DIY ventilation heat exchanger

[kostas]

The cardboard exchanger is very interesting, I wonder if there are any mildew problems...

[AC_Hacker]

Gasper,

Thanks for the links, especially the cardboard exchanger. What a concept!

Originally Posted by Gasper

...Temperature efficiency can be over 100% if there is high humidity in indoor air.

Please explain how efficiency can be over 100%... I thought entropy might be involved in HRVs also.

Originally Posted by Gasper

The recuperator from last link is working well for 2 years with no problem, no it's not mine.

...this one really interests me. However, I would think that the cellulose and the moisture together would become the perfect home to mold.

Not true?

Best,

-AC

[Piwoslaw]

Mold happens in damp environments without much airflow. There is lots of airflow inside the recuperator, but there may be corners inside the HX core with less ventilation.

Also, moist cardboard gets soft, so I would imagine it would start to change shape sooner or later. Especially since in many climates both incoming and outgoing air can be quite humid.

Originally Posted by AC_Hacker

Please explain how efficiency can be over 100%... I thought entropy might be involved in HRVs also.

I believe this is similar to how condensing boilers/furnaces can be more than 100% efficient: It is the sum of the purely thermal heat exchange and heat from condensation.

[AC_Hacker]

Originally Posted by Piwoslaw

I believe this is similar to how condensing boilers/furnaces can be more than 100% efficient: It is the sum of the purely thermal heat exchange and heat from condensation.

I don't wish to belabor the point, but if the efficiency of the HRV is above 100%, this would mean that the HRV would have a net positive effect on home energy, and that by operating it, the home would receive more energy than was used to run the HRV. If this was the case, homes could be heated by a bank of HRVs alone.

Do you actually think that this passes the 'smell test'?

And, I also question the idea that condensing furnaces can be more than 100% efficient.

Efficiency is defined as 'energy in' divided by 'energy out'.

So the energy in is the total heat content of the fuel divided by the heat produced by burning the fuel.

The total heat content also includes the latent heat of the moisture in the water by-product.

Assuming we are talking about gas heat, non-condensing furnaces were able to hit almost 85% efficiency (probably not considering the blowers, etc). The newer ones which scavenge the latent heat in the water vapor are able to get up to 93% maybe even 97% (still not considering the blowers).

I do realize that Europe has a better record on efficiency than the US, but exceeding 100% is in opposition to the laws of physics.

Heat Pumps are able to have an efficiency of better than 100% because they are only making a small portion (maybe 28%) of the heat they provide, and they are moving and 'upgrading' the rest of the energy (ultimately it is solar) from the ground or air or water, into the house. So heat pumps are open systems and their efficiency rating is a bit of a fiction, at least on the cosmic scale. As far as the home owner is concerned, she pays for 1 watt and receives 3.5 watts, and she becomes very happy.

Best,

-AC

[Piwoslaw]

Originally Posted by AC_Hacker

I don't wish to belabor the point, but if the efficiency of the HRV is above 100%, this would mean that the HRV would have a net positive effect on home energy, and that by operating it, the home would receive more energy than was used to run the HRV. If this was the case, homes could be heated by a bank of HRVs alone.

Do you actually think that this passes the 'smell test'?

And, I also question the idea that condensing furnaces can be more than 100% efficient.

I'm not claiming that this is an objectively sound way of describing HRV/boiler/furnace efficiency, it's just the way that they are marketed.
There was an efficiency scale used in the times before condensing units went mainstream, and on that scale 100% was the theoretical maximum of just the burner. When the new models became available, the same formula was used for the burner, then the condensing gain was added. This is more of a marketing scheme than physics.
My new boiler's (Ariston Premium Evo System) manual claims:

Efficiency of combustion: 97.8%
Efficiency at rated heat use (60/80°C) Hi/Hs: 97.5/87.5%
Efficiency at rated heat use (30/40°C) Hi/Hs: 105.8/95.3%
Efficiency at 30% at 30°C Hi/Hs: 107.7/97.0%
Efficiency at 30% at 30°C Hi/Hs: 97.7/88.0%
Efficiency at minimum heat use (60/80°C) Hi/Hs: 97.6/87.9%

I'm assuming that Hi is the 'old' way of calculating condenser efficiency, while Hs is the 'total energy' way

Originally Posted by AC_Hacker

I do realize that Europe has a better record on efficiency than the US, but exceeding 100% is in opposition to the laws of physics.

