DIY ventilation heat exchanger

[Student 07]

Hi,

I just have a few comments about combining a heat pump and a HRV. I am with AC, I would think a dehumidifier is the easiest to hack. I will have to keep an eye out for one. Although they use more power, I think the benefit of the HXs is a big plus. When combined with the HRV, the heat pump wouldn't have to work very hard.

Timsmall brings up a concern of mine: "At first glance, I think that running the heat recovery ventilation part efficiently (and quietly) is probably going to conflict with running the heat-pump part efficiently." I think airflow is going to be critical, too much and the HRV isn't efficient, too little and the Heat pump isn't efficient. Although, Piwoslaw pointed out that a larger condensor/ evaporator would solve the problem of low airflow. Maybe it would be best to use the HXs from the dehumidifier with a small cube fridge compressor

Piwoslaws drawing showed the heat pipe and other people have commented on using a solar air collector on the inlet side. But I don't think you can combine all these things. Air heated by a solar collector will be cooled in the HRV. Air flow though the solar collector would also have to be compatible with the airflow through the HRV. You could still use a solar air collector, I just wouldn't run it through the HRV.


A heat pipe would have to be very long to get any noticable gain. Think of the length of heat loops in a geothermal system. If the heat pipe was long enough it might help some if the temperature outside is below 50* (general ground temp), then the heat pipe may warm the air some. But, if the air outside is, say 55-65*f, then the heat pipe would be cooling your incoming air when you still need heat.

I think it would be best to rely on the HRV to do what it is designed to do: bring the temperature up (as much as possible), then use the auxilary heat source (efficient heat pump) to top off the temperature so the air coming into the house is the same temperature as the air being pumped out of the house.

To make a system like Piwoslaw drew work, sizing and airflow will be critical, but I would also have to factor in the amount of noise it may make. I need to do more research. Or, I could probably "reverse engineer" a commercial HRV then try to match a dehumidifier to the flow of the HRV. Such is life. Thanks for the comments.

[AC_Hacker]

Originally Posted by TimSmall

Any idea what sort of COP the de-humidifiers achieve?

If you're talking about de-humidifiers as de-humidifiers, it's probably not very high. I know there are some people who claim to be using them as heaters, I would estimate the COP in that case to be greater than 1 and less than 2.

The COP of an ASHP depends greatly on the heat content of the incoming air, the heat content is partly because of it's temperature, and partly because of the water vapor content. If you use a heat pump to extract the non-water vapor heat (sensible heat) you can capture that heat, if you use a heat pump to condense the water vapor out, you can retrieve that heat (latent heat), too. Heat pumps do both as at the same time.

So, typically ASHP COP ranges from 2.2 to 3.7 average, with some higher COPs if the incoming air is warmer (like if the ASHP is used to salvage heat from warm, moist indoor air in a power-assisted HRV).

Best,

-AC

[kostas]

Talking about motors, what do you think about this one?
It is a dual centrifugal with these specs:
130 W

2300 rmp

Fan diameter 80 mm (aluminum)

Fan length 83 mm

450 m3/h

static pression 237 PA

Exit port: 50x108 mm

I could use just one for both fluxes. 130w seems a good compromise...

[MN Renovator]

That motor is not a very efficient one since it's a shaded pole motor. Basically the cheapest motors around, lower end bathroom fans, kitchen forehead foggers, and very low end furnace motors use this type of motor.

[AC_Hacker]

Originally Posted by kostas

Talking about motors, what do you think about this one?
It is a dual centrifugal with these specs:
130 W
2300 rmp

I think that such a fan would make building your project pretty simple. In the beginning, simple is a very good thing. I'd say this is a good fan to use.

If your project works and solves your problem, then you might think about improvements. From my research, a fan that would lend itself to speed control is most desirable. I have seen Electronically Commutated Motors (ECM) that have a very good reputation for durability, are efficient, and lend themselves to control of speed.

Another good approach might be to look into HRVs that are similar to what you want to build. Download the manuals for the ones you like to see how they are built and get the parts lists, they will list the specifications for the fans.

Best,

-AC

[kostas]

ac_hacker yes, my thoughts exactly.
I saw those fans in a similar model that inspired my project. The advantage of this particular model is the price, roughly €65 which is very convenient. My only doubt is the air flow. A 50x100mm hose seems too narrow to do the work properly, but I guess I'll have to discover it by myself. Anyway, I can always get another pair in case one is not enough
In that case I could control two separate motors for more speed if necessary, for a total of 260W

[AC_Hacker]

Originally Posted by Student 07

...But, if the air outside is, say 55-65*f, then the heat pipe would be cooling your incoming air when you still need heat...

You might be over-thinking this thing a bit...

I don't know about your house, but my house is pretty much in balance when the air outside is 55-65 degrees, and just the additional heat generated by cooking, TV and very low power lighting keeps the place pretty comfortable.

A heat tube should be buried at least 3 ft deep (in our part of the world), more is much better. But during the days when the winter air temperature is hovering in the mid 30's to low 40's (as it does in the lovely Portland winters) the heat boost (underground temps here are around 54F) from an underground heat tube would be a very welcome addition to incoming fresh air temperature, before it hits the HRV.

And it's quite difficult to put a dollar value on the feeling of health and wellbeing that comes from breathing fresh, clean air.

Best,

-AC

[AC_Hacker]

Originally Posted by kostas

ac_hacker yes, my thoughts exactly.
I saw those fans in a similar model that inspired my project. The advantage of this particular model is the price, roughly €65 which is very convenient. My only doubt is the air flow. A 50x100mm hose seems too narrow to do the work properly, but I guess I'll have to discover it by myself. Anyway, I can always get another pair in case one is not enough
In that case I could control two separate motors for more speed if necessary, for a total of 260W

kostas,

Are you really in Italy? I wish I was rather than in this wet gloom that is my fate in the wintertime.

Another good source for high quality fans that I discovered just yesterday, is electronics recyclers. I was down at such a shop yesterday, and saw a really choice high efficiency German ECM fan that would be absolutely perfect for a HRV project. Unfortunately, the fan I looked at had a bad bearing, but these things are used in large computer installations... only the best for large computer installations, you know. And when they upgrade, the whole system (fans and all) comes out and new systems go in (fans and all).

Best,

-AC

[kostas]

ac_hacker yeah, Italy, Venice to be exact. The problem here is summertime, with an average 60-70 % RH for 1-2 months... After nearly 15 years here did not get used to it yet!
I already checked the local electronics recyclers here, not that much unfortunately. And the thing is that you need two identical fans for an HRV in order to have a balanced system, which is not very easy to find there...
Anyway, speaking about the aforementioned fan, I can choose wether to buy a double one or two separate with identical characteristics (but, of course, doubling the power consumption, 130w each). I wonder what are the pros and cons..
Sure a system with two motors is more flexible, i.e. if I decide to swap the air inputs and outputs.

[AC_Hacker]

Originally Posted by kostas

...I can choose wether to buy a double one or two separate with identical characteristics (but, of course, doubling the power consumption, 130w each). I wonder what are the pros and cons..
Sure a system with two motors is more flexible, i.e. if I decide to swap the air inputs and outputs.

If it were me, I'd go for the the configuration that offers greatest energy efficiency. I try hard to reduce every watt, and I'm sure that energy is even more dear where you live.

Best,

-AC