Most of my week is spent installing heat pumps, mainly oil-to-heat-pump conversions on period and rural properties across Devon, the sort of stone, cob and solid wall houses a lot of installers would rather not quote. One or two days a month I do something different. As a freelance NAPIT assessor, I go out to other companies’ finished jobs and check them against MCS standards: sizing, pipework, commissioning, documentation, the lot. It’s a strange position to be in. I’m judging other installers’ work by the same rules I hold my own jobs to, and four years into it, the same three faults keep turning up.
Undersized Emitters
A heat pump runs at a much lower flow temperature than the boiler it’s replacing, typically 45 to 50°C against 70 to 80°C for an oil or gas boiler. Radiators sized for the old system often can’t shift enough heat at that lower temperature, so the room that was warm on oil is lukewarm on a heat pump, and the customer is left thinking the technology doesn’t work.
On a proper install, someone has done a room-by-room heat loss calculation and upgraded only the radiators that actually need it. On a 1930s six-bedroom home we did in Exmouth, that meant seven radiators needed replacing to run comfortably at a 47°C design flow temperature, while the rest stayed exactly as they were. What I see more often on audit is the opposite: no heat loss survey on file, no radiator schedule, and an installer who’s swapped the boiler for a heat pump and left the emitters as a guess. Sometimes that guess is fine. Often it isn’t, and nobody finds out until the first cold snap.
Pipework That Was Never Resized or Insulated
Older properties are frequently still running the pipework installed for a high-temperature boiler decades ago, undersized for the flow rates a heat pump needs to operate efficiently. On one Newton Abbot job, a poorly insulated period property heated by a twenty-year-old Aga-style back boiler, the existing pipework throughout the house was restricting flow to the point that no heat pump would have worked properly on it without a full repipe. We replaced the distribution pipework, correctly sized it, and fitted fourteen new radiators including some in rooms that had never had heating at all. That’s not a small job, and it’s the sort of thing that gets skipped when a quote is being kept tight.
Insulation on the external run matters more with a heat pump too, since the losses along a badly lagged pipe eat into a system that’s already working at a lower temperature margin than a boiler. On our installs the external pipework goes into trunking, fully insulated end to end, partly for protection from the weather and partly because an uninsulated run outside is money leaking out of the system every hour it runs. On audit, that’s one of the first things I look at, and it’s one of the most common corners cut.
Commissioning That Never Really Happened
This is the one that catches most installers out, because it doesn’t show on the surface. Proper commissioning means checking flow rates, hydraulically balancing the system, setting the weather compensation curve to the property, checking the temperature difference across the heat pump, and producing a certificate with the actual figures recorded on it. What I find on audit is often a certificate that’s blank, generic, or clearly filled in from memory rather than from readings taken on the day. The system might run. It just runs on factory default settings that were never adjusted for the house it’s heating, which usually means it’s working harder than it needs to and costing the customer more than it should.
What to Check Before Sign-Off
For a site manager or a builder bringing in a subcontractor for a heat pump on a renovation, a few questions before sign-off will tell you most of what you need to know:
- Is there a room-by-room heat loss calculation and radiator schedule on file, or was sizing done by eye?
- Is the pipework, especially any external run, fully insulated, and was the existing pipework checked for capacity rather than reused by default?
- Is there a commissioning certificate with real recorded figures, not a template?
- Was any related electrical work, a consumer unit upgrade for instance, carried out and signed off alongside it?
- Is the installer MCS accredited, and does the paperwork match what’s actually in the plant room?
If the property being converted was heated by oil or LPG, it’s worth building the numbers with the current Boiler Upgrade Scheme in mind, which offers up to £9,000 towards an air source heat pump on those properties until March 2027.
None of this is scaremongering. Most heat pump installs I audit are fine, and the technology itself isn’t the issue in the ones that aren’t. When a job does go wrong, it’s very rarely a faulty unit. It’s one of these three things, and all three are checkable on site before the customer moves back in, not after they’ve been living with a cold room for a winter and blaming the heat pump for it.
It’s the same design discipline behind every job we take on at Eco Pro Renewables: survey properly, size it for the actual house, and commission it as if someone’s going to check the paperwork. On my audit days, someone usually is.
