We got a heat pump installed. Most of what we read beforehand made it sound like a boiler swap with extra steps. It wasn’t. Nothing went catastrophically wrong. But there were a handful of moments where we thought “why did nobody mention this.” So here’s the honest version — the stuff that happened, not the stuff that’s supposed to happen.
If you’re partway through your own install and something on this page sounds familiar, that’s exactly why we wrote it.
The sizing problem nobody warns you about
Before any of the physical work started, there was a sizing headache. Our house needed roughly 8kW of heating output — a completely normal number for a house our size.
The problem: one of the big manufacturers we looked at (Vaillant) jumps straight from a 7kW unit to a 10kW unit. There’s no 8kW option. So the actual choice on the table was:
Undersize to 7kW — slightly short of what the house needs, meaning it has to work harder on the coldest days
Oversize to 10kW — more than the house needs, which usually means it cycles on and off more than an efficient heat pump should, and costs more upfront
Neither is really “right.” It’s just a gap in that manufacturer’s range that happens to land exactly where a lot of ordinary houses sit. We ended up looking at Octopus’s own unit (the Cosy) instead. Partly because its output steps didn’t leave the same awkward gap for our house size.
The lesson, if there is one: ask your installer directly what the output steps are for any brand they’re quoting, and where your house’s number falls. Nobody tells you about the gaps until you’re the one stuck in one.
Two leaks, for two completely different reasons
We won’t pretend the install was leak-free, because it wasn’t — though neither leak was really about the heat pump itself.
Leak one: our system was originally non-pressurised. Switching it over to a pressurised system for the heat pump revealed a weak point that the old, low-pressure setup had never stressed. It leaked. This is apparently a known risk when converting an older system. The pipework’s been sitting quietly under low pressure for years, sometimes decades. Suddenly being asked to hold more pressure finds whatever weakness was already there.
Leak two: after the first leak was fixed, a second one turned up. The engineer had forgotten to replace a radiator valve while dealing with the first issue. Straightforward human error, sorted quickly. But it’s a good reminder that a multi-day install has a lot of moving parts — worth actually checking things over rather than assuming it’s all been done.
Neither leak was expensive or dramatic in the end. Both were mildly stressful in the moment, mostly because nobody had warned us they were even a possibility.
The pipework almost caused a much bigger job
Our house has 10mm microbore pipework — thin pipes, common in a lot of homes from a certain era. Heat pumps generally need slightly wider pipework to move enough water around efficiently, so this is a genuine checkpoint in any survey.
Ours passed — but only just. If the pipework had been marginally narrower, or the layout slightly less favourable, it would have meant a full pipework replacement. That’s a much bigger, messier, and more expensive job than the heat pump install itself.
If you’re getting a survey done: ask specifically where your pipework sits relative to that threshold, not just whether it’s a “pass.” Ours was a pass with very little room to spare, and that’s a very different situation to comfortably clearing it.
The cost nobody mentioned until it was our problem
Our old system ran on LPG, not mains gas. When it’s mains gas being removed, decommissioning it is usually just folded into the installer’s job. LPG is different. We had to separately find and pay for an approved engineer to disconnect the LPG connection before the heat pump install could go ahead.
It wasn’t a huge cost, but it also wasn’t mentioned anywhere until we asked directly why our quote didn’t seem to account for it. If you’re on LPG or oil rather than mains gas, this is worth raising with your installer early, not assuming it’s included.
Would we do it again?
Yes, honestly. None of this was a reason to regret it — it’s a reasonably involved job, and involved jobs have snags. What would have made the whole thing smoother wasn’t a different heat pump. It was simply knowing these four things existed as possibilities before we started, instead of discovering each one as it happened.
That’s really the only reason this page exists.
A couple of honest disclosures
We use Octopus Energy ourselves, both for our tariff and for the heat pump install. We have referral codes for both — using them can get you a discount, and gets us a small thank-you too. We’re not going to pretend that’s not the case.
Who this doesn’t suit: if you’re not already looking at Octopus as an option, or a different supplier or installer clearly makes more sense for your situation, don’t force it just because of a code below. The rest of this site is built to help you work out what’s actually right for your home. This page is the one place that’s personal rather than neutral — it’s clearly marked as such for exactly that reason.
If you do want to use them:
Energy switch referral code: A split of £100 50:50 with every sign up. £150 if you’re a business.
Heat pump install referral code: see how it works
Just Ask: Heatpump
Just Ask: Heatpump is an independent companion to Visit a Heat Pump and Get a Heat Pump. Built from real installation experience to give honest, practical guidance before you commit.
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