DIY ventilation heat exchanger

[AC_Hacker]

Originally Posted by mackstann

I'm a little concerned about condensate draining. Creating a sloped pan in the bottom of the enclosure seems like it might be tricky, and how do you keep it from leaking air? A trap will only work during times that create enough condensation to keep the trap full. During spring and fall, the trap might dry out, and then what? Air would be leaking out the condensate tube. Maybe it's a small enough leak (though a small long tube with lots of airflow resistance) to not care.

I think you should be more than a little concerned, HRV condensation is a major issue here in PDX.

A previous PDX poster to the thread made a simple cross flow chloroplast rig that was about 2 feet long and about 6" x 6", using a computer fan. He had it tilted a bit, so that it was self-draining.

It dripped and drained a bit, but it dripped and drained constantly.

If you had a strategically rigged drain pan and a small plastic tube, it would self-drain and there would be minimum air leakage.

BTW, I like your series cross-flow idea.

-AC_Hacker

[jeff5may]

Originally Posted by mackstann

I've been thinking more about design.

I originally wanted to make a counter-flow HX, for maximum efficiency, but it seems like sealing up the alternating channels at the ends would be really finicky and prone to leaking. A square cross-flow HX seems much easier, but efficiency isn't as good. So what I'm thinking about doing is simply making many small cross-flow HX's and putting them in series. It would get close to counter-flow efficiency, but is simpler to make. The hexagonal ones seem like a pain to make and still just a compromise between counter-flow and cross-flow. The counter-flow HX is just so elegant and simple, I find myself maybe irrationally biased towards it.

The concensus among most manufacturers is to use two square hx assemblies. Arranged in a "'double diamond", this seems to be the best trade-off between efficiency and expense.

It doesn't get that cold here in Portland, so I'm thinking I could just have a thermistor in the outside air inlet and shut down at the freezing point and wait until it warms back up before turning back on. The house leaks more during deep cold anyway, due to the stack effect. The most useful time for the HRV is during temperate times when there is no stack effect. Also, I could use a microcontroller to make the HRV run at higher CFM for a while when coming out of a below-freezing shutdown period. So during the winter it'd run at higher speed when above freezing, and then not at all when below freezing, while during warm times it would just run constantly at a low/moderate speed.

Include a defrost damper. When the outdoor air gets too cold, open the damper to prevent the core from freezing. This can be made dead simple or as complex as you would like to control. You can either blend the supply air with defrost air or bypass the supply air, substituting warm indoor air at the inlet or between the two cores. See the pics at the bottom of this post, a picture says a lotta words.

I'm a little concerned about condensate draining. Creating a sloped pan in the bottom of the enclosure seems like it might be tricky, and how do you keep it from leaking air? A trap will only work during times that create enough condensation to keep the trap full. During spring and fall, the trap might dry out, and then what? Air would be leaking out the condensate tube. Maybe it's a small enough leak (though a small long tube with lots of airflow resistance) to not care.

Any time you are doing heat transfer, there will be condensation. During the summer, the fresh warm air from outside will shed some water on its way in. During winter, the stale warm air will shed some water on its way out. The only exception is in the desert or dust bowl during some of the summer. Even there, if it's really hot out and you have air conditioning, some water will condense.

Make sure to include drip edges and a drain in your enclosure. Leave no space for water to stagnate, and put a p-trap in the drain. The small pressure difference (millimeters of H2O) will not overcome a half inch p-trap.

Also, make sure to put filters on the intake sides of both flow paths. These cores like to clog up with dust. Bugs love to make homes in the cores also (especially spiders), so put screens on the outdoor ports to keep them from invading.

The lifebreath unit:


Functional sketch of the lifebreath:

Notice the defrost damper on the fresh air inlet. I believe it will rotate to close off the fresh air port completely when defrosting.

The frigate unit:

Notice the fans are in the center chamber and no defrost damper. The unit defrosts by running exhaust air only.

You can route ducts for these things many ways. Check out this thread:
http://www.doityourself.com/forum/du...uestion.html#b

[Piwoslaw]

Run your intake air through a few meters of buried tube(s). That way it will be precooled in the summer (and slightly dehumidified) and prewarmed in the winter. Not only does this increase the system's efficiency, but also if sized correctly, the intake air will never be below freezing, so no defrost will be required.

