Efficiency of compressor assisted HRV

[montreal]

Hello this is my first post.

One HVAC product that is used in Europe but rarely found in North America is the heat-pump assisted heat recovery ventilator, such as the Nilan VPL-15.

So rare are these products where I live that I have had difficulty discussing this subject on local forums.

I have recently retired and have spent a fair amount of my hobby time trying to bulid a similar HRV by taking a small 5200 BTU window air conditioner and re-boxing it in a way that I can attach four flexible hoses to the new box in order to have access to the inlet and outlet of the evaporator and condensor coils.

I am operating on a shoestring budget so my AC did not come with accessories like expansion valves, filter/driers, etc. It is a cap tube type.

I have been operating my creation for a couple of weeks now and I am getting an exchange of air with heat recovery, how much, is hard to measure.

I am having difficulty determining what percentage of the heat in the outgoing stale air stream is being recovered and reintroduced into the incoming fresh air stream.

First, let me say that whatever electrical energy goes into driving my compressor and fan motor produces heat that is brought back into my house and I do not consider this heat as part of the equation of rating the efficiency of my HRV.

I rate the efficiency of my HRV by only comparing the energy extracted from the outgoing stream to the energy added to the incoming stream, independant of the heat from the compressor motor and fan motor.

Unfortunately, it looks like I am comparing apples and oranges. I find that the temperature of my incoming air stream rises more than the temperature of my outgoing air stream falls.

Intuitively, the rise in one should match the fall of the other, since energy cannot be created, only transferred. The wildcard in this equation is the role that humidity plays in the relationship between temperature and energy.

In my system, it appears to take less energy to heat dry air by say 30 degrees compared to the amount of energy that can be extracted when humid air is cooled by the same 30 degrees, and condensation results.

If my unit is to be exactly 100% efficient, then it would require that the outdoor temperature and humidity be such that after I heat this fresh exterior air and bring its humidity up to the same level as the interior ambiant humidity, then the final temperature of the fresh air would be the same as the temperature of the indoor air.

Essentially I would be starting with a pound of stale air at a certain temperature and humidity and exchange it for another pound of air from the outdoors restored to the same temperature and humidity, no more, no less.

For my unit to exceed 100%, I would need to be able to extract more energy from my stale air than I need to process the exterior fresh air back to the starting point.

The professional Nilan HRV claims to extract a certain number of watts of energy from the stale air stream. What they don't say is how much of that extracted energy is necessary to re-heat the fresh air.

Thus it is impossible for me to know if the Nilan meets or exceeds 100% recovery.

I liken the process to riding a bicycle down a hill in hope of coasting all the way up the other side of the valley. In a frictionless world, no pedaling would be required to achieve this. Gravity would do all the work. If one additionally pedaled with the right gearing, one might not only reach the top of the other side of the valley at the same altitude, but have some extra momentum stored up.

In heat pumps, I wonder if a similar analogy can be made? No matter how cold the temperature is outside, this low temperature makes it that much easier for the compressor to extract more energy from the outgoing air stream. I suppose there are limits to this due to the temperature that ***** will boil at in a perfect vacuum, but this aside, there seems to be the expectation that these devices may well exceed 100% recovery no matter if it is minus 20 degrees outside or plus 10.

So far, I have not found that my own creation exceeds 100% efficiency.

I am hoping to hear from any forum member who has some experience with these compressor assisted heat recovery ventilators and knows how well they perform.

Thanks.