The single biggest decision when replacing a boiler. Here's how we help customers in Bristol and Cardiff choose between a combi and a system boiler.
Nine out of ten boiler replacements we quote for come down to one question: combi or system? Both are excellent when specified correctly, and both are a poor choice when they aren't. The right answer depends on how many bathrooms you have, how many people live in the house, and what your incoming mains water pressure actually measures.
How a combi boiler works
A combi (combination) boiler heats water on demand straight from the mains. There is no hot water cylinder and no loft tanks, which frees up an airing cupboard and reduces standing heat losses. Turn a tap and the boiler fires; hot water arrives in seconds and never runs out.
The catch is flow rate. A 30kW combi typically delivers around 12 litres per minute at a 35°C rise. That is plenty for one shower, but run a second outlet at the same time and both drop noticeably.
How a system boiler works
A system boiler heats a stored cylinder of hot water. With an unvented cylinder at mains pressure, you can run two or three showers simultaneously without a drop in performance. The trade-off is space for the cylinder and a recovery period once the stored water is used up.
Our rule of thumb
- One bathroom, one to three people: combi, almost always.
- Two bathrooms used at different times: a well-sized combi still works.
- Two or more bathrooms used simultaneously: system boiler with an unvented cylinder.
- Mains flow below 12 litres per minute: a combi will disappoint, store instead.
- Existing cylinder in good order and a gravity system: consider a system boiler upgrade rather than a full replumb.
Get the flow rate measured first
We always measure incoming flow and static pressure before quoting. It takes two minutes with a weir gauge and it is the difference between a boiler you love and a boiler you resent every morning. If a company quotes you a boiler over the phone without asking about your water supply, be cautious.