MHRV

MHRV

Mechancial Heat Recovery Ventilation

I've installed a system in each of the last 4 houses I've owned.

The need for good ventilation provision in houses is one of those things that in the last 20 years is becoming better understood.
We've all seen pictures of mouldy walls and windows in bathrooms and probabl;y experienced condensation on windows and mouldt grouting in bathrooms and showers. Having too much water vapour in your home can damage furnishings, decoration and even the structure of the house.

With MHRV you'll never get mould issues, condensation on windows, lingering smells from cooking and in toilets. After a shower the cubicle will be dry within an hour and it'll be fine to dry your clothes on radiators within the house. Fresher air with normal levels of moisture in your home will bring noticeable health benefits.

An open window or trickle vents integrated into new windows do give some passive ventilation but in the winter heat will escape and you'll get cold drafts and good air circulation/replacement is not guaranteed.

Why ventilate?
Apart from having fresh healthy air in your house to breath, a noticeable lck of smells from cooking (and from the loo) the main benefit is the removal of water vapour. 

During the day the average person produces 3 litres of fluid in the form of water vapour from breathing and sweating. Add to this all the water that escapes into the air during cooking, washing up, showering (and the drying of the cubicle after) and drying clothes and you have a huge amout of water in the air in your house. If this water doesn't get out of your home it will be find it's way into your furnishings, wallls, roof space etc. and will condense on cooler surfaces. This can lead to mould growth and very unhealty living conditions. 

MHRV basically constantly replaces the air in your house with fresh air from outside. The air is replaced roughly once per hour.

Each room has a vent in the ceiling/wall. In the rooms with the greatest amount of water vapour - bathroom, kitchen - the air is sucked out. Then normally drier rooms like bedrooms, living room have air pumped in. Therefore there is a flow of air from those drier rooms into the wetter rooms. The flow of air is very gradual and you are never aware of drafts. Clearly if a room door is very tight and there is no gap underneath, this will inhibit the circulation of air within your house. Sometimes installers will put vents at the bottom of some doors if they are going to stay shut for a long time.

You may be thinking that surely if you suck out the warm moist air out of the wetter rooms and send it outside then surely the heat is lost and you're going to get cold air coming in to replace it.
No. The air that is sucked out of wetter rooms travels in ducting that all eventually join up and lead to the MHRV unit. Inside here, the warm wet air passes into a heat exchanger where at least 85% of the heat is passed onto the cold air coming in from outside. The two air steams do not mix as they are kept apart by the thin walls of the exchanger - only the heat passes from one to the other.
Yes some heat is lost, but this is far less than having windows partially open or other forms of passive ventilation like trickle vents. 

One of the myths about with ducted heating and ventilation is that it can lead to mould growth inside the ducting which could harbour germs and heath risks like legionella. The ducting never gets damp as it's insulated and the flow of air within ensures no condensation occurs. 

Fitting and costs.
The cost of fitting a system will be easily recovered in the coming years by reducing your heating costs, redecoration bills and avoiding potentially costly structural repairs to your property. Plus, it is likely your health will improve.
If you have a standard house with a ground floor and a 1st floor then the MHRV unit may be best positioned in the loft (roof space). Getting the ducting to 1st floor rooms is easy at it will simply  snake it's way within the roof void to each room. Routing the ducting to ground floor rooms will be more tricky and may require some light building work - boxing in the ducting as it travels through 1st floor rooms on it's way to the ceilings in the ground floor. One way of disguising the 'boxing in' is to simply extend a chimney breast by 250mm with some battening and plasterboaad - this way no one will be aware of any change. Other than that it might be possible to run the ducting down the corner of a room next to the doorway or wthin a built in wardrobe.

If your system is properly fitted few people will even know it's there. The small circular vents in the ceilings in each room are the only visible signs. 

The MHRV unit will produce water as the warm air removed from rooms can be saurated in water vapour. This may condense within the hear exhanger. It's all perfectly normal and every system has a way of collecting this and draining it out. Therefore you need to ensure your MHRV unit is placed in a convenient location for these condesates to be drained to the house drainage system - the bath/basin/shower waste for example in the bathroom.

Cooker hood: some advocate integrating the extractor from your cooker hood into the MHRV system. This can indeed be done as the heat from cooking can be utilised to heat up the incoming air. However, it is very important to have very good filters in the cooking hood (which are regularly cleaned or replaced) as you do not want vaporised oils and fat entering the ventialtion system as it will deposit on the ducting walls and the heat exchanger unit resulting in poor performance and eventually failure of the unit.