Three decisions matter here: the type, the size, and who installs it. Getting the first two wrong costs you money every day the boiler runs.
A boiler typically lasts ten to fifteen years, so this is a decision you live with for a long time. Here is how each part of it gets made.
The three types
Combi boilers
Heat water directly from the mains on demand. No tanks, no cylinder, everything in one unit.
Good because they save space, give unlimited hot water on demand with no waiting, cost less to install, and avoid the heat loss that comes with storing hot water. With good mains pressure they also give a strong shower.
Limited by flow rate. A combi struggles when several outlets run at once, and it depends on decent mains pressure. You cannot effectively run a bath and a shower simultaneously. It is not the right choice for a household with heavy hot water demand.
Suits one to three bedroom properties with one bathroom, flats, and small to medium families. That covers the majority of homes around Crawley and Horsham. Combis are the most popular type of boiler fitted in the UK.
System boilers
Heat water and store it in a separate cylinder. Pump and expansion vessel are built in, but you need somewhere for the cylinder, usually an airing cupboard.
Good because they supply several outlets at once, handle high hot water demand, give strong pressure to multiple bathrooms, and work with solar thermal. With no tank in the loft they are quicker to install than a regular boiler, and usually more economical to run than one.
Limited by cylinder size, since you can run out and then wait for it to reheat, and by needing the space for it. They also cost more to install than a combi, and some heat is lost from the stored water.
Suits three to five bedroom properties with two or more bathrooms, larger families, and anywhere several showers run at the same time.
Regular boilers
Also called conventional or heat-only. Need both a cold water tank, usually in the loft, and a hot water cylinder.
Good because they supply multiple outlets, work with low mains pressure, suit older heating systems, and are straightforward to replace like for like. They also work with solar thermal panels and immersion heaters, and cope well in large properties with high demand.
Limited by the space required, the number of components, heat loss from storage, and loft tanks being vulnerable to freezing. Installation usually costs more, and they make up a shrinking share of new installations.
Suits larger older properties with an existing conventional system, homes with poor mains pressure, and listed buildings where changes are restricted.
Which one, quickly
| Your situation | Usually |
|---|---|
| Flat or one to two bedrooms | Combi |
| Three bedrooms, one bathroom | Combi |
| Three to four bedrooms, two bathrooms | System, or a high-output combi |
| Four or more bedrooms, three plus bathrooms | System |
| Large family, heavy hot water use | System |
| Poor mains water pressure | System or regular |
| No space for tanks | Combi |
| Replacing an existing regular boiler | System, or keep the regular |
Sizing it correctly
Boiler output is measured in kilowatts. Too small and the house never warms properly. Too large and you pay more to buy it and more to run it, because it cycles inefficiently.
As a rough guide, small properties with up to ten radiators and one bathroom need around 24 to 27kW, medium three bedroom homes with ten to fifteen radiators need 28 to 34kW, and large four bedroom properties need 35kW upwards.
The bands assume one bathroom in a small property, one or two in a medium home, and two or more in a large one. A large house with more than fifteen radiators sits at the top of the range.
Those are starting points only. A proper heat loss calculation accounts for your actual room sizes, insulation, glazing and how you use the house. Oversizing to be safe is the single most common mistake in this trade, and you pay for it every day.
We do that calculation at survey, before recommending anything, as part of every boiler installation.
Choosing a brand
Worcester Bosch, Vaillant, Ideal, Viessmann and Baxi all make boilers we install and stand behind. Warranty length, price and availability of parts vary, and the right answer depends on your property and budget.
What matters more than the badge on the front is that the boiler is correctly sized, correctly installed, and properly commissioned on a clean system. A premium boiler fitted carelessly will underperform a mid-range one fitted properly, and it will fail sooner.
When you compare models, these are the things worth weighing:
- The guarantee, both its length and its conditions. Most depend on annual servicing and on the system being cleaned and protected when the boiler goes in. Up to 18-year guarantees are available on selected boilers, subject to manufacturer terms, correct system protection and annual servicing.