[AC_Hacker]

Originally Posted by Piwoslaw

...Run your intake air through a few meters of buried tube(s)...

Piwoslaw,

I agree with the concept for sure, but I think your idea of "a few meters" may be a bit naive. You may be underestimating by a factor of 6.

Most earth-tube designs I have seen call for lengths of around 30 meters.

But your principle is sound.

-AC

[aknowlesey]

Here are some photos of a DIY Heat Recovery System I built last summer with the help of info in this thread.

Like previous posts the core is made from Corex sheets and Corex spacer strips stuck together with double sided tape.
The case is made form laminated MDF, the fans are 48VDC ones I found on Ebay which are controlled by an Arduino, the gold box on top of the unit is the 48V PSU.

There are temp sensors in the four inlets/outlets which are read by the arduino and which can be monitored by myself via my PC.

I built the unit to bring down the relative humidity in my house during the winter, as I found myself opening all the windows during the cold weather for ventilation.

last winter was mild in the UK and the lowest input air temp I saw was about 1*C, with an extract temp of 20*C I was seeing about 16/17*C Supply air to the house, if I remember correctly. As long as the external temp was below 8*C the internal Relative Humidity was between 45-55% where it had been closer to 70% without the heat recovery.

The unit could still do with a bit of work with regards to insulating the ducts as it lives in my loft. You can see the aluminum drip tray which deals with most of the condensation, although I do get some condensation on the internal surfaces which may shorten the life of the MDF case. A better case design would allow all the condensation to run down to one point, or build it out of something water resistant.

When I first ran the unit I didnt connect the condensate drain which seemed fine until the outside temp dropped to below 10*C it then started producing about half pint of water in 24hours

When winter returns ill post some screen shots of the temp sensor readings

[Jim_UK]

This has turned into a long thread, but totally worthwhile.

Thanks to all contributors for the last 4 years or so of fantastic information and inspiration.

Apologies from me for making a long post, but on topics like this enough detail needs to be given to solicit quality responses without lots of follow up posts!

----

I'm researching the design/construction of a HRV for an old wooden boat we're completely restoring/renovating. I'd like to run some design aspects/choices past the audience and ask for feedback/advice please before they get locked in.

To empathise, as it's a boat, power (aka never enough of it, aka electricity) and keeping costs under tight control are the two major factors.

Also due to the nature, I need to plan all of the major works (electrical, plumbing, HVAC etc in whilst I'm working on the Hull/Superstructure). So important to get it 'mostly' right from the start!

So, to our own HRV design...

Proposed HRV design

Three Counter Cross Flow cores in series. Each ~243mm x 243mm x 346mm (~ 9.5" x 9.5" x 13.5"), constructed from 2mm Correx (extruded polypropylene, aka the estate agents sign material as discussed at length here). The size is partially determined by the size of the fans.

I appreciate that 3 cores is perhaps overkill and suffers from diminishing returns. Still, the Correx is very cheap and a small proportion of the costs of building a HRV. Lots of surface area and the 2mm has thin walls.

If air resistance of the 3rd core proves to be too high a cost to pay (compared to the extra ~10% efficiency) then I can leave it out but I'll start off with it in.

Airflow

4x 48V DC 13W 0.27A high efficency axial fans, each yields 226CFM. 172mm (6.75") in Dia. 47 db(A) at full tilt means they are fairly quiet and when they are enclosed with the HRV cores and tucked away in the 'mechanical' room means we'll never know they are running (cross fingers).

Two of these on the input and two on the output. A total of 452CFM for each of the flow and return BUT I only need 400CFM to acheive 6 Air Changes per Hour. I'll PWM these down to the 85% of flow for 6ACH, 42.5% for 3ACH (or drop down to 1 fan per leg) etc (whatever saves the most power).

Depending on time of day and temp/humidity conditions measured throughout will influence the tradeoff between ACH and maintaining the target temp/humidity selected on the 'thermostat'.

Filtration

A cheap, fairly coarse, air con/HRV disposable filter on the Fresh Air In, and I'm hoping to source a HEPA H14 filter for the Fresh Air Out.

A HEPA H14 grade filters down to 0.3µm so is good enough to stop Legionella spores. I don't know if the resistance of this filter will be a show stopper or not yet. I could add more fans (I can get them dirt cheap, and they are energy efficient).