If things can go faster than light, or be colder than absolute zero, then why not >100% efficiency?

[AC_Hacker]

OK, I think I am beginning to see things with the EuroMind now...

Originally Posted by Piwoslaw

If things can go faster than light...

But of course, this would be an Italian sports car...

Originally Posted by Piwoslaw

...or be colder than absolute zero

Naturally, this would be a Norwegian winter night...

Originally Posted by Piwoslaw

...then why not >100% efficiency?

OK, OK, I am moving to Europe!

-AC

P.S.: (...and all this time, I thought that Texans had a monopoly on exaggeration...)

[Gasper]

I just read it somewhere that temperature efficiency can be more than 100%, but only if inside humidity is high and outside very low, but in practice you will never have high inside humidity with HRV, because the air coming in will practically have 0% humidity.

My point was that with HRV you can easily have over 90% temperature efficiency, while with ERV you will be struggling to get 80%. Energy transfer is not measured only by air temperature but also transferred humidity.

This cardboard exchanger is by the words of his creator always dry, he had only one event when outside humidity was high 91% at 12°C and inside humidity was high >70%, he had a little humidity at air outlet, and this part of the exchanger was little bent.

Freezing can be avoided with defrosting, this can be done with ventilating air only out thru the exchanger, cold air intake is shut down or run thru a bypass, this is done only to defrost (eg. 1 min), after that you can run it normally for 30 min and then perform defrost again. The other option is of course electric heater.

[AC_Hacker]

Originally Posted by Gasper

I just read it somewhere that temperature efficiency can be more than 100%, but only if inside humidity is high and outside very low, but in practice you will never have high inside humidity with HRV, because the air coming in will practically have 0% humidity.

OK, so how do you measure temperature efficiency? What is the formula for temperature efficiency? What is the procedure for determining it?

Originally Posted by Gasper

...with HRV you can easily have over 90% temperature efficiency...

I'd like to know more about this...

(* I think one of the problems with the word "easily" is that it is too easily said *)

In my experience, 90% efficiency in anything is not easily achieved. In fact the run of the mill cross-flow HRVs struggle to hit 60%. From researching I have done, only the very best devices that have been carefully designed to salvage the heat energy in outgoing air are able to reach anything approaching 90+% and they do it by salvaging the latent heat in water vapor in this case they are called Energy Recovery Ventilators. The devices that do not utilize the latent heat in water vapor are called Heat Recovery Ventilators... at least that's the way Wikipedia uses the terms.

So, if you are convinced that a HRV can easily reach 90% efficiency, please send me your plans. I have time this weekend to easily whip one together. And please send me your efficiency formulas, for testing.

The world will be a much better place for it.

Best,

-AC

[JRMichler]

Combustion efficiency starts with the heating value of the fuel. Any fuel has two different heating values: Higher Heating Value (HHV) and Lower Heating Value (LHV).

The LHV is measured by burning an amount of the fuel in a bomb calorimeter and cooling the resulting gas to some temperature at which none of the water of combustion is condensed. The HHV is measured the same way, except that the gases of combustion are cooled to a lower temperature where most of the water is condensed.

If you have a high efficiency condensing boiler or furnace, and you compare the resulting heat to the LHV, then you can get over 100% efficiency. If you compare to the HHV, you will not get over 100% efficiency.

[AC_Hacker]

Originally Posted by JRMichler

If you have a high efficiency condensing boiler or furnace, and you compare the resulting heat to the LHV, then you can get over 100% efficiency. If you compare to the HHV, you will not get over 100% efficiency.

Checking the Wikipedia entry there is a historical reason for the confusion:

For historical reasons, the efficiency of power plants and combined heat and power plants in Europe may have once been calculated based on the LHV. However, this does not seem to be the case nowadays and most countries are tending to correctly use HHV for true efficiency figures, and if they are not already then they should be doing so. This is becoming noticeable in modern UK energy publications with the increase of energy awareness and based on the simple fact that it is correct. While in the US, values may have been reported to be generally based on the HHV, although any initial investigation may reveal that the US is still tending to use LHV in some circumstances, whether technically correct or not. This has the peculiar result that contemporary combined heat and power plants, where flue-gas condensation is implemented, may report efficiencies exceeding 100%. Using LHV in other energy calculations brings similar errors, especially when pulled (incorrectly) into electrolysis calculations etc.

Best,

-AC