It’s relatively easy to fit a cooling coil to the MVHR that will lower the temperature of the air being supplied. But that doesn’t mean it’ll lower the temperature of the building.
In terms of cooling potential, the entry-level is an evaporative cooling coil full of tap water, the mid-level is a condensing coil full of chilled water, normally generated by a reversible heat pump, and the top-level is an Air Conditioning DX coil full of liquid refrigerant gas. It can provide the most cooling because refrigerant gas carries more energy than water.
But none of these options will cool the building properly unless you put more air in as well as cooler air. Moving more air through the building quietly and efficiently is the challenge, and it’s also where most of the cost is.
To meet the minimum background ventilation rate in a domestic property you must replace all of the air inside the property every 2.5 hours. Or put another way, achieve 0.4 Air Changes per Hour (ACH). But if you’re cooling the air, that’s not enough air to significantly lower the temperature inside the building. Not so you’d notice. It’ll be like trying to cool a sauna with an ice cube. For MVHR cooling to work properly the ventilation rate needs to be 1.0 ACH. Effectively, you have to design the MVHR to ventilate a property 2.5 times bigger than it actually is.
To do that you need more pipes, bigger pipes, and a more powerful machine. You also need to insulate all the supply pipes so the air doesn’t warm up by the time it reaches the rooms you’re trying to cool. The lagging also prevents condensation problems. Because you can’t retrofit more pipes or lag pipes without returning to First Fix, you can’t retrofit MVHR cooling. Simply bolting on a coil and turning up the fans risks overwhelming the system and exceeding the acoustic and energy regulations. It’ll just make a racket until the motors wear out.
Full-blown Air Conditioning works at 4.0 ACH, so ten times the background ventilation rate. It can therefore provide more cooling to individual rooms, whereas MVHR cooling lowers the temperature in the whole house, only by a lesser amount. Done properly, MVHR cooling should capable of dropping the room temperature by up to 10oC. But beware of providers achieve an average indoor temperature at least 5oC cooler than outdoors, although more cooling is certainly possible.
Note: Under building regulations AC doesn’t qualify as ‘ventilation’, so if you want it you’ll need MVHR too. And because one system moves considerably more air than the other, when you have both in the same property the ventilation can be compromised. This is one reasons why MVHR with ‘comfort’ cooling is an attractive option.
What is the difference between MVHR and Air Con? from Airflow Developments on Vimeo.
Before you plan any ‘active’ cooling for the property you need a ‘passive’ cooling strategy. It makes a lot more sense to keep the heat out than paying to deal with it after it’s got in. Simple interventions that stop the property overheating in the first place. Heat reflecting underlay, solar control glazing, shading and blinds, effective insulation etc. Prevention is better than the cure, and certainly less expensive. Otherwise, the modest cooling available through MVHR will be overwhelmed by outside influences like the sun, teenagers playing 1kW X-boxes, or 3kW rack servers to name a few.
Since the introduction of Part-O regulations in June 2022, your ventilation designer is now responsible for assessing potential overheating risks. Or they should be. TM59 Thermal Modelling may be needed in certain parts of the country, particularly city centres. They should be able to guide to your architect and build team to help them identify and mitigate potential over-heating risks. Someone familiar with Passivhaus principles would be invaluable on your design and build team. We are living with Global Warming after all.
What is Part O and why is it important? from Airflow Developments on Vimeo.
