Building Services Engineer Alan Clarke explains how he has designed and specified the ventilation, heating, and hot and cold water systems for Ben's new house.
Interview with Alan Clarke
We first spoke with Alan Clarke back in 2015 where he helped answer our question of whether a Passivhaus needs a heating system. Alan's role is to engineer the design of the plumbing, heating and ventilation within a house, and in this podcast we focus on the designs he has created for Ben's Passivhaus self build.
Alan's aim is to hand over a specification that is as short and easy to follow as possible. On one hand it will act as a shopping list for a contractor to price from, but also provide basic instructions on things to be installed in particular ways, such as pipe size, thickness of insulation, types of terminals for the ventilation unit etc. It will be a comprehensive list of all of the kit required, the instructions on how to get it commissioned, and what flow rates to commission the ventilation system to.
Start by getting the ventilation system right
Firstly the design must make physical space for the required services, and make them work well. Alan will develop the ventilation scheme by looking at the plans and understanding the airflow, establishing which are the supply rooms and which they will need to extract from. At this early stage he will feed the information into the PHPP energy analysis programme, along with heat loss information which will help when it comes to working out the heat load details for the heating system design.
The ventilation systems Alan uses are all Passivhaus certified. Using a certified unit tends to be cost effective as their efficiency is tested under the methodology that fits in with the energy calculation in PHPP. Some non-certified units tend to shut down the supply air in sub-zero temperatures, but the certified units have frost protection. They also have the commissioning controls to allow you to set them up in a properly balanced way, so you're not wasting heat through unbalanced supply and extract.
Larger ducts can produce a more efficient system
With very little ceiling space the ducts can be made flat to be squeezed in unobtrusively. The downside to these is that more fan pressure is required to push the air through the narrower ducts. More fan power requires greater energy and more noise is generated at the unit. It also tends to be a system which doesn't come with well sealed junctions in the duct work.
The alternative approach is to use slightly bigger, round ducts which are inherently easier to make airtight junctions. This approach tends to produce quiet and efficient systems, but does require thinking more cleverly about how to make space for them all.
In Ben's house the ducts are spread around easily on the lower floor because there are open web floor joists which are engineered joists with a metal zig-zag, giving plenty of space to fit services through.
Upstairs there is a truss rafter roof. In a Passivhaus they make the air barrier the base of the trusses. Lots of insulation goes on top and Alan wouldn't want to be putting lots of ducts through that air barrier and into the cold loft space. Keeping them inside the warm, airtight envelope means that there needs to be a dropped ceiling somewhere to run the ducts through. They will be lowering the landing ceiling to accommodate the ducts, by virtue of it connecting the rooms together and not having windows so there will be no loss of window head height. The ducts stop at the bedrooms where there is a step up in ceiling height. Here they will use a wall mounted, side throw air terminal to push the air into the rooms.
Ben will be using a Zehnder Passivhaus-Certified MVHR
It has a fairly large capacity so is ideally suited for family houses. It has been designed with integral airflow measuring inside the unit, meaning that whatever the conditions it will maintain the selected ventilation rate without losing performance.
A change to the size of the kitchen and utility areas has meant that the MVHR will now be outside of the thermal envelope and inside the garage. It will be on the house wall of the garage to make the ducting run as short as possible, and the ducts will be insulated to reduce the heat losses.
The house will have a mainstream boiler
Alan is seeing more projects using air source heat pumps, which although they have a high capital outlay do benefit from low running costs. Using specialist imported systems can create problems when there is a fault, with a lack of qualified tradespeople available to fix them. A mainstream boiler won't have those problems when it comes to parts and technical support.
The hot water will have a flow restriction
The system is designed to get hot water to the taps as quickly as possible without circulating hot water that is losing heat to the rest of the house. Entering information into PHPP on pipe sizes and how they will match up with the flow rates and available pressure will help calculate the heat loss.
Ben does have a concern about how the restricted flow will actually feel in the shower pressure, so it will be important to select a shower head that has been designed to work in conjunction with flow regulation.
Radiators have been installed to complement the flexible spaces
Alan has considered the design of the house and how it will be used, to conclude that there will be radiators in most of the rooms. For example, one bedroom has three external walls and a balcony door so a radiator would seem sensible to accommodate people's different comfort levels. Another spare room might have different temperature requirements depending on whether it is being used as a guest bedroom or a study.
Underfloor heating wouldn't be cost effective
Alan explains that it is inherently a high output system, with the whole floor available to heat the space. By virtue of it being a Passivhaus the heating demand will be reduced by a factor of ten, so paying full rate to cover the whole floor with underfloor heating doesn't seem logical.
Find out more
Download a transcript of the interview with Alan Clarke.