Oil boiler installation

Most replacement boilers are too big

The standard approach to a boiler replacement is to read the output off the old appliance’s data plate and fit something similar. It is quick, it is what most quotes are based on, and it is usually wrong – because the old boiler was very likely oversized when it went in, and because the house has almost certainly had insulation, glazing or draught-proofing done since.

An oversized boiler does not heat your house faster in any meaningful sense. What it does is short-cycle: it fires, satisfies the demand quickly, shuts down, cools, and fires again. Every one of those cycles has a purge and an ignition, and the appliance spends a large share of its life in the least efficient part of its operating range. On a condensing oil boiler the penalty is worse, because an oversized appliance runs at return temperatures too high to condense, which is exactly the thing you paid extra for.

A heat loss calculation is not a long job and it changes the answer. It also feeds directly into whether your existing radiators are adequate, which is a question most replacement quotes never raise.

What the job actually involves

1. Survey and heat lossRoom-by-room heat loss for the building as it is now, not as it was in 1994. Plus a look at the existing radiators, pipework, controls, tank, oil line and flue route.
2. The honest conversation about the systemWhether your radiators suit the new appliance, whether the pipework can carry the flow, where the condensate can realistically go, and whether the tank is fit to stay.
3. A written, itemised quoteAppliance, flue, condensate, system treatment, controls, electrical work, making good, certification. So you can see what you are comparing when you look at someone else’s.
4. System clean before the new boiler goes onA new heat exchanger connected to a dirty system is a new heat exchanger with a short life. Cleaning first, then inhibitor, then a filter – in that order.
5. Removal and installationOld appliance out, new one in, flue and condensate installed to the manufacturer’s instructions and the relevant standards, fire valve and oil line checked or renewed.
6. ControlsCorrect controls fitted and actually set up, rather than left on the factory defaults with a note to read the manual.
7. Commissioning with an analyserPump pressure to the data plate, combustion analysed, readings recorded, and the benchmark and commissioning documentation completed.
8. HandoverWe show you how it works, where the reset is, where the isolation is and what the service interval is – in person, not as a folder left on the side.

The details that get skipped

Condensate

Modern oil boilers are condensing appliances, which means they produce an acidic condensate that has to be drained. Where that pipe goes is one of the most common sources of trouble in the years after an installation. An external run in 22mm plastic on a cold night is a boiler that will shut down in January. The right answer is an internal termination wherever the building allows it, and where it genuinely must go outside, oversized pipe, insulation, the shortest possible run and a decent fall.

Flue route and terminal position

Terminal positions are governed by minimum clearances to openings, boundaries and adjacent surfaces. This frequently constrains where the boiler can go, and it is much better to discover that at survey than after the old one is out. Plume from a condensing oil boiler is visible and it goes somewhere – usually across a path, a window or a neighbour’s fence unless somebody thinks about it.

The oil line and fire valve

A replacement is the right moment to deal with an oil supply line that has been patched, a flexible hose that is past its life, or a fire valve whose sensor has been cable-tied somewhere convenient rather than somewhere correct. These are small items on a quote and large items when they fail.

The tank

If the tank is in poor condition, fitting a new boiler in front of it is building on sand. Sediment and water from a failing tank go straight into a new pump and a new nozzle. We will tell you honestly whether the tank needs doing at the same time – see oil tank replacement.

Building Regulations, certification and what you should be given

Replacing an oil-fired appliance is notifiable work under the Building Regulations. It can be self-certified through the competent person scheme, which means you should receive a certificate confirming the installation was notified. Keep it – it is what a solicitor asks for when you sell the house, and reconstructing it years later is difficult and sometimes impossible.

You should also end up holding: the manufacturer’s installation and user instructions, a completed commissioning record with the combustion readings, the benchmark or equivalent service record book, and the warranty registration. If you are handed a boiler and nothing else, something has been skipped.

Should you replace oil with something else?

It is a fair question and the honest answer is that it depends on the building. A well-insulated house with generous radiators and somewhere sensible to put an outdoor unit is a reasonable candidate for a heat pump. A solid-wall Norfolk cottage with small panel radiators and a stone floor is a much harder proposition, and pretending otherwise does nobody any favours.

We have written the comparison out properly, including where heat pumps genuinely win, in oil vs heat pump for an off-grid Norfolk home. What we will not do is tell you an oil boiler is your only option when it is not, or sell you a heat pump into a building that will be cold and expensive to run.