What to Look For in a New Boiler: 10 Checks Before You Buy

Most people buy a boiler once every ten to fifteen years, so it is hard to know what matters and what is sales talk. The brand on the front matters less than whether the boiler is the right type, the right size, properly protected and set up with the right controls. These are the ten checks worth making before you agree to a new boiler.

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1. Is It the Right Type of Boiler for Your Home?

There are three main types:

• Combi: heats hot water on demand from the mains. No cylinder, no loft tank. Best for flats and homes with one bathroom.
• System: heats a sealed hot water cylinder fed from the mains. Better when two showers run at once.
• Conventional (regular or heat-only): works with a cylinder and a cold water tank in the loft. Common in older TW-area homes.

Switching type is possible, but it changes the job a lot. A family with two bathrooms and teenagers may be unhappy with a combi, however good the brand. If you are not sure what you have now, read our guide to combi, system and conventional boilers.

2. Is the Output Right in kW?

Bigger is not better. A boiler that is too large for the home cycles on and off, which wastes gas and wears parts. A combi has two figures:

• Central heating output: should match the heat loss of your home.
• Hot water output: decides how fast hot water comes through.

A good engineer will size the boiler from your home, not just copy the number on the old one.

3. What Is the Hot Water Flow Rate?

For a combi, the flow rate in litres per minute matters more than the kW figure on its own. As an example, Ideal lists its Logic Max Combi2 at 9.9 litres per minute (C24) up to 14.5 (C35), and its Vogue Max Combi at 10.6 (C26) up to 16.4 (C40), all at a 35°C temperature rise.

The boiler can only deliver what your mains water supply allows. If your mains flow is poor, a bigger combi will not fix it. Ask the engineer to check the incoming flow before choosing a model.

4. How Efficient Is It?

In England, every new gas boiler must reach at least 92% ErP seasonal efficiency. That has been the rule since Boiler Plus came in during April 2018. Most modern condensing boilers are A-rated for heating. The real-world difference between good boilers comes more from correct sizing, low flow temperatures and good controls than from the last percentage point on the label.

5. What Does the Warranty Actually Require?

A “10 year warranty” or “12 year warranty” nearly always comes with conditions. Typical ones are:

• Fitted by an installer in the manufacturer’s accredited scheme.
• A specific magnetic filter fitted and registered at the same time.
• Registered within a deadline, often 30 days.
• Serviced every year by a Gas Safe registered engineer, with the service recorded.

For example, Baxi says the 10 year warranty on its 800 Combi 2 is only valid if the Adey Micro 2 filter is installed and registered with the boiler. Viessmann says a Vitodens that is not registered within 30 days falls back to a basic 2 year warranty. Read the conditions, not just the number, and keep up with your annual service.

6. Does It Meet Boiler Plus Rules for Combis in England?

Yes, it must. Under Boiler Plus, every gas and oil boiler needs time and temperature controls. A new combi boiler in England also needs at least one of these extra measures:

• Weather compensation: an outdoor sensor lowers the flow temperature in milder weather.
• Load compensation: the boiler lowers its output as rooms approach the set temperature.
• Smart thermostat with automation and optimisation: it learns how long your home takes to heat and starts at the right time.
• Flue gas heat recovery: uses heat from the flue to pre-warm incoming cold water.

Since June 2022, replacing a boiler in England also means each heated room should have its own thermostatic control, usually a TRV on each radiator. Working TRVs you already have do not need replacing. Building Regulations guidance is updated from time to time, so your engineer should fit to the rules in force on the day.

7. Are the Controls Compatible?

Good controls save more gas than a fancier boiler. Things to ask about:

• OpenTherm or the manufacturer’s own modulating protocol, which lets the thermostat adjust the flow temperature instead of switching the boiler on and off.
• Weather or load compensation, which many boilers support with the right sensor or thermostat.
• Smart thermostats. Check the one you want works with the boiler you choose, especially if you want modulation rather than simple on and off.

8. Is a Magnetic Filter Included?

A magnetic system filter catches sludge and debris before they reach the boiler. Many manufacturers require one for their longer warranties, and some require their own brand. Make sure the filter is on the quote, and that the system will be cleaned and treated with inhibitor when the new boiler goes in.

9. Will It Fit Where You Want It?

Measure the space, including clearance for servicing. Then think about:

• Flue position. The flue must meet distance rules from windows, doors, boundaries and other openings. Moving the boiler often means a new flue route.
• Condensate pipe. Condensing boilers produce acidic water that has to drain safely. External pipes that freeze are a common cause of winter breakdowns.
• Gas supply. A larger combi may need a bigger gas pipe than the old boiler had.

10. Will Parts Be Easy to Get?

Mainstream models from Worcester Bosch, Vaillant, Ideal, Baxi and Viessmann are widely fitted, so local engineers and merchants tend to carry common parts. That matters at 7am on a cold January morning. Very new, rare or imported models can be harder to repair quickly.

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Should You Buy a Hydrogen-Ready Boiler?

You do not need to pay extra for it. Some manufacturers say their boilers can already run on a blend of up to 20% hydrogen with natural gas. Worcester Bosch, for example, says this of all its Greenstar condensing gas boilers made since 2000. But the gas supplied to homes in the TW area today is not a hydrogen blend, and the government has not required new boilers to be hydrogen-ready. In March 2024 it said it would only consider requiring that from 2030, and only if it decides hydrogen should play a substantive role in home heating. Choose a boiler on today’s needs.

What Is the Best Boiler Brand?

The best boiler is the one sized and fitted properly for your home, from a brand your engineer knows well, with a warranty whose conditions you can meet. Our guide to the best combi boilers for 2026 compares the main models and their boiler-only prices.

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Get a No-Obligation Quote

For a no-obligation quote, call 07360 268948 or send photos of your current boiler on WhatsApp, and we’ll put you in touch with a local Gas Safe registered engineer who will check these points for your home. Find out more about boiler replacement in the TW area.