
Back working on the house again. This time fitting the Heat Recovery Ventilation system (
HRV). It's a Vent
Axia HR250 from
ebay; £250 brand new with speed controller.
All houses need to be ventilated. Highly sealed SIP houses like this one need mechanical ventilation (passive is hard to design and control and cannot recover heat) otherwise you get condensation and poor air quality problems.
An
HRV exchanges the heat energy from stale exhaust air to the incoming fresh air. This creates a rats nest of pipes at the
HRV unit. Fresh air in, stale out, two feeds of warmed air to rooms, extracted air from bath room, small toilet room and, experimentally, a filtered cooker hood.

I've made my own control unit. Essentially it gets three boost signals to raise the unit from L1 (default trickle level) to L2 for small toilet (90m3/h @ 36w), L3 bathroom (112 m3/h @ 51w), Cooker hood (220 m3/h @190W).
The fans in the toilet and bathroom are
icon 30 and icon 60 fans which automatically open when triggered by
humidistats, IR sensors and lights. They each have in built timers. I disabled the icon fans and just use the boosted
HRV to pull the air through the units. This makes them practically silent in operation now. Boosting is achieved with three 240v coil relays on
IDC carriers in a small consumer unit, connected to 3 of the six 6 transformer taps in the speed controller (white box above). The relays are
daisy chained to pass the highest boost level currently on demand to the HR250. Later I might 'break' the
default L1 trickle ventilation rate if the outside air temp is very low or
PIR sensor detects the house is empty.
It all works really well, is quiet and makes the air much fresher, especially in the mezzanine areas of two bedrooms which were poorly ventilated and uncomfortable before. As the fan is balanced (pulls air in while pushing the same
amount out) it runs MUCH quieter than the noisy cooker and bath room fans it made redundant and more efficiently. The bathroom
humidistat runs for a noticeably shorter time. The HR250 has two exhaust ducts. I'm using one on the cooker hood via a filter and I hope that stops it from clogging the heat recovery unit. I'm sure it will be OK as the cooker hood has run for year, and the pipes are clear.
Now how much energy do these things save over just exhaust fans? Well it's not easy to calculate, but here goes (making lots of assumptions):
Taking the volumetric specific heat capacity of air as 1300 J/m3K and the average outdoor temp here as 12
degC, indoor temp 21
degC, the designed ventilation rate for this house 30 l/s or 108 m3/h according to building regs, HR250 has a 70% efficiency.
- The temperature of the incoming air is raised: 21-12 x 70% = 5.6 degC
- The energy reclaimed = (1300 x 108 x 5.6)/3600 = 218W/h
- Now the fan will use 51W to achieve this ventilation rate, so energy saved = 218 - 51 = 167W/h
- Assuming continuous operation that's = (167x24x365)/1000 = 1,462 kWh
- At 0.12p per kWh, that's £175 per year or about 750Kg of carbon saved.
Now that sounds good, but I don't run it at 30l/s, I run at less than half that rate in the summer due to high air infiltration from open windows and doors, and I use boosts when required, so perhaps the savings are going to be more? When running at lower speeds the system seems more efficient. I have measured > 80% efficiency at L1. Later, when the house is empty I want the system to shutdown, so not ventilating at all. I don't know how to calculate all these variables and you start to see why it's hard to work out the saving. I might know next year when I get my full year energy bill.
The system cost £800 when you include the HR250, ducts, vents and controls. As you have to ventilated the house to reduce condensation, smells and pollution it's definitely better than just simply ventilating with no heat recovery.
Tips:
- Put a fresh air inlet close to the entrance of the kitchen to make cooking smell extraction more efficient and encourage air to going into the kitchen.
- To reduce noise don't put a T duct junction closer than 1.5m from HRV unit, and use a soft Y junction.
- Use acoustic ducting between rooms if there is 'cross talk' via the ducting, or for short outlet ducting. It is expensive if you get the proper stuff - it HAS to be acoustic ducting, don't accept insulated ducting with acoustic properties. This can cost 3x more than insulated ducting.
- If you connect the cooker extract, disable the cooker hood fan which locally accelerates grease around the ducts and use the fan signal to demand maximum HRV boost. Use an additional in line-filter if there is any doubt about grease as this is not recommended operation and regularly check and clean the exchanger. Note as the system is balanced you will not need to run it at such a violent rate as before to get the same effect. I find the default trickle vent enough for cooking with lids on pots in summer.
- Build time about 3-4 days for this job.
- I choose the Vent Axia HR250, because it older technology, cheap, robust, easy to speed control, has multiple extract vents, and the company have a good and long reputation. I'm not sure the newer versions with pollen control and summer by pass filters, electronic control systems really justify an extra £1K.
- In winter when the UNICO forced air heating system is on and the windows are normally closed, the HRV uses the house as a giant air mixing box. The UNICO system pushes the air to all corners of the house. I have seen designs with the HRV air inlet feeds only into the return of the forced air system to save using two sets of ducting. This really only works for forced air systems with cooling and heating ie. 24x7x365 operation. I do not use the UNICO system in the summer (The UNICO fan is 200W and I have no cooling option - not very eco!), so I need two sets of ducting. Fortunately my HRV ducting is very limited and 80% is in the loft room.