Combi's don't work in larger houses - still true?

[garyo]

I'm considering some layout changes and at the same time addressing the poor hot water pressure from my vented hot water tank.

One option is to do away with my 30 year old oil boiler and existing vented hot water tank, and get a big combi like a HeatslaveII 25/32 that claims 22 litres flow on the hot water.

Assuming I have the incoming mains flow & pressure, do they actually work reasonably well if two people are having a shower?

I had a WB-28Cdi in my last house so I'm pretty happy with the other aspects of living with a combi - such as the lag time as you wait for the hot water to flow through the pipe.

Thanks

 

[Combicert]

I have a 6 bed over 3 storeys - old converted church, so has a large footprint. 17 rads and a WB 42cdi. All works well.

2 showers, it struggles - but both out shower outlets are on a 22mm feed - rather than the more usual 15mm. We installed a third shower which is electric - during such instances.

Unvented cylinder is the way to go...

 

[garyo]

Unvented was the plan in my head, but given that I'm replacing the boiler anyway it'd avoids a lot of complexity if there's a new generation of combis that address the problem.

Mine's a bit smaller than yours - 2 or 3 showers, all 15mm, over 2 storeys.

 

[Nozzle]

Address the problem that it takes a lot of energy to heat water?

And a huge amount of power if you want it do quickly.

Nozzle

 

[ollski]

Does your water supply provide 22lpm? and what is the temperature rise on that?

 

[muggles]

A combi is more complex than a boiler & cylinder setup. Especially if you're oil fired, boiler & unvented is the way to go

 

[garyo]

Address the problem that it takes a lot of energy to heat water?

And a huge amount of power if you want it do quickly.

No that's just physics - you can't change it. I'm talking about evolution of product design, and whether there's been any or if it's all just marketing bluff.

 

[garyo]

Does your water supply provide 22lpm? and what is the temperature rise on that?

It varies with time of day. 22lpm I've seen claimed by some manufacturers is at a 35 degree delta, so that's not a realistic figure in winter.

 

[Combicert]

I was at worcester bosch a few days ago doing a training course. There was a really knowledgable guy doing the course - focussing on the cdi... during the day he said that a new boiler range was ‘pretty much there’ and would be a ‘game changer’ (his words...). Now, i don’t know any timescales but i got the impression it was imminent as he was talking about potential courses.

Not much help perhaps in your circumstance, i guess. But it does make you wonder about possible multiple plate heat exchangers and extra diverters or something... the imagination wandered...!

If i were planning a new extension i think i’d be tempted to pipe showers to an accessible place close to heat source - just incase something new comes out in a few months/years

Anyway, many will see this as a totally irrelevant post, perhaps it is... but you get the feeling something’s brewing...

 

[oldbuffer]

1. It really depends on usage. If you have two showers using 10 lpm each, then a 22 lpm flow would be fine. If each used 15 lpm one or both would suffer.
2. As garyo says, it might be 22 lpm at delta 35 degrees. If the incoming water is just above freezing, that would be stretching it for a 38 - 40 degree shower.
3. I would assume the manufacturers mean 22 lpm flow out of the boiler into an "open pipe". Your pipework could well offer restrictions to reduce the available flow.

 

[John D v2.0]

There's nothing fundamental stopping 2 good showers from one boiler.
With a normal gas meter you can run two 30kw combis which most people could get one standard shower from each and be happy.
Incoming water you're using the same supply whether it's unvented or Combi.
There is no fundamental limit for heat exchangers or burners that would make a large Combi not work.
Just need to watch out for any resistance in the dhw side of the boiler.

If you have a cylinder with an accumulator and massive pipesp you can charge it up and dump it all in one minute into a bath (or on your head). That would be impossible on a Combi.

 

[John D v2.0]

2 showers, it struggles

Is that struggles in temperature or struggles with enough flow?

 

[fezster]

1. It really depends on usage. If you have two showers using 10 lpm each, then a 22 lpm flow would be fine. If each used 15 lpm one or both would suffer.
2. As garyo says, it might be 22 lpm at delta 35 degrees. If the incoming water is just above freezing, that would be stretching it for a 38 - 40 degree shower.
3. I would assume the manufacturers mean 22 lpm flow out of the boiler into an "open pipe". Your pipework could well offer restrictions to reduce the available flow.

To add to this, in the current weather, you may be using 60% hot and 40% cold to give a comfortable 38 deg shower. This equates to 22 lpm hot and 14 lpm cold, giving a combined 36 lpm. Plenty enough for even 2 powerful showers. This is assuming your cold main can deliver this flow.

With outside temps at close to zero, you'd be using 100% hot and getting only 22 lpm to (just about) reach your 38 deg shower.

Something to keep in mind.

 

[garyo]

Thanks for all the feedback. I've just been to check - The outside tap is currently at 20lpm. This is 1 metre away from the blue 25mm mains pipe coming in to the house, and is only fed by a 1 metre 15mm copper and a couple of turns. Currently the whole house is 15mm from this point - another problem for me to address.

Depending on the layout I might be able to fit in an unvented cylinder downstairs next to the boiler - I'll give that some thought.

 

[John D v2.0]

With outside temps at close to zero, you'd be using 100% hot and getting only 22 lpm to (just about) reach your 38 deg shower

Agree although if you have a combi it's pointless to heat the water to 60 and mix with cold straight away. You'll get better efficiency if you turn the boiler to 35c or whatever your favourite temperature for a shower is.

In winter I gas rated our shower on hot only at 17kw heat input on a 24kw boiler, haven't clocked it recently in the warm though.
That proves that with a second shower it would need 34kw+ to cope in winter (or one of those waste water heat exchanger stacks might work out cheaper overall)