Mitsubishi Ecodan vs Vaillant aroTHERM for a Semi-Detached Retrofit with Old Microbore

An 8mm microbore leg carries roughly 0.09 litres per second before velocity noise and pressure drop become the binding constraint. A heat pump running at a 5K delta-T across the system wants closer to 0.12 to 0.15 l/s per emitter at the same output. The Mitsubishi Ecodan R32 8.5kW, model PUZ-WM85VAA, and the Vaillant aroTHERM plus VWL 75/6 both nominally cover a semi-detached heat loss of 6 to 7kW at minus 2C external. The Vaillant’s advertised low-flow-temperature operation at 55C has limited value if the return is strangled before the far bedroom radiator receives enough water.

Microbore appeared across large areas of UK housing stock between roughly 1970 and 1985, normally as 8mm or 10mm soft copper in a manifold-fed radial layout. A gas boiler at 70C flow copes with the small bore because the larger temperature difference carries the room load. Once flow temperature drops to 45C for a heat pump, the required mass flow roughly doubles for the same kilowatt output, and many 8mm legs cannot provide it.

The microbore verdict

Keep the 8mm throughout and both units are likely to short-cycle while the extremities underheat. The distribution becomes the limiting component before either compressor reaches its useful envelope.

Why the aroTHERM plus loses its edge in the wall

The VWL 75/6 uses R290 propane refrigerant and can genuinely produce 55C to 60C flow at low outdoor temperatures with a seasonal COP in the region of 3.5 to 4.5 under favourable conditions. That headroom encourages a lighter retrofit, because higher flow temperature reduces the amount of emitter upsizing needed. In a property with 15mm and 22mm copper feeding ordinary single and double panel radiators, the argument is sound and can reduce the job by a four-figure sum.

The old radial legs change the calculation. Even at 55C flow, a far radiator on an 8mm run still needs its design mass flow. Over a 9 to 12 metre length, 8mm soft copper develops a pressure drop that the aroTHERM’s internal circulator plus a typical UPM3 secondary pump cannot overcome cleanly. Pushing velocity past 1 m/s brings erosion risk and audible flow noise, often most noticeable in a bedroom during the night.

Running the Vaillant hotter to mask that restriction costs efficiency. A VWL 75/6 forced to 58C flow to get useful heat through microbore will settle at a real-world SCOP near 2.8, no better than the Ecodan at 45C on properly sized pipe. On this pipework, the aroTHERM’s flow-temperature ceiling is a weaker advantage than the brochure suggests. Deliverable output at the radiator is governed by bore, run length, pump head and balancing.

What the Ecodan setup asks for

Mitsubishi’s FTC controller and the Ecodan Melcloud interface lean heavily on weather compensation. On a retrofitted semi, the usual approach is a curve that tracks about 40C flow at 7C outdoors and 48C at minus 2C, with the room thermostat left as a high-limit backstop instead of a cycling on-off control. The FTC5 or FTC6 board handles that curve natively.

Thin pipe dislikes recovery surges, which is why the setback strategy carries more weight than it would on a boiler. A deep overnight setback of 4C or more forces a hard morning lift, and the system then demands high mass flow at exactly the moment the pipework is least able to supply it. Holding to a shallow 1C to 1.5C setback, or dropping setback altogether in favour of a flat low-temperature curve, keeps water moving at a steadier rate. The Ecodan’s modulating inverter compressor can hold a low continuous output down to around 3kW, which suits a 6kW heat loss property operating over long periods.

Commissioning on surviving microbore needs to be gentle and continuous. Any emitters left on old runs still have to be sized for 45C to 48C flow, which commonly means replacing single panels with double-panel-double-convector radiators or using fan-assisted emitters in the weakest rooms.

