Why two units?

When designing a Heat Recovery Ventilation system, one of the first questions that comes up is which unit and how many of them. The answers to these questions lie in the size of the house. Like many HRV units on the market, the PA600LI is in its comfort zone serving a house of C200m². Due to the nature of the electronically commutated direct current fan motors used, it is possible to accurately control the speed at any point between zero and one hundred per cent. Because of this, theoretically, the unit can serve anything from a one bed apartment of 40m² up to a large villa of 400m² in floor area. Neither of these is practical. The small flat won’t have the space available for a large unit and it won’t be cost effective in such a small housing unit. Conversely the very large house will be better served by two PA600LI units for the many reasons as follows;

1. One unit serving this size house will run continuously at full speed.
2. There will be no boost facility available to deal with higher-than-normal water use such as showers.
3. Noise levels in the vicinity of the unit will be excessive.
4. Maintenance will be higher due to the high throughput of air dirtying the filters quicker.
5. The thermal efficiency will be down due to the velocity of the air through the exchanger.
6. Life expectancy of the unit will be much reduced.
7. The power consumed will be disproportional to the amount of work done.

Obviously, one unit is not appropriate for this size house. The question arises, what size should be the maximum that one PA600LI will serve? 260m² ? .  Let’s look at this size house. 2019 regs say that we need supply and extract 302 ^m3 /hr to/from this house, while not exceeding 75% of unit capacity. This might be possible but not practical or cost optimal, as all the ducting and air terminals would need to be doubled in size to minimise the static pressure in the system.

If we did succeed in getting this house to comply, what about the economics? It would use 145 watts continuously, which equates to €238/annum at today’s energy prices.

Dividing this house into two will have a usage of 60 watts and a saving of €140/annum.

Taking into consideration the extra cost of larger ducting, the difference in price for this house would be no more than €1,000. This extra cost would be recouped in less than seven years and at that stage the two units will be still performing almost as new. There may be an argument for using only one unit but not a very strong argument.