DIY ventilation heat exchanger

[AC_Hacker]

Originally Posted by Viking House

Well, I suppose I could sell you some heat exchangers at cost (€150 each plus transport) if you want to try them out?

Viking House,

I sent a private message to Viking House via the EcoRenovator message system, and I also sent an email to seamus at Viking House Ireland, expressing an interest in buying a FiWiHex heat exchanger.

Oddly, neither did the emails bounce, nor did anyone respond to the emails.

I have also sent a query to Viking House US to see if they know anything about this matter.

Viking House, has your health suddenly deteriorated?

-AC_Hacker

[MN Renovator]

"so -11C. That is about 22 below freezing in F."

-11c is 12.2 degrees F.
-20f would be -29c.
0f is -18c

Remember celcius has 0 at freezing while we have it at 32f and our resolution is higher so the numbers get a little confusing in the negatives sometimes.

[Student 07]

Hi
I am new to the site, so I am still learning (a lot). Thanks everyone.
I do have a question though. I am interested in the DIY heat recovery ventilator. I don't think the commercial HRVs are very efficient (50-80%) so I haven't purchased one yet.

In an earlier post somebody had the idea of using a hacked AC to cool the outgoing air and heating the incoming air, with a heat exchanger in between them. It sounds like a great idea to me.

My question is- wouldn't it be better to run the stale air through the condensor (?), then straight outside, and run the fresh air from outside straight through the evaporator(?)? My point is I don't see the benefit of using the heat exchanger.
It cools the air going to the condensor (which is trying to gather heat). Although the heat exchanger sends 50- 80% of the incoming heat to the fresh air side, wouldn't it be better to let the pump operate at a higher COP and create more heat? The way I see it the hx saves heat and the pump multiplys heat, using the hx lowers the pumps ability to increase heat. I hope I explained things correctly, I am a long ways from being an expert. Just my $.02.

[Piwoslaw]

Welcome to ER

Originally Posted by Student 07

In an earlier post somebody had the idea of using a hacked AC to cool the outgoing air and heating the incoming air, with a heat exchanger in between them. It sounds like a great idea to me.

My question is- wouldn't it be better to run the stale air through the condensor (?), then straight outside, and run the fresh air from outside straight through the evaporator(?)? My point is I don't see the benefit of using the heat exchanger.
It cools the air going to the condensor (which is trying to gather heat). Although the heat exchanger sends 50- 80% of the incoming heat to the fresh air side, wouldn't it be better to let the pump operate at a higher COP and create more heat? The way I see it the hx saves heat and the pump multiplys heat, using the hx lowers the pumps ability to increase heat.

Actually, if you put a heat exchanger before the heat pump it will have much less work to do (much less heat to move). The smaller the temperature difference between airstreams, the smaller the heat pump can be (= less energy + cheaper).

Putting the HP between airstreams with a large temperature differential will raise the COP (assuming heat is being pumped from warm to cool), but at the expense of using much more energy to move all that heat. Let a HX do most of the work, while a small HP only tops off the system's efficiency.

Also, a HX can partially get rid of the moisture, reducing the risk of icing up the HP. Tempering the intake air with an underground HX helps even more, and allows an even smaller HP.

[Student 07]

Hi Piwoslaw,
Great drawing, that describes what I am talking about. I should probably explain the application I am considering. It is winter and I have my crawlspace closed. It would be good to still ventilate it and inexpensively heat the crawl space.
If I understand you correctly, you're saying collect the heat from the stale air before it goes into the HX? I know an ASHP isn't very effective when it is very cold outside, there has to be some heat in the air for it to collect.? I am all for using the HX to save as much heat as possible before it gets vented to outside, but the HX would supply cooled stale air to the HP. I would think that you would want warm stale air going into the heat pump so it can push hot fresh air out.
Some HRVs use an auxillary heating element in the duct that vents fresh air into the home, but it would be better to use an ASHP with a high COP. From my understanding HPs have a higher COP, (produce more heat) when there is lots of heat to be collected. That is why I am thinking that a HE is counterproductive. With a HP the temp on one side depends on the temp of the other side.?
I agree that a smaller heat pump would be better, but between two heat pumps I would choose the one with a higher COP, and I think by supplying warm air to the HP it would achieve a higher COP.
I like your idea of the ground heat exchanger. I did some informal research on an "Earth tube". Most people who use them report mixed results. Some say the moist warm air cools on it's wall and causes condensation in the pipe, which collects dust and eventually mold. I would think you could clean it like a chinmey, so a little maintenance isn't really a problem. One guy I spoke with said it didn't really change the temperature much. He recommended that instead of one large pipe it would have been better if he had used many smaller pipes connected to a manifold. Of course the required lengths and depths of the tubes is a whole other topic.
I will say though that I have considered connecting a HRV to a solar air collector. But like the HP, I think the HE would cool the air coming from the solar collector, and be counterproductive to what I am trying to accomplish. Would it be better to not try to combine all these things?

