Alan Clarke explains the role of a mechanical services engineer on a low energy house build, in designing a heating and ventilation system that works efficiently and is affordable.
Interview with Alan Clarke
We've been fortunate enough to have Alan Clarke on the podcast before. Back in episode HPH062 we discussed with him whether a Passivhaus needs a heating system. In a later episode, he also explained the designs for the heating, ventilation, and hot and cold water systems for Ben's own self build.
Today, we welcome him back to conclude our mini-series, looking at different professions that we may need on our self build or retrofit project. Alan is a mechanical services engineer, with particular expertise in Passivhaus buildings.
Most mechanical services engineers will have a background in engineering
A mechanical services engineer will cover the design of the heating and ventilation systems of a building. Some practices would also do M&E (mechanical and electrical) or MEP (mechanical engineering and public health) work, which would cover all of the building services. As well as the heating and ventilation, that will be the electrical services, drainage and plumbing too. The majority of people practicing it professionally would come from an engineering training background.
Relevant parts of the building regulations are Part L and Part F
Part L of the building regulations covers the energy of the house as a whole, including the heating system. An electrical installer would be expected to be qualified and understand how to install a system that is going to be in line with the wiring regulations and building regulations compliance. This might include energy modeling of the building too.
Part F of the building regulations is to do with ventilation. Ventilation systems in houses are relatively new, and there isn't quite the same level of qualification and supervision.
Pre-planning is the ideal time to involve your mechanical services engineer
Not all architects will be familiar with integrating a mechanical ventilation system so that it works efficiently and is easy to maintain. There would be some spacial requirements at Stage 3 of the RIBA Stage of Works, but pre planning is the ideal time for an engineer to come in and think about where the main items of equipment are going to be located, and how the distribution through the building will work. Consideration also needs to be given in the design to accommodate the ductwork, although the detailed design for that would come later. If it's for a Passivhaus, there will also need to be PHPP energy modeling at this stage, before the planning application goes in. This will have an impact on the form of the building, window sizes, etc.
It is important to make sure the overall system is efficient
The energy efficiency of the overall system can be broken down into separate parts:
- The MVHR unit – this must be an efficient piece of kit with good heat recovery efficiency and quiet, efficient fans. If the fans aren't quiet, people won't use it.
- Good ductwork design – fan performance can be scuppered by putting in ducts that are too small, which means the fans have to work harder. They use more energy to run, and you get more noise. A good approach to the design of the ductwork is therefore important.
A further consideration in the Passivhaus analysis is heat loss, looking at how long the ducts are, and also how well insulated they are. Ideally, these should be as short as possible, just one or two metres length, which has implications on where to position the MVHR.
An MVHR unit also needs to have regular filter changes. Therefore, there's no point putting it somewhere inaccessible, where you can't see what you're doing!
There are two main ducting systems
The basic principles of a balanced mechanical ventilation system, are to bring in and supply fresh air to the rooms where we spend the most time, and take out stale air, particularly from the smelliest and wettest parts of the building. The extracted air passes through the MVHR which has a heat exchanger. This transfers practically all the warmth from that extracted air to the incoming air. Ductwork connects the ventilation unit to the rooms in the house.
A radial system uses lots of separate ducts that run to each room. If you want more air to go to or from a particular room then you run two ducts. Also, if it's got a long way to go, you might increase the number of ducts to a distant room to make it easier to get air from there to the MVHR.
The other system used is a branched system. You start with the biggest duct at the MVHR and run to the first room. At this terminal, it tees off to another room before branching off again, and so on. The duct size reduces as you go along.
Generally, the radial system ducts will have a diameter of 75 or 90mm, which is easier to thread through than the branched system, but you do need more of them. The branched system starts off at around 150-160mm, before dropping down to 100mm for individual rooms.
Space for the ducting will need to be allowed for in the structural and architectural design of the building, so it's good to know what system is going to be used as early as possible.
