Replacing the whole heating system, not just the boiler

Two jobs get asked about in the same sentence and they are not the same job at all. One is a boiler swap: the old appliance comes off the wall, a new one goes on, and everything else in the house stays where it is. The other is a full heating system replacement, where the radiators and the pipework come out too, usually because the floors are already up for something else.

The second is a design exercise. The first is not. That difference decides what the regulations ask for, what belongs on the quote, and why two prices for what sounds like the same work come back looking nothing alike.

What counts as replacing the whole system

There is a real line here and it is drawn in the building regulations guidance. Approved Document L says that where a wet heating system is newly installed, or fully replaced including the appliance, the emitters and the associated pipework, all parts of it should be sized to run at a maximum flow temperature of 55C or lower.

Lift a boiler off the wall and leave the radiators and pipes alone and you are outside that trigger. Take the lot out together and you are inside it. Same house, same installer, different set of rules.

The guidance is realistic about older houses. Where 55C is genuinely not feasible, for example because there is not the wall space for larger radiators, the system should be designed to the lowest flow temperature that will still meet the heating needs of the home. That fallback is a design decision somebody has to make and write down, not a reason to skip the sums.

Source: Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, paragraph 5.10

The word everybody skips in that rule

Read the trigger again and you will find a word that almost never makes it into the advice written about it. Pipework.

Everyone hears 55C and thinks bigger radiators. The rule says all parts of the system, and the trade press summarising it for engineers spells out that this means pipework and radiators both. That matters far more than it sounds, because of how heat actually gets around a house.

Heat delivered into a room is the flow rate multiplied by the temperature drop across the radiator. Lower the flow temperature and, for the same output, the water has to move faster. The Energy Saving Trust puts a working range on that for installers: flow velocity should stay between 0.5 and 1.5 metres per second, with around 0.9 to 1.0 as best practice. Push more water down a narrow pipe and velocity climbs out of that range, and you get noise, pressure drop and a pump working harder than it should.

So a house plumbed in 8mm or 10mm microbore is the one to ask about early. Sometimes the runs are fine and the sums say so. Sometimes the last few metres need upsizing. Sometimes a long run has to come out. Nobody can tell you which from the doorstep, and an installer who has not raised it has not thought about it.

Source: Energy Saving Trust, Green Heat Toolkit: how to make microbore pipes work; Registered Gas Engineer, Industry Standard Update 110: Approved Document L Volume 1

The design should be in the quote, not in somebody's head

A boiler swap is priced from an appliance and a day or two of labour. A full system replacement cannot be, because until somebody has worked out how much heat each room loses, nobody knows what size radiator belongs in it or what size pipe feeds it.

That calculation is the job. The trade guidance on the 55C requirement says plainly that an appropriate heat loss calculation for the dwelling will need to be carried out to achieve it. Room by room, not a rule of thumb off the old radiator sizes, because the old radiators were sized for a boiler running flat out.

So the question to put to anyone quoting is simple and it separates the field fast. Ask what the design flow temperature is, ask whether the radiator sizes come from a heat loss calculation, and ask to see the room by room schedule. An installer doing this properly has the numbers and will happily show you. One who has priced it as a boiler swap with radiators bolted on will change the subject.

Source: Registered Gas Engineer, Industry Standard Update 110: Approved Document L Volume 1

Why two full system quotes look nothing alike

On a boiler swap the appliance is the most comparable line on the page. On a full system it is close to the least important. These are the lines that actually move a price, and they are the ones to line up against each other:

• Whether a heat loss calculation and a radiator schedule are included, or whether the radiators are like for like in the same positions.
• Whether the pipework is renewed, partly renewed or reused, and who decided which.
• Floors. Lifting boards, chipboard or engineered flooring and putting them back is real work, and making good is where quotes quietly differ. Ask what is included and what is left to you.
• Cleaning. A system that is being partly reused should be thoroughly cleaned and flushed, with an inhibitor added, before a new appliance runs on it.
• Controls and zoning. A full replacement is the one moment zoning is cheap to do, because the pipes are already open.
• How long the house is without heating and hot water, and whether temporary heating is part of the offer.

When you do not need the whole system

This is the part a firm selling a full system will not lead with. Plenty of the complaints that send people looking for one are not system-wide problems at all.

One radiator cold at the bottom is usually sludge in that radiator. A house that heats unevenly is often a balancing job nobody ever did. A boiler that keeps locking out has a cause, and it is more often water quality or a control fault than the age of the pipes. None of that needs the floors up.

The honest test is whether the problem sits in one place or everywhere. In one place, fix that place first. If the whole house is slow to warm, or the pipework genuinely cannot carry what you need, the full job earns its keep.

The thing worth doing while the floors are up

If you are replacing the emitters and the pipework anyway, designing the system around a lower flow temperature costs very little extra at that moment and is close to impossible to retrofit later without doing it all again.

A system sized for 55C runs a condensing boiler in the condition it was designed for, which is the whole point of a condensing boiler. It is also the system that could take a different heat source later without the radiators coming off the walls again. We place leads with installers rather than sell appliances, so there is nothing in it for us either way, which is why it is worth saying plainly.

The paperwork

Installing a new or replacement heating system is notifiable work. If the installer is registered with a competent person scheme, Gas Safe Register among them, they notify the local authority themselves and issue you a certificate of compliance. You should end up with that certificate, the manufacturer commissioning record for the appliance, and the guarantee registered in your name.

Anything touching the gas circuit is a Gas Safe registered engineer's work, without exception. Check the card and check the categories on the back of it, because registration is not a single qualification.

Source: GOV.UK, Building work, replacements and repairs to your home

Where to start

Get the survey before you get attached to a number. On a job this size the survey is the product: it is where the heat loss calculation, the pipework decision and the radiator schedule come from, and a price given without one is a guess dressed up as an offer.

Three quotes from installers who have all actually been in the house, all working to a stated design flow temperature, are comparable. Three quotes where one has surveyed and two have not are not comparable at all, whatever the numbers say.