Category Archives: Plot

Posts relating to the building plot

Validation of Passivhaus-Level Heating Performance

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The main criterion for Passivhaus-level performance is achieving a space heating requirement better than 15 kWh per m2 of floor area per year (see e.g. this page from the Passivhaus Institut) when heating to an internal temperature of 20C.

One benefit of having a heating system listed on the Heatpump Monitor website is the Heatpump + Fabric dashboard, which orders systems by kWh/m2 and thus provides a direct view of whether buildings are performing at this level.

Screenshot of the “Heatpump + Fabric” dashboard on Heatpumpmonitor.org

A few points to note:

  • By default, this list is sorted by Electricity Input (Elec kWh/m2) rather than Heat Output (Heat kWh/m2) but that’s easy to change.
  • It’s not obvious but the Elec and Heat figures relate to the Combined performance of the heat pump, when delivering Space Heating and also when heating Domestic Hot Water.
  • There is no indication of the internal temperature achieved, so some buildings might be hotter (or cooler) than the nominal 20C used for the Passivhaus assessment.

The second point tends to increase the consumption figure – potentially quite significantly, because high-performance buildings can use just as much DHW as low-performance buildings, and the higher temperatures required for stored hot water (compared with space heating) results in lower efficiency from a heat pump.

Some systems listed on heatpumpmonitor.org provide additional data on whether they are delivering Space Heating or Hot Water at a particular point in time. If this is accurate, it allows the Hot Water heating periods to be filtered out from the results, providing a more representative view of the heat required for Space Heating. However, it’s clear that many systems don’t accurately report Space Heating mode – so the overall list is unrepresentative. It can be useful to filter out Hot Water production for a single system where the data is known to be accurate.

In my case, looking only at Space Heating drops the kWh / m2 from 13.8 to 13.1, demonstrating that this is even more compliant with the 15.0 kWh / m2 Passivhaus limit (even though the target indoor temperature is 21C, rather than 20C).

Outbuildings: Week 19, Day 3

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Outbuildings: Week 19, Day 3

Back to better weather again. The internal walls on the Garage block are now completed up to ridge height, meaning it’s no longer necessary to omit a few roof trusses to maintain access to build those walls – so the whole set of roof trusses are now in-situ, with a bit more work required to fully brace them in place. The north-eastern gable wall is also nearly complete, leaving just the south-west facing gable (right hand side of the overview photo) to finish off.

The ‘missing’ roof trusses over the Garage have been installed, now the access they were providing to build the internal walls is no longer required

The likely installer for the Air-to-Air Heat Pump also did their first site visit, to advise on placement of the three Internal units and the one External unit. Every model of external unit has a maximum pipe run it can accommodate so the best plan is generally to locate the external unit so as to minimise the pipe runs to the internal units. By chance, this results in the External unit facing due East, which is good in terms of helping to manage the frost build up when running in heating mode on winter mornings.