Every Day is a School Day with an ASHP
Owning an air source heat pump (ASHP) is a constant learning experience. We’ve installed plenty over the years, but only one with a full energy monitoring system. Normally, we rely on manufacturer data from their various apps and online portals—which, let’s be honest, never quite tell the full story.
Recently, I installed a Vaillant aroTHERM plus 10kW three-phase ASHP at my own home, complete with full monitoring hardware, so I can finally see what’s actually happening with the system. I deliberately went for an oversized unit because—well, my gut told me to. Come summer, we’ll be linking the house to an older building, so at the moment the ASHP is nearly twice the size it could or should be?!
The main house itself is a new build, with 285m² of ground floor space and an additional 15m² mezzanine above the master bedroom. About 50% of the house has ceilings averaging 3…5 metres high, so there’s plenty of air to heat. It’s super-insulated, has high thermal mass, and an airtightness of 1.9m3 /(h·m2 ) @ 50Pa —no heat recovery, no trickle vents, just high-powered extractors in each wet area as we use good old-fashioned Stoßlüften (shock ventilation), which works brilliantly in combination with the building’s thermal mass.
We’ve got underfloor heating (UFH) throughout with 100mm pipe centres buried in 96mm of beautifully polished concrete**. This not only provides a fantastic heat store but also makes for a seriously nice-looking floor—because why shouldn’t heating be both efficient and stylish?
The older building we’ll be linking up adds another 80m², with similar U-values, though it won’t match the airtightness or thermal mass of the main house. The total footprint will be 380m², with a calculated heat loss of 6.86:
17 W/m² for the main house (17 × 300m²) = 5.1kW
22 W/m² for the older building (22 × 80m²) = 1.76kW
Total heat loss = 6.86kW
So why did I go bigger than necessary? Because my gut said so.
We did all the usual heat loss calculations, Vaillant did theirs, and even our merchant confirmed the numbers. Every bit of logic pointed to a 7kW system being the perfect fit. But despite all that, I went with my instincts and opted for the 10kW unit instead.
I’m always the first to say, “Trust the numbers”—but this time, I didn’t.
The system was installed in December, with the heat monitoring linked on the 17th Jan, meaning we now have nearly a full month of data—aside from the 36-hour blackout courtesy of Storm Éowyn, which apparently decided we didn’t need electricity for a while. That little hiccup aside, the data is starting to paint a very interesting picture of how the system is actually performing.
The initial results looked promising, with the first week averaging a COP of 6.1 and a mean outside temperature of 7°C. Given that this cold wee country averages around 10°C over the year, that’s a very encouraging start.
Over the following three weeks, we’ve seen a 3-4°C drop in outdoor temperatures, in addition to tweaking the system to see how I can achieve the best COP for domestic hot water (DHW). As a result, our overall SCOP (30-day average) is now sitting at 5.3—still respectable, but there’s plenty more experimenting to be done!
So, back to the oversizing…
Today has been the warmest day since installation, and the charts and graphs on Emoncms Heat Pump Monitor make for some interesting reading. In addition to the increase in outdoor temperature, I did two things yesterday:
- Dropped the heat curve from 0.3 to 0.25
- Set the system to Eco Mode
The result? Eco Mode has nearly doubled the DHW COP—but is this purely down to Eco Mode, or is it just the effect of the warmer outdoor temperature? Something to investigate further.
Another interesting—or slightly concerning—occurrence is that the ASHP appears to be switching off or at least modulating or cycling for extended periods. Between 2:10 am and 5:10 am this morning, the heat pump was basically off.
Is this a sign that the system is too oversized for the milder weather, or is it simply a normal response to lower heat demand? Will adding the extra 80m² of building help balance things out, or have I set myself up for a troublesome time?
Only time will tell whether gut instinct beats maths… but for now, the system is running well, and the learning continues!
