Here is a synopsis of the HRV system from Patrick at Snow Temp Heating solutions:
Many systems call themselves heat recovery systems but stay clear of calling themselves a Heat Recovery Ventilator as this machine is defined by many HVAC industries worldwide such as ASME American Society of Mechanical Engineers IRHACE, RACCA etc. The system is defined as having two fans and a heat exchanger core.
Companies such as HRV (Marc Ellis) say their letters stand for Heat recovery ventilation but do not state anywhere they use a heat recovery ventilator, as this claim would be false and liable for prosecution under the fair trading act. They may as well call their product a heat pump as it “pumps” “heat” from the roof space. Marketing people use a pretty loose license. We have had countless people contact us after they have done their research on ventilation systems and think the company H.R.V actually sells what they have researched when they find it is simply an attic fan system.
New Zealand is a unique case in that the attic air systems are the #1 selling ventilation system. In America and Europe the true HRV or ERV system is preferred whilst the PPV systems are banned because of the danger they represent during a house fire where smoke in the roof space is then pushed back into sleeping areas.
The condensation problem is a simple one, it is allowing high humidity air to cool onto a cold surface: two actions are required to fix this.
1) Remove the built up human caused high humidity air from the house (technically the removal of high absolute or latent humidity)
2) Replace it with drier air
Bring in outdoor air.
Outdoor air has less water in it than indoor air, sounds silly as outdoor RH (Relative Humidity) readings could be 98% at 2C and indoor only 50% at 20C but if you bring that wet cold outdoor air into the house and then apply heat to it the humidity of the introduced air falls to just 25%.
Outdoor airs Absolute or total humidity measured by weight is almost always lower than indoor humidity.
At 25% RH and 20C the surface temperature of a surface (say aluminium window frames) is minus 11 and considering the air inside also can keep the frame a bit warmer the outdoor air temperature needs to be almost arctic conditions.
Quite different if no ventilation was present.
Typical new house w=that is not ventilated would have say a minimum overnight temp of 15C and a RH of 50% in these conditions the aluminium frame or glass only needs to reach 4C for moisture to occur so if we have a frost condensation is very likely.
In rooms where the bedroom door is kept closed and breathing increases humidity it can condensate on even mild winter nights.
The cleanaire HRV has two fans one sucking air out of the house via strategically placed exhaust vents and then a second fan bringing in outdoor air. These air flows pass through the HRV unit into a heat exchanger made of wafer thin aluminium plates the air flows don’t mix but pass very close to each other rubbing over the aluminium plates and this transfers heat energy from one air stream to the other.
The result is that the air coming into the house is very close in temperature to that leaving, up to 95%. E.g. 20C indoors.0C outdoors new air is supplied at 19.5C. NO HEATER REQUIRED.
All areas where moisture is a problem requires ventilation if you don’t have a flow through of air how can it be removed and replaced. The PPV systems are not reliable enough as they will push air into the house but it all may go out in one direction as air will go to the easiest exit sometimes this is back up into the roof via downlights.
The Cleanaire system design calculates where airflow is required and then it is sucked out one vent and returns through another, this makes it controllable and targeted.
Keeping the air warm in the house is very important as if we bring air into a house that is kept as the same temperature as outdoors no lowering of RH will occur
Cost wise the systems typically are installed and supplied for between $5000-$6000 dollars, sounds a lot but nothing made in china here.
The heat exchanger core, which is the critical part, is sourced from Europe’s largest heat exchanger company the fans are German and then the metal casing made in Christchurch. Ducting is all acoustic insulated and typically around 80mtrs add 10 vents Y branches, dampers, 20 hrs labour and electrical and the price is more than reasonable.