can do away with the cistern in the attic but you will need the cylinder space.

Thanks for the feedback. The water tank is on the apartment building roof and isnt going anywhere soon as most apartments still use it (bar the ones that went to combi), I want to get rid of my reliance on it (due to the buildings plumbing). What I'd like is, at a minimum, to have hot water supplied to 1 shower with at least the same pressure as I have now. Which seems to be 10l per min. And ideally, to have 2 useable showers at once, as in more than a dribble. If I can lose my ideal mexico, and hot water tank I get 2 large cupboards back which would be a bonus.
I suppose worst case I could go for a Viessmann 111-W with the built in tank, but I really dont want to if I can help it.

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).

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