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How can ventilation with heat recovery help your home?

Posted by Helena Ripley on 18 September 2015 at 2:01 pm

The basic theory of a heat exchange or heat recovery system is that heat from one fluid (either a liquid or a gas) is transferred to another fluid (also either a liquid or a gas) without the two fluids mixing.

Q. How does a ventilation system with heat recovery work?

A. For an explanation of airtightness and ventilation in homes see our recent blogs: Why does my home need ventilation and what are the options? and Which retrofit ventilation system - passive or mechanical? Most homes are heated by a localised heat source (like a fire, a radiator or underfloor heating) that warms the air around it; the warm air then moves around the room. So, when this air leaves the building, the warmth is lost. This can result in the house feeling cold or needing extra energy to heat it. To prevent these issues, some ventilation systems are available with built-in heat recovery. In these systems, the extracted warm air flows past incoming cold air and the heat is exchanged between them, warming the incoming air and cooling the outgoing air.

Q. How much heat can be transferred?

A. In theory, this process could be 100% efficient, if all the heat was transferred; in practice though it is around 75% to 95%. This is still significant, as it can both keep the house at a comfortable temperature and save on the amount of heating required.

Q. Can both passive and mechanical ventilation systems recover heat?

A. In theory, yes. There are both passive ventilation with heat recovery (PVHR) and mechanical ventilation with heat recovery (MVHR) systems that can do this. However, the PVHR systems are less efficient as they don’t have as much force behind the air to drive it through the small tubes of the heat transfer unit. MVHR systems have far more efficient heat exchange, but the house must be airtight for MVHR to be most effective. If your house is not draught-proofed and, say, only half of the air goes through the heat exchange unit, the actual efficiency of your heat recovery unit will be less than 45%.

Q. Doesn’t MVHR use a lot of energy?

A. A well-installed mechanical ventilation system with heat recovery will use considerably less energy to run than it will save through heat recovery. But be aware that many systems have a 1- 2kW electric heater element inside the box. By default this will automatically switch on to pre-heat sub zero air from outdoors in the depths of winter unless you overide this feature.

Q. What are the benefits of ventilation with heat exchange?

A. Ventilation with heat exchange is particularly useful in the winter as it provides easily controllable ventilation without any nasty draughts, and it stops heat from escaping. The heat exchange means that the air entering the house is pre-warmed, which prevents cold areas forming. As mentioned above, this can also help to reduce your energy bills as you might not need to have the heating on as frequently or as high. Since MVHR requires a high standard of airtightness, the house will be free from draughts, which also helps it to be comfortable at lower internal temperatures.

Q. Can you turn ventilation systems off?  

A. Some people with mechanical ventilation systems turn them off in the summer and use passive ventilation – for example, opening the windows to ventilate the house. If you are going to turn off a mechanical ventilation system be aware that you will need to ventilate the house by some other means as the high standard of air tightness will cause the indoor air to become unhealthy. Also, if there is condensation in the system and it is off for a long time, mould may grow. This is a particular problem when the system is turned back on again as it could result in mould spores being blown into the house.

The effectiveness of passive stack ventilation or wind-driven ventilation systems can be reduced by closing the vents; again you need to make sure that the house is adequately ventilated by some other means if you do this.

You can witness whole house mechanical ventilation with heat recovery and smaller heat recovery units in action at free SuperHome Open Days in September 2015.

Photo credit: Centre for Sustainable Energy

More information about ventilation and draughts on YouGen

Find a ventilation and draught proofing installer.

Related blogs

Why does my home need ventilation and what are the options?

Which retrofit ventilation system - passive or mechanical?

Need help with any Jargon?

If you have a question about anything in the above blog, please ask it in the comments section below.

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