Heating/Cooling

I've also pencilled in two water/air heat exchangers (Honda motorcycle 'radiators') that I'll be passing water through.

Hot through one, cold through the other (not at the same time, motorised ball valves under microprocessor control which, if either, is on). I'm expecting the hot radiator to add 2-6°C depending on air flow. The cold radiator won't be nearly as effective, possibly 1-2°C due to a much lower deltaT, but every single degree of cooling helps.

(Two radiators make the plumbing design simple)

Condensate drainage

I've allowed space for the condensate to collect and the microcontroller will monitor the water level and activate a small DC pump to remove that from time to time.

Humidifier/De-humidifier

Pulling apart a cheap ultrasonic humidifier could be a nice touch. Removing the integral large water reservoir and feeding the water from an external source make them quite compact.

A cheap one can inject 480 ml/hr. In rough numbers that is enough to drop the temp by 2.5°C (I think). However 3 ACH's drop this to @1°C of course, 6ACH's = ~0.5°C. Perhaps something that can inject 1000 ml/hr may be a better choice.

Of course maintaining RH is important but remember this is a boat so higher than average RH on the input compared to a house.

De-humidifier. I'm not so keen on integrating this internally. I can always add this 'feature' at some point if humidity is ever a problem by using one of the portable units as my gut tells me it may only be an issue in rare circumstances (given the number of ACH's the HRV can do, after all a HRV acts somewhat like a dehumidifier as a side effect).

Some background info/factors:

I've roughly calculated that due to the volume size, the amount of insulation etc. that a 5KW water heater (for heating & domestic hot water supply) will ensure a ~20°C internal temp IF we fit a HRV that manages 50% or better under external conditions down to ~-5°C.

Volume: It's not a massive boat, in round numbers 4,000 Cu. Ft.
Insulation: Good, not far off a typical house (double glazing etc).
Location: UK (so Winters can be cold - say -10°C. Summers can be hot - 32°C yesterday for example)
Power: 48VDC (from batteries, generator & solar - shoreline 230VAC at times too)
Air Changes per Hour: between 1 & 6, depending on time of day and environmental circumstances (IE it will be dynamic)
Control: Microprocessor controlled, Temp/Humidity constantly measured everywhere of course.
Heating: As mentioned, a 5KW diesel water heater provides hot water. This would be used for domestic hot water and underfloor radiant heating.
Cooling: As it's a boat, we're sitting on an unlimited supply of 'cold' water. 5-12°C is typical. This would be utilised via a water/water heat exchanger to the 'dirty' water isn't used directly inside the boat of course. This cooling source will be used for other things too.

[AC_Hacker]

Originally Posted by Jim_UK

...Thanks to all contributors for the last 4 years or so of fantastic information and inspiration. ...

I prefer this kind of fan, it has the ECM motor integrated into the hub, and is quieter and more efficient.

-AC

[Jim_UK]

What make/model (URL if available please) are those? How many ACH's etc does your setup do? Any issues with maintaining flow/pressure?

I've already bought the fans but it would still be good to compare/contrast. I can't do inline images or URL's yet (due to low post count) but images.google for 109E1748H502 will quickly show what I have (quite common in top of rack cooling solutions so plentiful from IT refurbishers/recyclers)

[AC_Hacker]

Originally Posted by Jim_UK

What make/model (URL if available please) are those? How many ACH's etc does your setup do? Any issues with maintaining flow/pressure?...

The fan type is "radial", EBM Pabst make an assortment of them.

Ebay has a pretty good listing.

They tend to be very expensive on ebay.

I found mine (I got 2 different-sized pairs) by constant, relentless ebay searching.

The answers to your technical questions could best be found in a EBM Pabst catalog.

Best,

-AC

[NiHaoMike]

Originally Posted by Jim_UK

Power: 48VDC (from batteries, generator & solar - shoreline 230VAC at times too)
...
Heating: As mentioned, a 5KW diesel water heater provides hot water. This would be used for domestic hot water and underfloor radiant heating.

You could build a water source heat pump to supply some of the heat, with the generator supplying the rest. Should be far more efficient than idling the "APU" purely for heat. Also, when connected to mains (maybe not often enough for it to really matter), the APU wouldn't have to run at all.