Two boiler quotes, £400 apart.
What's actually different?

Most people we sit down with have two or three quotes on the kitchen table by the time we get there, and the first thing they usually say is that one of them is cheaper. Sometimes by a few hundred pounds, sometimes by considerably more. The question they are really asking is whether the cheaper one is a bargain or a problem, and there is no way to answer that from the price alone, because the price is a single number sitting at the bottom of a page that may or may not describe the same job.

So the useful thing is to compare what is being done, and to leave the totals until you have done that.

Check 1: are the two quotes
for the same job?

A quote for a boiler swap and a quote for a boiler swap that also cleans out the system, fits a filter, deals with the water coming into the property and brings the controls up to current standards are not the same job, and they should not cost the same. When there is a gap of several hundred pounds between two quotes for what looks like an identical boiler, the gap is usually a difference in what each installer intends to do, and one of them has left things out.

It works both ways. A more expensive quote is only better if the extra money is buying something, and the way to find out is to read what each quote says it will do, which takes longer than reading the price but is the only part of the exercise that tells you anything.

Two things need to be on the page before that comparison is even possible. The first is the exact boiler model, and the brand on its own is not enough. "Worcester Bosch" tells you nothing, because a Worcester Bosch 8000 is a very different machine from a Worcester Bosch 1000 with a price gap to match, so a quote that only names the brand cannot be compared with one that names the model, and sometimes that is the reason the model has been left off.

The second is the warranty, in writing. The extended manufacturer warranties only apply where the installer is on that manufacturer's installer scheme and registers the boiler with them after the installation, so the three things to look for are the length of the warranty, who is registering the boiler, and what you actually receive in writing at the end. A long warranty on a piece of paper you never receive is worth nothing.

What to do: put the two quotes side by side and read the work descriptions, check the exact model is named on both, and check the warranty length and who registers it. If one of them does not describe the work in enough detail to compare, that tells you something about the paperwork at least.

Check 2: is the system being
cleaned, and is a filter going on?

This is the one that surprises people most, because it sounds like an add-on, and on most quotes it is treated as one, when the guidance and the manufacturer both expect it as part of the job.

Cleaning is in the building regulations guidance. The Domestic Building Services Compliance Guide, which is the document Part L points to for wet heating systems, says central heating systems should be thoroughly cleaned and flushed out before installing a new boiler, and it names BS 7593 as the standard for how the water is treated afterwards. And it is not only the building regulations guidance. Boiler manufacturers insist on a clean system as well, and a dirty system can void the warranty. Manufacturers have refused to carry out warranty work before now until the system has been cleaned. So a quote that puts a new boiler onto an old system without cleaning it has left out something the guidance asks for and something the manufacturer insists on, and that is where some of the saving has come from.

A magnetic filter is a manufacturer's requirement, and it comes from the installation instructions and not from the building regulations, which is a distinction that matters. Boiler manufacturers set out in their installation instructions what has to be in place for the appliance to be installed correctly and for the warranty to stand, and a filter is commonly part of that. A gas appliance has to be installed in line with the manufacturer's instructions, so on most installations a filter is a requirement (it is worth being accurate about where the requirement comes from, which is the instructions and not Part L).

The practical reason underneath both of them is the same. An old system holds years of iron oxide, and if you put a new heat exchanger on the front of it, that debris now runs through a much narrower waterway than the one it came out of.

What to do: ask each quote what it does about the existing system water, and whether a filter is included and where it will go.

Check 3: what is the quote doing about
London's hard water?

The Compliance Guide is specific about this one. Where the mains total water hardness exceeds 200 parts per million, provision should be made to treat the feed water to water heaters and to the hot water circuit of combination boilers, to slow the rate at which limescale builds up.

London is well over that threshold across effectively the whole city, so on a London combi installation you should expect to see it on the quote as a matter of course. You can see the figure for your own postcode on Thames Water's water quality checker at https://www.thameswater.co.uk/help/water-and-waste-help/water-quality/check-your-water-quality.

There is a second item that often sits next to it. Where a water meter is fitted, and most London properties are getting them now, the incoming supply behaves differently and a system can start to hammer when a valve closes quickly. The requirement to deal with that sits under the Water Supply (Water Fittings) Regulations 1999 rather than under Part L. Schedule 2 of those regulations says every water fitting has to be built to resist damage from pressure surges, among other things, and a shock arrestor is the usual way of taking the surge out of a metered supply when a valve snaps shut. So it is answering a different rule from the scale reducer.

What to do: ask whether a scale reducer is included, and if there is a water meter on the property, ask about a shock arrestor.

Check 4: does the quote meet
Boiler Plus?

Boiler Plus has applied in England since April 2018 and it sits inside Part L. Every gas boiler installation needs time and temperature control. A combi installation needs one of four additional measures on top of that: flue gas heat recovery, weather compensation, load compensation, or smart controls with both automation and optimisation.

Two things follow from that. A quote which does not mention controls at all has not addressed a requirement, and a filter does not satisfy Boiler Plus, which is the one we see most often. A filter is a good thing to fit and it is usually a manufacturer's requirement, but it is not one of the four measures, and anyone presenting it as the Boiler Plus item has either misread the requirement or is hoping you will not check.

What to do: ask which of the four measures the quote is using, by name.

Check 5: whose Gas Safe number is on it?

This takes about two minutes and it costs nothing, and we would encourage you to do it on every quote including ours.

Every Gas Safe registered business has a registration number, and you can put that number into the Gas Safe Register website and see the business it belongs to and what work that business is registered to carry out. A registration covers particular categories of work, so a business can be genuinely registered and still not be registered for the thing you are asking it to do.

The part worth paying attention to, as much as the result of the check, is how the person reacts when you say you are going to do it, because somebody who is properly registered will tell you the number without hesitating and will not mind at all.

What to do: ask for the number, and check it.

The short version

Cheapest and best value are two different measurements, and on a boiler installation they come apart in a very specific way, because most of what separates a good installation from a poor one is invisible six months later and unarguable six years later.

Four of the five checks above come from a rule or an instruction that already exists. Cleaning the system is in the building regulations guidance. Scale treatment above 200 ppm is in the same guidance. Boiler Plus is in Part L. A filter is in the manufacturer's instructions. None of them are things a company invents to push a price up, which is why it is worth knowing which is which before you decide that the cheaper quote has simply found a better way to do it.

Go through the five, item by item, on each quote you have. It takes about ten minutes and it turns two numbers into two descriptions of work, which is a far easier thing to choose between.

We are not here to compete and give you the cheapest price possible. We try and compete in quality of service and customer care. We hope to have a long term relationship with you where you can call on us with confidence whenever you need our help.

Quick answers