Upgrading combi vs. moving to unvented

[Participant]

Hi all

I just want to run this thinking past you knowledgeables to check if it is sound or not.

We currently have a Ravenheat 120csi combi boiler in our house, which was there when we moved in. More than one outlet in use at any one time (e.g. someone uses a tap when the shower is running) causes poor performance from both outlets.

We've managed so far as there were only really two of us. However, our eldest is now three and we have a three month old, so our hot water demands are going to increase, with regular baths and showers becoming more of a feature.

House is a four bed with two bathrooms and a cloakroom.

Our choices seem to be: upgrade the combi to a beast with more power that can deliver a greater flow of hot water, or go unvented and swap the combi to a system boiler.

I'm fairly sure that the combi will be the cheaper option, but I love my super duper power showers, and both showers need to be usable with a strong flow at once.

Assuming that my flow and dynamic pressure is sufficient for both options, which would you go for? I have space for a cylinder if needed, but unless it'll blow the socks off a top combi, I'm not sure I want the added complication and expense.

Many thanks.

 

[Participant]

Cheers guys. Sound like I need to measure my cold water flow rate and get someone out to assess for an unvented solution.

Will an unvented support those body jet / rainfall showers? Or would I need a pumped solution for that?

 

[Participant]

You would need to know the flow rate /pressure required for the shower, an unvented cylinder will supply hot water matching the flow rate of your mains supply limited to a max pressure of 3-3.5bar. A quick google shows a jet /rain shower will use 16-30l/min so should not be a problem for an unvented system as long as your mains can keep up. A combi boiler wouldn't be able to supply a shower over around 15l/min

Cheers for that.

I've just measured flow rate at the garden tap and it's 21 l/min. According to the internet, this only just fits the minimum requirement for unvented.

What I'm not clear on is what level of performance I could expect from 21 l/min? Since it's minimum, does that mean performance would only just be adequate?

Your comment earlier about 11 l/min for each shower seems to suggest this would be OK for running multiple outlets simultaneously at reasonable flow rates.

 

[Participant]

Was your garden tap fed off 10mm pipe? The un-vented will be fed off a 22mm pipe as close as possible to where the water comes in for the maximum performance.

Garden tap is fed by 15mm sir. My water main is 15mm too, as far as I can see it anyway, which is basically just before the stop **** before it goes under concrete.

EDIT: 16-30L/hour :O your wallet wont like one of those either.

True, but I want the option, and the cost of water doesn't overly concern me cause it's pretty cheap .