Air should be supplied into the rooms in a way that you don't feel a draft
For a flat ceiling, there would normally be a disc in the ceiling that spreads the air horizontally so that you don't feel it blowing on you.
The other type of supply valve is designed to go in a wall. This also spreads the air horizontally. The air flows are relatively small and mix with the room air quite quickly, meaning it's unnoticeable to occupants.
Around 2.5 litres of moisture is generated into the house per person per day
This is produced in a number of ways, such as:
- Breathing and perspiration – this accounts for around a fifth of the moisture that we put in the house
- Bathing – not just the hot water steam but also the towel and bath mat will continue drying back out into the room too
- Water evaporation from the dishwasher drying cycle
- Cooking – such as steam from boiling water
- Laundry – drying washing indoors can mean there are several kilos of moisture that have to go somewhere
Without some way of extracting all this moisture, that's a lot of water that is just sitting in our houses. It can however be extracted through ventilation. Even on a wet day, the water vapour content of the external air is relatively low, and much less than the air indoors. Simply by bringing it in and exhausting the same volume of inside air, you can expel the water vapour.
Part F of the building regulations has guidance for calculating the ventilation flow rate that's needed. This is inferred from the number of bedrooms. A Passivhaus calculation is more specific and done on a per person basis.
It is more efficient to run the ventilation constantly
Many houses in the UK have intermittent extract fans, which just draw the vapour out when you have a bath or shower. Evidence shows however that this isn't effective in reducing humidity levels enough, leading to condensation and mould potential.
A house with an MVHR does focus on the wet areas, but has the ventilation running all of the time. There's not much to be gained by turning it off, as you're not losing any heat through it. Keeping the low power fans running continuously, rather than being off and then having to work hard for a while, is more energy efficient.
The primary benefit is to our health
Through living in an enclosed space, we generate a number of pollutants, some of which are a side effect of the moisture. The condensation and mould can be extremely detrimental to our health. Even at lower humidity levels dust mites can prevail, causing allergic reactions and asthma.
By ventilating a building we can also dilute and remove the VOCs and chemicals in the air that inevitably come out of the products.
Heat pump systems need to be designed carefully
Traditionally, a gas boiler with radiators has been a fairly resilient system for heating. Also, it's one which doesn't need too much oversight from an installer or heating engineer.
Although a Passivhaus only requires a small amount of heat, this will need to be delivered somehow. Alan commonly specifies heat pumps these days, however, he stresses that they must be designed and installed well.
Put a hot water cylinder where you are going to be using your hot water
It sounds obvious but is an important consideration. Positioning the cylinder centrally to where the hot water will be needed in the kitchen and bathrooms, means that you can have shorter pipe runs. Subsequently, there will be less heat lost through the pipes, and the hot water will reach the taps more quickly.
Most of a mechanical services engineer's work is before construction
A typical day for a mechanical services engineer might involve:
- A discussion with the client to understand their requirements and how they might live in their house
- A review of architect's designs
- Planning the building services, distribution, emitters, ventilation terminals and key bits of plant on the layout drawings
- Drawing up a specification. This is the shopping list for the contractors for the equipment needed. Also, it's the particular requirements for the installation, workmanship and commissioning
- Working on the drawings/revisions – looking at where ventilation and hot water kit will go, positioning a heat pump, and considering any constraints around where that might go
When a client is building a house for the first time, the plans can sometimes evolve later in the process as they think more about what they really want. Ideally you should be executing the plans you have developed in the design stage during the construction stage, not altering them. This will make it harder for the whole team and more expensive for you.
Your mechanical services engineer needs to understand your priorities
They should listen and understand what their brief is. It's not helpful to have someone come in with preconceived ideas of a technically complicated piece of engineering, when the client just wanted something simple. It's a waste of their time and money.
Ideally, you also want your engineer to have enough experience to be able to look back on other projects to see what has and hasn't worked well.
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