Why can’t MVHR be used for cooling?

With homes being built more efficiently and more airtight than ever, the demand for summer cooling options in UK homes is increasing. As an MVHR manufacturer we’re often asked if our systems can help to cool as well as recover heat. The short answer is no; MVHR is primarily a heat recovery system – cooling just isn’t what it’s designed for. But why?

Fundamentally, MVHR systems just don’t move enough air to achieve effective cooling. The systems are designed to achieve 0.5 air changes per hour (ac/h) – this means it takes two hours for the air in your house to be completely replaced with fresh air. At the other end of the spectrum, air conditioning units deliver roughly four air changes per hour – that’s eight times more air movement than an MVHR system provides. Essentially this means that your MVHR system would have to work eight times harder to achieve the same number of air changes needed for effective cooling. Besides the obvious factors like increased power consumption and noise, there’s another, much more integral reason why this isn’t practical…

MVHR units are designed to bring fresh air in from the outside using ‘replacement air’ whilst AC systems recirculate the air already in the house. Using ‘recirculated air’ enables the AC to keep the cool air in your house and have a higher number of ac/h without wasting energy. If you were to increase the number of air changes of the MVHR system to match that of the AC, the air in the house would be completely replaced every 15 minutes. This results in a massive waste of energy and over-ventilation. In this instance, adding cooling to your MVHR would be ineffective as the cool air supplied by the system would be quickly extracted and exhausted to outside. The alternative is keeping the system on its designated speed setting (0.5 ac/h) and increasing the amount of cooling you’re adding to the air. Unfortunately, this isn’t a great option either. At this speed, your MVHR system would need to supply much colder air (somewhere around -20°C) to achieve the same level of cooling as air conditioning. Not only is this impractical but it would result in some pretty nasty draughts in your home.

Put simply, adding a chiller unit to your MVHR system would be like trying to heat your whole house with one tiny electric heater – it just doesn’t work. Besides, all the ducts would need to be insulated to avoid condensation forming in the duct system. With that idea redundant, let’s look at what other options are available. The good news is that MVHR and air conditioning work great together (while acting as separate systems, of course). With the AC cooling down the temperature of your house, you might think that the MVHR extracting this air would be conflictive. In normal operation, the MVHR uses a heat exchanger to recover and keep as much heat in the house as possible. However, if the air in your house is cooler than the air outside, the heat exchanger will work in reverse. This means it’s using the air (cooled down by the AC) to further cool the incoming warm air, thereby keeping your house cool and fresh. While they seem to have opposing goals, having MVHR to work alongside your AC can actually compliment the system and aid the cooling of your house.

We’re often asked if adding a summer bypass to the system helps with cooling. This answer isn’t so black and white. One thing to remember about MVHR is that it can only work with the two temperatures it has – inside and outside temperature. It neither creates heat nor adds cooling; it just balances the temperatures it’s given to create a pleasant atmosphere. The use of the summer bypass means that in summer, the air bypasses the heat exchanger to prevent exacerbating the temperature in the house. In the circumstance that the temperature outside is cooler than that of the house, the system will bring the cool air straight in from the outside. This means the MVHR system is essentially cooling your house down whilst not

creating any cold air itself. So yes, the summer bypass can help to cool your house but only if there is a cooler temperature available for it to use.

Fig. 1 - Diagram of the Summer Bypass. The blue route shows the direction of air flow with the summer bypass fitted. The red route shows how the system works in ‘normal’ operation (or without the bypass).

Whilst both of these options can help to keep your house cool in different ways, its good to bear in mind that there is no additional benefit to having both air conditioning and a summer bypass installed. If the AC is active, the temperature in the house should (hopefully) be lower than the temperature outside. This means the MVHR will continue to use the heat exchanger to reduce the temperature of the warmer incoming air with the cooler internal air, creating very little opportunity for the summer bypass to activate.

In summary, we wouldn’t recommend attempting to integrate summer cooling with your MVHR system as it will be highly inefficient and not cost effective. Instead, you can opt for a separate air conditioning system or add a summer bypass onto the system.

Have more questions about cooling in relation to MVHR? Give us a call to discuss.