The pipework bill and the balancing work

Replacing microbore radial legs with 15mm from a repositioned manifold is the biggest variable line in this retrofit. In a two-storey semi with six to eight radiators, chasing walls or lifting first-floor boards and rerunning to 15mm commonly lands in a broad band of three to six thousand pounds. Access, plaster reinstatement and the ground-floor structure drive the spread. The lower end of that band holds when boards lift easily and the manifold sits in an airing cupboard, keeping the run short and the disruption modest. Solid floor screed is the complication that pushes the figure up: legs buried in screed cannot be pulled cleanly, and partial rerouting via skirting-level pipe often becomes the pragmatic route.

Balancing has more influence than it did on the gas system. Once 15mm runs and resized emitters are in place, each radiator lockshield valve should be set with a differential thermometer clipped to the flow and return tails. The target is a consistent 5K to 7K drop across each emitter at steady state.

On the Ecodan, that check is compared with the FTC flow-rate readout in the service menu. If the system delivers the design delta-T at the target flow temperature, the replacement pipework has done enough. Poor balancing lets one favoured radiator take excess flow, raises return temperature and can cut SCOP by a fifth with no obvious symptom beyond the electricity bill.

Opening the system also disturbs old contamination. Cutting out sections of microbore and adding fresh 15mm copper releases magnetite and sludge from legs that may have been untouched for decades. A power flush before commissioning, followed by dosing to the correct concentration of Sentinel X100 inhibitor, is standard on this type of heat pump retrofit because the tighter internal waterways of a plate heat exchanger foul faster than a gas boiler’s did.

Sentinel X100 is dosed at one litre for roughly 100 litres of system volume. A typical semi-detached system with new pipework and resized emitters holds somewhere between 60 and 110 litres, so a single one-litre bottle usually covers it with margin. A magnetic filter such as an Adey MagnaClean or Fernox TF1 belongs on the return before the heat pump.

The inhibitor concentration should be checked at first-year service with a test kit. A topped-up leak can dilute the chemistry and is a common cause of premature exchanger scaling. The heat pump plate exchanger has far less thermal mass and far tighter passages than the old cast-iron gas boiler heat exchanger it replaced, so contamination that caused little trouble before can damage performance quickly.

Fabric work changes the heat pump and the pipe plan

Cavity wall injection on a 1970s semi with unfilled 50mm cavities typically costs a few hundred pounds per property for bonded bead or blown mineral fibre. It cuts wall heat loss substantially and often removes 0.6 to 1.0kW from the design load of a mid-terrace-adjacent semi. Kingspan Kooltherm K7 at the loft, or more commonly 270mm of mineral wool topped up to current standard, removes another meaningful slice. If both are completed before the final heat loss calculation, a design load of 7kW can fall closer to 5kW.

That smaller load changes how much old pipe can remain. A 5kW house load spread across the same emitters permits lower flow temperatures for the same room output, so the surviving 8mm legs to smaller rooms, a bathroom towel rail or a box room radiator may cope without replacement. The insulation spend can reduce the pipe surgery as well as the heat pump size. Filling the cavity and topping up the loft before the MCS heat loss survey can be the difference between rerunning eight radial legs and rerunning five, shifting the pipe upgrade cost by a four-figure margin. On this housing type, fabric work often pays back faster than the difference in unit price between the two heat pumps.

Where the comparison lands

Give both machines upgraded distribution with resized emitters running a 45C flow, and the Ecodan 8.5kW and the aroTHERM 75/6 return broadly similar seasonal efficiency. At that point the choice comes down to installer familiarity, the refrigerant leak-safety siting rules for R290 propane versus R32, and preference between Melcloud and Vaillant sensoAPP controls. The picture changes on surviving or lightly upgraded microbore, where a buyer hoping to run hot and skip the pipework will find both units drifting toward mediocre performance, because the radiator circuit simply cannot move enough water cleanly.

The safer specification is an Ecodan with a flat low-temperature weather-compensation curve and a shallow setback, paired with 15mm replacement runs wherever the room-by-room calculation says the old legs fall short. Emitters should be selected for 45C operation after the cavity and loft work have been included in the heat loss survey. What the plan cannot settle in advance is how far the compromise between old and new distribution can be pushed before efficiency quietly erodes, and that question tends to answer itself only once the floors are up and the true state of the buried runs is finally in view.