Which Boiler Do I Actually Need?

Nearly every boiler conversation I have starts with a brand name. Someone has been told Worcester, or their neighbour swears by Vaillant, and the question arrives as “how much for a new Worcester?” That is the last question, not the first. The brand matters far less than getting the type and the size right, and both of those are decided by your house rather than by a brochure.

Here is how I work through it on a first visit, so you can do most of it yourself before anyone comes out.

1. Work out which of the three systems you have

Almost every home in Cardiff and Caerphilly runs one of three arrangements. You do not need to open anything up to tell them apart — you need to look in two places: the airing cupboard and the loft.

That matters because swapping like for like is a fundamentally different job from changing type. Combi for combi is a day's work in most houses. Turning a heat-only system into a combi means taking out the cylinder, taking out the loft tanks, re-running pipework and making good afterwards — the boiler is the cheapest part of that list.

2. Swap or conversion? Know which one you are buying

This is where quotes stop being comparable. Two prices for “a new boiler” can describe two completely different amounts of work, and the cheaper one is not always the one that leaves you with a working house at the end of the week.

3. The bucket test — before anyone sells you a combi

A combi makes hot water on demand, straight off the mains. That means its hot water performance is capped by what your incoming main can actually deliver, and no amount of boiler kilowatts changes that. If the street main and your supply pipe can only give you nine litres a minute, a bigger combi gives you nine litres a minute of slightly hotter disappointment.

You can measure it yourself in about a minute. Take a container you know the volume of — a two-litre jug is ideal, a bucket you have measured is fine — open the kitchen cold tap fully, and time how long it takes to fill. Do it with nothing else running. Litres divided by seconds, multiplied by 60, gives you litres per minute.

The other half of the question is how many people want hot water at the same time. One bathroom and a kitchen is combi territory. Two bathrooms where someone showers while the bath is filling is where a cylinder starts to earn its space back, whatever the flow rate says.

4. How big does it need to be? (yours is probably too big)

Oversizing is the default fault in British housing. A boiler that is far bigger than the house needs does not heat it faster — it reaches temperature, shuts down, cools, and fires again, over and over. That short cycling wears the boiler out and wastes gas, and it is why a like-for-like “same kW as the old one” replacement often repeats a mistake made twenty years ago.

The proper method is a heat loss calculation for your actual house — construction, glazing, insulation, room by room. As a starting sanity check before anyone arrives, the trade rough guide runs something like this:

Those are rules of thumb for a conversation, not a specification — and note what the right-hand column is really describing on a combi: hot water output. The heating side of a combi in an average Cardiff semi rarely needs more than about half of it. That is the whole reason the numbers look so large next to the actual heat loss of the building.

5. What the rules actually require — in Wales

Most boiler advice online is written for England, and on this subject England and Wales genuinely differ. Replacing a boiler is notifiable work under the Building Regulations either side of the border, but the standard it has to meet is not the same.

The Welsh Government's own guidance sets the minimum efficiency for a replacement at 86% for gas and 85% for oil, expects gas replacements to be condensing boilers, and points installers at Approved Document J for air supply, flues and chimney positions. Work must be done by a Gas Safe registered installer, who notifies it himself; you should be given a commissioning certificate, and Building Control then issues a Building Regulations completion certificate. If an installer is not registered, the homeowner has to notify the council in advance and pay a fee — which tells you everything about why you check the register first.

None of that is an argument for doing less. Weather compensation and a decent smart control are genuinely worth having, and I will happily fit them. It is an argument for knowing which parts of a quote are required and which parts are recommended, because the two get blurred a lot.

6. Grants: what genuinely exists in Wales

“Free boiler” adverts are mostly lead-generation sites that sell your details on. The real schemes are simpler than they make them look, and you can reach both without going through anybody.

7. Why I will not price it over the phone

You will find plenty of sites that give you a fixed price in ninety seconds. What they are pricing is a boiler on a wall in an average house, and the difference between that and your house is where the money actually goes:

• Where the flue can go. Terminal positions are set by Approved Document J — distance from windows, doors, boundaries and openings. On a mid-terrace with a rear extension that is often the whole design problem.
• Whether the gas supply is big enough. Older properties are frequently on a pipe that was sized for a much smaller appliance. Upgrading the run from the meter is real work.
• Where the condensate can drain. Internal is best. An external run needs the right diameter and insulation, or it freezes and locks the boiler out on the coldest morning of the year.
• What state the system is in. A new boiler on a system full of sludge is a warranty claim waiting to happen — which is why a proper flush and a magnetic filter are on my quotes rather than in the small print.
• Whether it is a swap or a conversion — see the second diagram.

So I come and look, and you get a price agreed before I start. If the honest answer is that your existing boiler has years left and needs a service rather than replacing, that is what I will tell you.

8. The warranty small print, in one paragraph

Long manufacturer warranties are real, but they are conditional. The two conditions that catch people out are an annual service, documented, and the system being clean and treated — flushed properly at installation, with inhibitor and usually a magnetic filter fitted and cleaned. Miss the services and the guarantee that sold you the boiler quietly stops being worth anything. Keep the paperwork somewhere you will find it in five years.