[Piwoslaw]

Originally Posted by Student 07

If I understand you correctly, you're saying collect the heat from the stale air before it goes into the HX?

No, collect the heat after it has gone through the HX, ie the heat pump is between the two outgoing streams - one going out of the house (cooled stale air) and one going out of the HX into the house (warmed fresh air).

Look at it this way: Assume that the air in the house is at 20°C/68°F, while outdoor temp is 0°C/32°F. If the HX is 80% efficient, then the airstreams leaving it should have temps: 16°C/61°F (housebound) and 4°C/39°F (outbound). Now put a HP on those two to extract heat from the cold stream and cool it to 0°C/32°F, warming the housebound stream to 20°C/68°F. The HP will be pumping "against the temp gradient" to carry over those 4°C/7°F, so its COP won't be too high (though that can be slightly improved by increasing the size of the evaporator and condenser coils).

Now, imagine a HP which has to totally switch the temperatures of the two ingoing streams: It has to cool one stream from 20°C/68°C to 0°C/32°F and warm the other by the same amount. This may seem like good potential for a higher COP, but that is only an illusion. It would be true if you were taking up to 10°C from the warm stream and feeding it to the cold, but you'll still be using electrical energy to do that - it won't flow for free unless you use a HX. Once the two streams' temperatures evened out you'd start pumping "against the temp gradient" again, only with each degree that gradient will get larger. At some point (assuming you are increasing the size of the heat pump) you'll have the warm, stale stream cooled to 4°C/39°F and the fresh stream warmed to 16°C/61°F. Now what's left is the same situation as in the HX case: a 4°C/7°F temperature difference against the gradient. So, summing it all up, you'll be doing all the work as in the first case, plus lots of extra work just to cool the warm stream from 20°C/68°F to 4°C/39°F and to warm the cool stream by the same amount. At the beginning the COP may have been high for a short time, but in all you used much more energy just to do what a heat exchanger does for free.

The energy needed to counter extra pumping losses through the HX is negligible.

[AC_Hacker]

Originally Posted by Ko_deZ

93% efficiency, 300m³ per hour, 20C inside 0C outside.
The outgoing air will have a temperature of 0+(20*0.07) = 1.4C. Difference is 1.4K

Ko_deZ,

As I recall, you said that your heat exchanger is made of aluminum. and that it is about 1 meter long and about 10 cm by 10 cm, and that it is counter-flow, is that correct?

I wanted to build an experimental version similar in construction to your, only from plastic (I know it will not be as good).

I easily can see how I would build the counter-flow, but I don't see how I would get the air flows into the counter-flow structure.

Could you post a photo of your HX core? I am drawing a blank here...

Thanks,

-AC_Hacker

[Ko_deZ]

This is the one I have, with the front door open:
http://ener.no/filestore/E101600K.pdf

The pipes going out of it is 200mm, so I guess the heat sinks are ca 20cmx20cmx100cm.

The principle is like the added drawing. By alternating the airflow with the airflow switch (SW in the drawing), the heat-sinks are heated and cooled. The switch is done just before the heatsink is completely cooled/heated. As you can imagine, there will be an area where most of the temperature change happens that is not very long on the heat sinks (guessing 10-20cm) that will move towards one side or the other depending on airflow direction. In practice you get a counter flow HX, only that you do this using two heat sinks and alternating the direction instead.

I like the principle. It avoids having the constantly moving wheel and airleaks that a rotating HRV has, and also avoids the humidity loss/gain and icing problems that a standard counter-flow unit has. This thing will do humidity recovery which significantly reduces the icing, and in a humid summer day reduces the energy needed for cooling (less condensation). There still might be a little icing in this unit, but until now, I have not seen any. We have not been below -10C (10F?), so I cannot say for sure yet. I can say for sure though, that a normal counter flow unit would have had icing problems already. That unit Viking was talking about would certainly be icing up in our current conditions. Wonder why he never answered my questions regarding that

[AC_Hacker]

Ko_deZ,

Thanks for the picture.

From that photo and this diagram:

I estimated that the cores are 100cm x 20 cm x 50 cm. The 50 cm dimension is if the HRV does not have a false back, I can't tell from the photograph, but the drawing indicates that the cabinet is 52 cm deep, so I assume that the cores go full depth.

I guess the "switch" is a rotating wheel with baffles that allow air to alternately flow through one exchanger and then another.

Here is another similar HRV that uses a similar air flow principle:

Here's inside view:

They even have a video of the thing in action... it is not youtube, so it takes a while to load.

I think the one you have is obviously designed for better efficiency.

-AC_Hacker

[Ko_deZ]

Hi.

There is a false back. That is how the airflow gets where it is supposed to. The doors have rubber gaskets on them ensuring an airtight seal. They might be as much as 30cm deep though. Cannot say for sure.

Interesting video. Yes, same principle, but way too small heat sinks. It is the length that is the secret I believe. My unit switches about every 50 second. Also, having the input/output vents beside each other will give you a bit of air just looping and coming back in.

-Ko_deZ-