- Parts and servicing. A boiler that local engineers know well, with parts that are easy to get hold of, is quicker and cheaper to keep running.
- Noise, if it is going in a kitchen, a living space or near a bedroom.
- Controls, and whether it works with the smart thermostat or weather compensation you want.
If the existing system is full of sludge, it needs cleaning before a new boiler goes on it, usually with a powerflush. Fitting a new boiler to dirty system water shortens its life and can put the guarantee at risk.
Efficiency ratings
Every new boiler carries an energy efficiency label, the same idea as the ones on fridges and washing machines.
- A-rated: ninety percent or better, which covers all modern condensing boilers
- B-rated: eighty-six to ninety percent, older condensing models
- C and below: seventy to eighty-six percent, old non-condensing units that can no longer legally be installed
An A-rated boiler turns ninety pence of every pound of gas into heat. Upgrading from a seventy percent efficient boiler typically saves twenty to thirty percent on heating costs, which on an average three bedroom home is a few hundred pounds a year.
Pairing it with smart controls and weather compensation adds another ten to fifteen percent, and that part is a setup job rather than a product.
Features worth having
Essential: A-rated efficiency, a decent manufacturer warranty, frost protection, pump overrun, and a modulating burner.
Worth having: weather compensation, smart control compatibility, quiet operation, a built-in timer, and dry fire protection.
Nice to have: wifi connectivity, a clear display, a holiday mode, and self-diagnostics.
What the less obvious ones actually do:
- Frost protection fires the boiler briefly if it gets close to freezing, protecting the boiler and the pipework near it.
- Pump overrun keeps the pump running for a short time after the burner stops, carrying leftover heat away from the heat exchanger.
- A modulating burner turns the flame up and down to match demand, rather than switching on and off at full power.
- Weather compensation uses an outside sensor to lower the water temperature on milder days, which keeps the boiler in its most efficient range for more of the year.
- Dry fire protection stops the boiler firing if there is not enough water in it.
Ten questions to ask any installer
- What output do I actually need, and how did you work that out?
- What is the total installed cost, including everything?
- How long is the manufacturer guarantee, and what conditions apply to it?
- Are you Gas Safe registered, and can I see the card?
- Does the price include a system flush if the system needs one?
- Will I get a written quote broken down?
- What controls are included, and are smart controls an option?
- How long will the installation take?
- Will you notify Building Regulations and register the guarantee?
- Do you offer annual servicing, and what does it cost?
If an installer cannot answer the first question with reference to your actual property, be careful. That is the question that decides whether the boiler is right.
Frequently asked
How do I know what size boiler I need?
Properly, by a heat loss calculation that accounts for room sizes, insulation, window types and how the house is used. Rules of thumb based on bedroom count are a starting point, not an answer. An oversized boiler costs you money every day it runs.
Can I switch from a regular boiler to a combi?
Often yes, and it frees up the loft tank and airing cupboard. The deciding factor is your incoming water flow rate and how many bathrooms need to run at once. A combi cannot serve three simultaneous showers regardless of its stated output.
Which brand should I choose?
Worcester Bosch, Vaillant, Ideal and Viessmann all make genuinely good boilers, and the right one depends on your property and budget. What matters more than the badge is that it is correctly sized, correctly installed and properly commissioned. A premium boiler fitted badly will underperform a mid-range one fitted well.
How long should a new boiler last?
Typically ten to fifteen years, and it will reach the upper end of that if it is serviced every year and the system water is kept clean. That is why it is worth getting the type and size right rather than choosing on price alone.
Do I need a hot water cylinder?
Only with a system or regular boiler. A combi heats water on demand from the mains, so there is no cylinder at all. If several people need hot water at the same time, a cylinder is usually the better answer, and it is worth finding the space for one.
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