Commercial boiler replacement
A boiler replacement is a plant condition decision. It locks in fossil heating for the life of the plant, so a heat pump is considered first.
What it actually is
A failing boiler is replaced with a modern condensing unit. Efficiency improves. Reliability improves more.
Boilers and heat pumps are alternatives, never both.
A boiler replacement swaps failing or inefficient heating plant for a modern condensing unit. Efficiency improves, and reliability improves more, which in a building where heating failure stops trading is often the real reason for doing it.
It is a plant condition decision rather than an energy decision. The saving from a new boiler against an old one is real but modest, and it is rarely large enough to justify replacing plant that still has years left in it.
The important thing about a boiler replacement is what it forecloses. A new boiler is an asset with a long life, and installing one usually closes the option of electrified heating until it fails in turn. So the heat pump question has to be answered before the boiler is ordered, not afterwards.
Where a heat pump has been assessed and genuinely ruled out on flow temperature, electrical capacity, space or cost, a modern condensing boiler with a proper control strategy is a sound decision. Make that assessment explicit and keep it, because you will be asked about it later.
Which buildings it suits, and which it does not
It suits buildings where the plant is near failure and a heat pump has been ruled out on flow temperature, capacity or space.
It suits you less where the plant has years left, or where a heat pump is viable.
- Buildings with failing plant and no electrical headroomGood fitWhere a heat pump would require a supply upgrade that dwarfs the project, a boiler is the practical answer.
- High temperature systems in older buildingsGood fitWhere emitters cannot be changed economically, the flow temperature required rules a heat pump out.
- Buildings with a genuine high temperature process needGood fitSome hot water and process requirements are outside the range of standard commercial heat pumps.
- Buildings with a carbon commitmentDependsA new fossil asset sits against that commitment for its whole life. It may still be right, but it should be a considered decision.
- Buildings with a poor EPC bandDependsPlant efficiency is modelled, so a replacement moves a rating, though usually less than electrifying the heat would.
- Buildings where the plant has years leftRarelyThe efficiency gain alone rarely justifies early replacement. Spend the money on controls or fabric instead.
- Buildings where a heat pump is viable and unassessedRarelyDo not close the option by default. Get it assessed, then decide.
What it typically costs and saves
- Installed cost
- An installer prices this one
What drives the variation
- •Thermal output required, which follows the heat loss and not the old rating.
- •Whether one unit or a modular arrangement is used, and what resilience you need.
- •Flue routes, condensate drainage and any plant room alterations.
- •Removal and disposal of the old plant, and access to get it out.
- •Controls, metering and commissioning.
- •Whether temporary heating is needed while the work is done.
How this appears on your building
- No gas consumption is held for this building, so no saving has been produced.
What decides whether it works on your building
The age and condition of the existing plant
Genuine end of life, with parts unavailable or repeated failures, is what makes the case. A service history is more useful here than an age.
Whether a heat pump has been properly assessed
Properly means a heat loss survey, a flow temperature, an electrical capacity check and a price. Not a remark in a covering letter.
The heat loss of the building
The new unit should be sized to what the building needs now, which is often much less than the old boiler was rated at, particularly if any fabric work has been done since.
Flue routes, plant room space and gas supply
Modern condensing plant has different flue and condensate requirements from what it replaces, and plant rooms are often tight.
The control strategy
Weather compensation, zoning and time control usually deliver more than the efficiency of the boiler itself. If the controls stay as they were, much of the gain does not appear.
Any carbon commitment or future compliance
Consider the whole life of the asset, not just the year you install it.
How long it takes and what it disrupts
Assessment
Heat loss, heat pump comparison and a plant room survey. Short, and it is the stage that determines whether you are buying the right thing at all.
Installation
Confined to the plant room and the flue route for most of the work, so the rest of the building carries on.
The changeover
The building is without heating during the switch. In an occupied building this is planned for warmer weather or covered with temporary heat.
Commissioning and controls
Setting the compensation curve and the time schedule is where the efficiency is realised. Ask for a review visit in cold weather.
What usually goes wrong or gets missed
Sized to the old boiler
Old plant was routinely oversized, and copying the rating means an oversized new boiler that cycles and never reaches condensing operation.
The heat pump option never properly assessed
The option then closes for the life of the new plant, and it closes without a decision ever being made.
Controls left on the old strategy
A modern boiler run on an old time clock gives back a large share of the efficiency you paid for.
System not designed to condense
A condensing boiler only condenses when the return temperature is low enough. If the system runs hot, you have bought the efficiency and will not see it.
System water quality ignored
Old debris in the system damages new plant quickly. Ask what flushing and water treatment is included.
No resilience considered
In a building where heating failure stops trading, a single unit is a risk. Modular plant costs more and fails softly.
- •It is sized to the old boiler rather than the building's heat loss.
- •A heat pump is never properly assessed, and the option closes for the life of the new plant.
- •Controls are left as they were, so the new boiler runs on the old strategy.
What to ask an installer
- 01What is the heat loss of the building, and how did you calculate it?
- 02What output are you proposing, and why is that different from the old boiler's rating?
- 03Has a heat pump been assessed for this building, and can I see that comparison in writing?
- 04Will the system actually run cool enough for the boiler to condense?
- 05What controls are included, and does that include weather compensation?
- 06What flushing and water treatment is included?
- 07Is temporary heating included during the changeover, and when will it be done?
- 08Is removal and disposal of the old plant in the price?
- 09What resilience does this give me if a unit fails in January?
- 10What annual maintenance does it need and what does that cost?
How it interacts with other measures
With a heat pump
They are substitutes and never both. The heat pump question is answered first, because ordering a boiler answers it by default for the life of the plant.
With insulation
Insulate first if the programme allows, then size the boiler to the improved building. Doing it the other way round leaves you with oversized plant.
With controls
The largest gain in most boiler projects is the control strategy rather than the boiler. Make sure the controls are in the scope, not an option.
With an EPC rating
Heating plant efficiency is a modelled input to a non domestic assessment, so a replacement can move a band.
Does it move an EPC rating
Heating plant efficiency is a modelled input to a non domestic EPC, so a replacement can move a rating.
Ratings matter most if you let the building. Our guide to MEES and the commercial EPC standard sets out the thresholds, the dates and what to do if a building falls short.
Common questions
When should I replace a commercial boiler?
When it is at the end of its life, when parts are becoming difficult, or when failures are affecting the building. Replacing working plant purely for the efficiency gain rarely pays.
Should I fit a heat pump instead?
Assess it before you order a boiler, because a new boiler closes the option for the life of the plant. The assessment needs a heat loss survey, a design flow temperature, an electrical capacity check and a price, so that you are comparing two real options.
How much will a new boiler save?
Less than most people expect from the boiler itself. The larger share of the gain usually comes from the control strategy and from sizing the plant correctly, which is why both belong in the scope.
Will a new boiler improve my EPC rating?
It can, because heating plant efficiency is modelled in a non domestic assessment. Electrifying the heat generally moves it further.
Why is my new boiler not saving what I was told?
The two usual reasons are that it is oversized and cycling, or that the system runs too hot for it to condense. Both are design issues rather than product faults.
Related reading
Where these figures come from
Show the provenance
- Installed cost: An installer prices this one
- The plant room, flue, controls and heat loss have to be assessed first.
The figures on this page are modelled ranges for UK commercial buildings, not a quote for yours. Add your building and we will model it on your own roof, load and rating.