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Kirkburton Passivhaus: Heating & MVHR

We’re delighted that the Kirkburton project is nearly completed and will be ready in time to be open for the Passivhaus Open Days in November.  We’re a bit behind on our blogging about the project, so here are some updates from the build.

Heating & MVHR strategy

The MVHR system was designed to be simple and stand-alone and separate from the heating system. Our MVHR team designed a simple root and branch layout and uses a PAUL Focus 200 unit.

Heating deliberations

We had long discussions about the heating strategy with the clients and project team. The clients have always been motivated by environmental concerns and wanted to have a heating system with as minimal impact as possible. We are finding that the question of heating sources (gas vs electricity)  is coming up more and more among our clients more generally as everyone works out how we transition away from fossil fuels.

There is obviously the question of the approach our clients would want to take, and inevitably they would be balancing all considerations, particularly cost and CO2 emissions.   But of course, this also needs to be considered against the backdrop of UK energy policy and particularly the role of electrification.  Can we electrify ourselves out of the crisis?  Can renewables be ramped up sufficiently against the rapidly increasing demand for electricity for heating and transport?  Can increased localised generation (PV) at scale and increased demand be accommodated with a sufficient capacity national grid?  Will a hydrogen grid become feasible – and where will the hydrogen come from if not hydrolysis, that is electrical energy from yet more renewables?  And so on.

In looking at the heating strategy in PHPP, the project was designed to 14 kwh/m2/ annum, as part of the value engineering strategy.  Due to the closeness of all the numbers for achieving Passivhaus certification, it ruled out direct electrical heating for the project, as the numbers wouldn’t have stacked up in PHPP.  However, we suspect that taking into account our musings about electrification, direct electrical heating is unlikely to make sense in this context anyway, so we could happily rule it out. 

ASHP or mains gas?

In the end, the clients had a choice. With the house design, as it was,  we could have had gas or air source heat pump (ASHP) as the heating source.  The additional capital cost and aesthetic impact of an ASHP system in the garden meant that the clients made the pragmatic decision not to go for ASHP at this point, although they wanted a system that could possibly be later switched to ASHP.

A larger heating system than is needed

The house has a heating load of 1.2 kW. We installed the smallest Vaillant Ecotec gas boiler available (with a 12 kW capacity) a cheap simple system that feeds 6 kW total capacity of radiators in the house. We oversized the radiators to a nominal 6 kW for several reasons.  The first reason was to allow an even distribution of the small amounts of heat needed.  More importantly, the high water volume allowed the boiler to work effectively and for the return temperature to be very low, optimising the efficiency of the boiler.  Although we have governed the boiler down to 4 kW this is still hugely over capacity with the house’s heating load of only 1.2 kW. The oversizing means a happy and efficient boiler and a very simple system.  The house doesn’t overheat, it just turns off quicker.

Adaptation to ASHP later on

If and when the heating system is changed to an ASHP in the future, the radiator system needs the capacity to take the supply side at a lower temperature of 35 degrees, as ASHP works most efficiently at that temperature. The oversized radiator system will be ideal, so we could use the same radiators and just switch the heat source to ASHP.  Working with an ASHP the radiators would only be using half temperature output, but this still is well above what the house needs.

Hot water tank

In a Passivhaus the domestic hot water needs are greater than space heating, so most of the heating system will be contributing to the house’s hot water.  There is a 250 litre hot water tank for the house, which has been set up to accommodate future input from solar PV. This is a significant heat source for the house, so to prevent overheating we have ensured that this is highly insulated, with careful insulating of associated pipework,  to cut down on incidental gains in the house.

Renewable energy

Our clients were very keen to have solar PV panels  on their roof and the roof has been designed to accommodate a large array. These have not yet been installed. We are partly waiting to see what happens with Government policy on feed-in-tariffs etc. Current Government policies appear to make it uneconomic to install solar panels although there is a positive announcement recently from Ofgem that renewable energy suppliers would be financially rewarded for supplying electricity to the grid. Watch this space!

Air testing

We’ve had the final airtightness test and are comfortably well within the Passivhaus target of 0.6 ach@50Pa, and made our own 0.4 target, which very conveniently is the level we have been working to in PHPP. This time, we were working with a local airtightness testing company, rather than Leeds Beckett university who have done all our previous testing. The air testers  were over the moon about the low reading, something that they had not encountered before. We could have gone lower, and have done on other projects, but this project has been all about value engineering and so getting a level of 0.4 is was all we needed to achieve. We’re always impressed with the ultra low airtightness results, but to achieve Passivhaus a result of 0.4 is absolutely spot on.

Our airtightness strategy has been much as it was at our Denby Dale and Golcar Passivhaus projects. One small change we made was trying out Pro Clima Aerosana Visconn airtightness spray instead of sand cement parging.  The Aerosana requires three coats and is ideal for fiddly places (eg behind electrical sockets, behind wall plates or on walls where partition wall stud work for partition walls are fixed). We also used it in our ground floor detail (see our earlier blog on the ground floor) which was an airtightness weak spot and using the Aerosana was a belt and braces approach.

For us parging and wet plaster would always be our first choice for our airtightness strategy but for certain strategic applications, such as chasers behind the electrical metal back boxes, we have found the Aerosana very useful.

In the next blog, I’ll be looking at the detailing for the project’s flat roof.

Visit the Kirkburton Passivhaus during the International Passivhaus Open Days 8-9 November 2019.

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