The final say, combi boilers and two showers....

Can a combi boiler supply 2 showers with no noticeable drop in performance at the same time.

  • YES, a combi boiler can supply two showers at the same time.

    Votes: 2 7.7%
  • NO, a combi boiler cannot supply two showers at the same time.

    Votes: 9 34.6%
  • Maybe, too many variables, this is a rubbish question.

    Votes: 15 57.7%

  • Total voters
    26
  • Poll closed .
Joined
18 Oct 2005
Messages
326
Reaction score
17
Location
Essex
Country
United Kingdom
So after going round in circles, reading manufactures websites with quotes such as:

"Suitable for larger homes with one or two bathrooms"
"Suitable for homes with more than one bathroom"
"appropriate for 3 or 4 bedroom properties, with 1 or 2 bathrooms"
"for large 3-4 bedroom properties with more than 2 bathrooms"

So with good incoming water pressure, and 2 "average" showers can it be done?

Cheers
 
Sponsored Links
Joined
3 Nov 2006
Messages
26,613
Reaction score
2,820
Location
Bedfordshire
Country
United Kingdom
reading manufactures websites
all intended to sell their product to the general public most of whom have no idea of how to evaluate the real data tucked away in the small print.

( not a comment about installers but about the commercial pressure exerted by manufacturers to sell their products. )
 
Joined
28 Oct 2005
Messages
13,547
Reaction score
3,189
Location
Daventry
Country
United Kingdom
A Vaillant Green iQ 843 would probably run two carefully selected showers at the same time, but the purchase price is more than the combined cost of a regular boiler and unvented cylinder, so the only real advantage is a bit of space saving
 
D

Doggit

As with everything, manufactures don't live in the real world. Will the incoming flow and pressure handle 2 showers at the same time, and you need to make sure that the showers aren't going to take too much flow from each other.

Pretty much any combi will handle 2 showers in a house, just as long as they aren't on at the same time. You can get a combi that will do it, but the mains supply may need a new 25mm MDPE pipe first. I recently did a new 20mm pipe, and you can now open a cold tap whilst the showers on, not the hot tap.

I'd be inclined to go with muggles suggestion and fit an indirect cylinder when there's more than one shower required - but still upgrade in the incoming supply pipe.
 
Joined
26 Jun 2004
Messages
64,092
Reaction score
4,620
Location
London
Country
United Kingdom
You cannot expect to use words like "good" to define an engineering term.

Tell us the dynamic flow rate in litres per minute @ 1.0 bar and we can tell you what your choice of boiler can give.

Most larger boilers give a flow rate of about 13 li/min.

That leaves just 6.5 li/min per shower. Just a bit more than an electric shower can give.

Tony
 

DP

Joined
10 Dec 2003
Messages
15,548
Reaction score
3,500
Location
Glasgow
Country
United Kingdom
That leaves just 6.5 li/min per shower. Just a bit more than an electric shower can give

To be precise and exact, that would be the case if pipe lengths to the showers were at the same elevation and distance from the boiler i.e. total pipe resistance was the same so delivery from the boiler was divided equally:whistle:
 
Joined
18 Oct 2005
Messages
326
Reaction score
17
Location
Essex
Country
United Kingdom
Cheers guys.

7 to 1 saying no, although 8 said it was a stupid question, heh. The reason for asking was I have had two quotes to install a system boiler with an unvented cylinder, at 4.6k and 4.2k, which is a little over what I was budgeting. Was hoping the combi boiler would be a suitable option as it would be cheaper overall.

Looks like I will have to start saving the pennies!

Thanks
 
Joined
10 Mar 2007
Messages
10,374
Reaction score
2,124
Location
Poole, Dorset
Country
United Kingdom
So with good incoming water pressure, and 2 "average" showers can it be done?
Good pressure, good flow, two mediocre showers and the largest gas guzzling combi possible are the option.

Manufacturers don't care in the slightest - and what really matters is how the bathrooms are used, not how many there are.
A place with 3 bathrooms could have a combi and the two occupants would probably be happy with it.
Same place with 2 adults and 4 children is a different matter.
 
Sponsored Links
D

Doggit

With those prices (plus the annual cost of the tank services, as the Vaillant Green iQ 843 is £2054 from plumbnation, why not ask the installers to quote on that instead. But check to see if you can get the 18ltrs per min to supply it - but you'll need to do that for the indirect cylinder as well.

Maybe what you need is an interlock on the showers, so that only one can be used at a time.
 
Joined
18 Oct 2005
Messages
326
Reaction score
17
Location
Essex
Country
United Kingdom
Small update. Had someone round that was "impressed" with both the flow rate and pressure of the mains. Can't remember the figures but he put some contraption on the tap. So in usual internet forum etiquette I've ignored the majority and gone for a Baxi Duo-tec 40 Combi. Stay tuned for post install tears.....
 
Last edited:
Joined
16 Nov 2007
Messages
6,225
Reaction score
2,755
Location
Northamptonshire
Country
United Kingdom
You cannot expect to use words like "good" to define an engineering term.

Tell us the dynamic flow rate in litres per minute @ 1.0 bar and we can tell you what your choice of boiler can give.

Most larger boilers give a flow rate of about 13 li/min.

That leaves just 6.5 li/min per shower. Just a bit more than an electric shower can give.

Tony


That's assuming the OP wants to shower at 55 degrees Tony, most people will need to mix a little cold in ;)
 
Joined
26 Jun 2004
Messages
64,092
Reaction score
4,620
Location
London
Country
United Kingdom
That's assuming the OP wants to shower at 55 degrees Tony, most people will need to mix a little cold in ;)

Where are you getting your 55 C from?

The standard combi specification is to quote the flow rate at a temperature increase of 35 C.

With the current incoming water temperature at about 8 C that gives a boiler outlet temperature of 43 C. But remember that you lose a degree or two on the way to the taps. But still about the right showering temperature at the stated boiler flow rate!
 
Joined
16 Nov 2007
Messages
6,225
Reaction score
2,755
Location
Northamptonshire
Country
United Kingdom
Where are you getting your 55 C from?

The standard combi specification is to quote the flow rate at a temperature increase of 35 C.

Rubbish Tony, almost every boiler I see is set up to give over 50 degrees.

Remind me what the early condensing juniors that didn't have a temp control were set at again??
 
Joined
18 Oct 2005
Messages
326
Reaction score
17
Location
Essex
Country
United Kingdom
Update time:

Went for the Baxi Duo-tec 40 Combi in the end. So far so good. There is a drop in temperature when you are using the shower and someone else uses a hot tap/shower but not a massive drop and you can either live with it or turn the shower round to hotter and all is good again in the world. Hopefully this drop will be cured by decent thermostatic showers once I get round to the bathrooms....

Cheers.
 
Sponsored Links
Top