Baxi combi, system or heat only: which fits which home

What is the difference between a Baxi combi, system and heat only boiler?

Where the hot water comes from, and how much kit the property has to house. Baxi's own descriptions are plain enough. A combi delivers instantaneous hot water and needs no additional tanks or hot water storage cylinder, and because it is fitted to a sealed system it can be a kitchen cupboard fit where space is tight. A system boiler is installed alongside a hot water storage cylinder and also runs on a sealed system, so there is no feed and expansion tank in the roof space, and Baxi builds the expansion vessel and circulating pump into the appliance to simplify the install. A heat only boiler needs two tanks in the loft, a cylinder and a pump. One loft tank draws off the mains and fills the cylinder. The other, the feed and expansion tank, keeps the heating circuit at the right level and gives the water somewhere to go when it heats up.

Which Baxi boilers are combis?

Most of the current line-up. Baxi's professional listing carries the 400 Combi 2.1, the 600 Combi 2, the 800 Combi 2, the Assure 500 Combi 2 and the Main Eco Compact Combi. That spread means a combi swap rarely forces a change of tier, because the numbered ranges and the installer-focused Assure line both cover the format. Baxi calls the combi the most popular boiler type currently installed in the UK, largely because it loses the cylinder and the loft tanks and takes up the least room in the property.

Which Baxi boilers are system boilers?

Four of them, and the 400 is not one. Baxi's system listing runs the 600 System 2, the 800 System 2, the Assure 500 System 2 and the Main Eco Compact System. So a customer on a 400 who wants to keep a cylinder but lose the loft tanks moves up to a 600 or across to Assure, which is worth knowing before you quote a straight tier-for-tier swap. Baxi sells the format on flow rate: the built in pump is there to give quick filling baths and a strong shower, and the range is designed to work with unvented cylinders.

Which Baxi boilers are heat only?

The 400 Heat, the 600 Heat, the 800 Heat, the Assure Heat and the Main Eco Compact Heat. Heat only has the widest coverage of the three formats across the Baxi range, which matters on an older property where the honest answer is a like for like swap. Baxi points at two cases in particular. An older heating system may not cope with the higher water pressure a combi or system boiler delivers, and a home in a low pressure area will not get an acceptable flow rate out of a combi at any output.

When does a bigger combi not fix a weak flow rate?

When the cold mains is the limit rather than the boiler. Baxi's own training guidance puts numbers on it. A 24kW combi delivers 9.8 litres per minute at a 35 degree temperature rise and a 40kW combi delivers 16.4 litres per minute at the same rise, but because a combi is mains fed the incoming supply caps what any of them can do. Baxi's worked example is direct: where the cold mains can only supply 12 litres per minute, any combi above 28kW is out of the question, and the choice is to sort the supply out or fit stored hot water instead. Baxi puts heat only systems at properties running roughly 15 to 20 litres per minute, and sets the minimum for an unvented cylinder at 1.5 bar and 20 litres per minute.

What should you check before quoting a configuration change?

• The data plate: read the make, model and output off the appliance rather than taking the customer's word for the format.
• The incoming mains: measure flow rate and pressure at the kitchen tap before promising a combi of any output.
• The loft: a cold water cistern and a smaller feed and expansion tank up there means the existing appliance is heat only.
• The cylinder: if it stays, confirm it suits the format, since Baxi's system and heat only boilers are sold on cylinder compatibility.
• The controls: since the Building Regulations changed in April 2018, every boiler installation has to include a timer and room thermostat.

Whichever format goes on the wall, the record keeping is the same job every time. Manifold builds the CP12 and the service record from the check on your phone, reads the make, model and GC number off the data plate with PlateScan, and keeps the appliance history against the property so the next visit starts with what is already fitted.