The eternal question: Unvented cylinder or combi?

[Blackburn5]

As part of an upcoming loft conversion, we're looking to upgrade our current vented boiler system. We've reached out to multiple plumbers and received conflicting advice, so I'm looking to get some more opinions...

We are a 2 adult + 2 child household, soon to be 4 bed, 3 bathroom, 13 radiators across 3 floors. We have been told our current mains flow rate is 22 litres. Ideally we'd like to have a system capable of supplying a max of two showers simultaneously if necessary although this is likely not a frequent need, as well as the usual heating/water requirements. We have space to fit a megaflo in the current airing cupboard.

We have been told that an unvented cylinder system with a megaflo or equivalent would be the best option. And we have also been told be another plumber that a combi is infact the best option. Thoughts please and thank you in advance!

 

[ericmark]

1) I would not install a cylinder needing annual testing, I would go for open vented.
2) A combi-boiler able to supply 30 kW may not modulate as low as one providing 20 kW so getting a combi-boiler to satisfy shower requirements may be far too large for heating the house.

Your house around the same size as mine, I have a 20 kW oil fired boiler, which is ample. But design is so important, my house the design is poor, may be worth having a heating and ventilation engineer design the system. Level 5 or over educated, not level 3 as with most plumbers and electricians.

 

[ETAF]

having recently changed to combi , in a bungalow , wished i had done when first moved in, so much better , have a shower over bath and nolonger a trickle having tried LOTS of things to improve that , when kids vome back with grandkids to stay no issue on using bath or shower

but my daughter has a very large tank in the cupboard and 3 showers all pumped and that works ok, BUT they are all pumped and the tank is the size of the airing cupboard
2 kids , 2 adults and at the moment 3 adults

 

[justigator]

I am not a plumber but I doubt a combi would work well for a 4-person household with three bathrooms. The North London semi I bought in 2019 (from a 4-person household) had a loft conversion in 2014 making it a 4-bed from a 3. At the same time, the hot water system was converted to mains pressure with a large accumulator tank in the loft conversion. All fine for the family who left and for me but then I am on my own 99% of the time. Next door is identical except that it is only a 3-bed, not having a loft conversion. They have just replaced their system boiler with a combi, but then there's only one old chap and his adult son. A larger family might struggle. I don't find having a pressurised hot water cylinder a problem as the G3-qualified Gas Safe/plumber guy I've used for over 20 years checks it every year when he checks the boiler, and he says the check is very quick and IIRC mainly just to ensure the tundish is working OK. He did spend longer the first time checking out the size and layout of the overflow arrangements, which he pronounced OK if of a smaller pipe bore than he would have installed himself. If I were looking to buy a house your size, having a combi might not necessarily be a deal-breaker for me but would certainly be a minus point.