Why heat pumps are less forgiving
A gas boiler runs flow temperatures around 70–80°C and has plenty of output in reserve. A heat pump typically runs 35–50°C and is sized much more precisely to the heat loss of the property. That difference has consequences:
- There is no headroom. A boiler can brute-force its way past a partially restricted system by running hotter. A heat pump cannot. Any loss of circulation shows up directly as rooms that do not reach temperature.
- Flow rates matter far more. Lower flow temperatures mean more water has to move to deliver the same heat. Sludge that a boiler tolerated becomes a real constraint.
- Radiators are already working at their limit. Existing radiators output substantially less heat at 45°C than at 75°C. Losing the bottom third of a panel to sediment is a much bigger proportional loss.
- The equipment is more sensitive and more expensive. Plate heat exchangers in heat pumps have tighter waterways than a boiler's, and they foul faster.
What the cleanse should involve
The same BS 7593:2019 principles apply, but the standard expected is higher, and MCS-certified installers work to documented water quality requirements as a condition of certification. In practice:
- A full power flush of the existing system, not a chemical circulate — the system is about to run at conditions it has never run at.
- A permanent in-line filter, sized for the flow rate the heat pump needs rather than a standard boiler unit.
- Inhibitor suitable for a heat pump system, dosed for the actual system volume.
- Water quality tested and recorded, often with a laboratory sample rather than a test strip.
- Glycol, where there is any frost risk to external pipework — which changes the inhibitor selection.
- Full rebalancing at the new, lower flow temperature.
The honest part
A system cleanse will not rescue a heat pump retrofit that was badly specified. If the radiators are undersized for low-temperature operation, or the heat loss calculation was optimistic, a spotless system will still underperform — it will just underperform for a different reason.
If you are getting heat pump quotes, the cleanse should be itemised on them. An installer who has not mentioned it either intends to skip it or intends to bill for it later.
If you are somewhere in this process
Three common situations, and what usually makes sense:
- Considering a heat pump, existing system is old. A flush is going to be needed either way. Doing it now means you find out about leaks and weak radiators before the heat pump quote is finalised, not after.
- Heat pump quoted, cleanse not mentioned. Ask. It is a legitimate question and the answer is informative.
- Heat pump installed, rooms not getting warm. Worth having the system water tested before assuming the heat pump is undersized. Sometimes it is circulation.
Tell us where you are in the process and we will put you in front of an engineer who works on both sides of this.
Common questions
Do I need a power flush before a heat pump is installed?
If the heat pump is being connected to existing radiators and pipework, then almost always yes. Heat pumps run at much lower flow temperatures with far less output in reserve than a gas boiler, so any restriction in circulation directly affects whether rooms reach temperature. MCS-certified installers work to documented water quality requirements.
Can a heat pump use my existing radiators?
Often, but they output considerably less heat at the lower flow temperatures a heat pump runs at, so some usually need upsizing. This is determined by a room-by-room heat loss calculation, and it is a separate question from whether the system needs cleaning — though a sludged system makes an already marginal radiator worse.