How to turn a lovley old house with solid walls into a warm efficient home




The heat recovery system and ducting should be sealed within the home. The only venting of the sytem is to the outside where you draw in fresh air and expell stale. It is important, therefore, that the system be balanced. There are two fans within the MVHR unit, one drawing air in and one pushing air out. First off you need to ensure your supply and extract rates match. If they don't and you are, for example, extracting air from the house at a faster rate than you are supplying air, you are effectively creating a negative pressure within the house. In such circumstances air will be drawn into the house via any way possible - tiny gaps around windows, doors or even through the fabric of the building, creating draughts. Likewise if a positive pressure is created then your warm air will be forced out of the house through nooks, crannies etc.

Anemometer, MVHR, heat recovery, eco, home, greenIt is also important to supply and extract air from rooms at an appropriate rate for the room. Supply and extraction rates will be designed by your MVHR supplier. If you are having the unit installed for you then balancing of the system will be done by your contractor. For anyone that has followed any of my blogs you will know that I prefer to do things the hard (cheaper) way. I purchased the system but installed it myself so I needed to find a way of balancing.

A contractor specialising in such installations uses a vane anemometer to measure air flow. However, as we need to know air volume we need an anemometer with a hood so all air is forced through the anemometer, otherwise you don't know what you are measuring. I beleive such devices can be hired, though as it is quite specialist equipment I found nowhere that I could get one at a sensible price for a sensible length of time. Naturally, I was unsure how long it would take and how much fiddling I would need to do. I also considered it would be nice to have such a device for future use in case things are changed or just to recheck periodically.

Unable to put my hands on a dedicated and calibrated unit designed specifically for the purpose I turned to ebay.

Anemometer, Green, heat recovery, eco, homeMany anemometers were available but none that came with a hood. Eventually I purchased a Mastech MS6252A and waited for it to get here from China. It seemed to be capable of measuring all I needed so I would make my own hood and do the necessary calculations and conversions to determine air volume. I created a card cone with a plastic rim on the top to which I fixed a foam door seal strip so that when it was pushed against the ceiling it would seal well and force all the air through the anemometer.

The plastic rim is the top of a bucket and the connecting section between the card cone and the anemometer is an old filler tub, but it sealed and works just dandy.

It is true that my equipment is not calibrated so I can not claim it to be as accurate as a dedicated device. However, I was confident with my calculations and that is was capable of a reasonabley accurate measurement. I also reasoned that the exact rates of supply and extract are less important than balancing The MVHR unit is capable of extracting 100L/sec whilst still heat exchanging efficiently. For my standard setting my supplier had suggest I have a rate of 58L/sec. Even if my device was 10% out and I was extracting slightly more than necessary the system would still be fine and the heat exchanged, and crucially, it would be 10% out on all measurements so it would certainly serve to balance the system.

Balancing is achieved by screwing the ceiling vents in or out to allow or restrict the passage of air. I had a supply/extract figure for each room so it was a matter of going to each room in turn, fitting the hood over the vent and taking the anemometer reading. It's a fiddly business because all the vents on each route are in series, so if I opened up a vent at the start of the extraction run it would effect the other vents. However, by going from vent to vent and increasing or decreadsing the fan speed on the MVHR unit you slowly start getting closer where vent adjustments become smaller and have less of a knock on effect. It was not difficult but just took a little time going from room to room. 

Having had the system running for some time now I am aware that when in the bathroom (the unit is in the loft directly above the bathroom and I have low ceilings) I can hear a slight hum of the unit. This is not excessive, but can be noticed in the background if all is quiet. However, No guests have ever noticed it or commented and I am confident I could use some acoustic insulation material within the loft to reduce this further or even eliminate it completely. I also have one supply vent in my kitchen (I both extract and supply to the kitchen as it is a large room). If I stand directly beneath the vent I again hear a slight hum. However, take even half a step to the side and you hear nothing. the vent is not in an area one woudl normally stand so it doesn't bother me.

I am also aware that in my bathroom and en-suite, the mirrors do not mist up as the moist air is being extracted. Historically, it was common for us to have a bedroom window open even in the winter as we don't like stuffy, warm bedrooms. Now however, we rely on the heat recovery system to supply fresh air without losing heat through the window.

© Christopher Thompson