DIY ventilation heat exchanger

[TimSmall]

So, as far as I can see, this problem can be split broadly into two different areas:

1. Heat Exchanger (plus housing, condensate drainage, duct ports etc.).
2. Fans
3. Control system (fan balancing, condensation control, CO2 control, noise control, freezing mitigation etc.).

There are quite a few different possibilities for the heat exchanger and enclosure, and the fans - and the optimum solution for each project will vary according to quite a few project-specific factors, it should be possible to make a pretty good universal (and I would prefer open-source) modular control system.

I'm thinking of buying an off-the-shelf exchanger, and doing the rest myself.

Why?

At the moment, there are hundreds of companies worldwide making their own units (a few big players, and many small ones). They all write their own control systems (or don't have on at all, just "dumb" speed setting). This sort of reminds me of the market for "small" computers in the early 80s - they all came with their own software, and overall the cost was high and the quality poor. Unless you were a big customer, modification of the operating system was expensive (and possibly difficult / inconvenient, or even impossible).

Here are just a few of the features which I think we could all have with a good open source system, some of these are present in the best commercial designs, but none of them have all those features...

. Automatic fan speed balancing to overcome external wind conditions, duct resistances etc.
. Intelligent condensation control (based on dew point calcs)
. Speed adjustment to minimise energy usage and noise

Those would be an excellent start, and you can quite easily spend lots of money on units which don't do all of those. Once you have that, you could add in:

. Internal air quality control via CO2 sensors
. Freeze control and recovery
. Realtime efficiency calculation
. Error alerting (fan failures, efficiency loss etc.)
. Remote dampers or auxiliary fans
. Noise control (based on time of day)
. Multiple sensor options (built-in, or remote, wired or wireless)
. Filter / duct cleaning reminders based on volume
. Summer cooling (over-night heat exchanger by-pass options etc.)
. Decent user interfaces (control panels or Android, iPhone, Web) and data logging options

Some of the higher-end models have some of those features, but I bet that none of them have all those features, and if the software became good enough (and was correctly licensed), then commercial manufacturers (especially the smaller ones) may well even pick up the software and begin using it themselves...

Any thoughts (overly ambitious maybe? )

Tim.

[TimSmall]

Originally Posted by Student 07

I really like the idea of using aluminum plates, they should increase efficiency.

They will a bit, but they'll also increase the cost (for a given volume of exchanger) and embodied energy. I believe that the limiting factor is the conductivity of the air, so an important thing is to get good turbulent air flow through the exchanger. Many commercial units use plastic, or paper based exchangers for this reason.

Originally Posted by Student 07

I still have a problem with HRVs. Even at 80% efficiency you are venting warm air and replacing it with cooler air. If the house is at 68*F you would be venting that air and replacing it with 61*F air. You would be cooling your house in the winter.

Yes, but that's still 80% less than without "heat-recovered" ventilation, and it's quite reasonable to get 90% efficiency or more at lower air flows.

There's a nice calculator to play with here which gives efficiency vs. air flow for one manufacturers' exchangers:

http://www.recair.com/recuperator_models.php

People need outdoor air to keep indoor CO2 levels under control, and also to avoid indoor pollutant build-up (the standard for UK schools is something like 10 litres of air per pupil per second or 1500 parts per million CO2 max daily average). We aren't well evolved to indoor living, and CO2 build-up does all sorts of odd things to our bodies.

At night, it might be reasonable to drop that figure to 5 l/s per sleeping person (e.g. 55 m³/hr == 32 cfm for 3 sleeping people). If your house leaks enough that you already get that amount of air in "by accident" (depending on wind speed), then you probably need to fix that before fitting an HRV...

[TimSmall]

Originally Posted by Student 07

If you could use PWM to control the speed- a variable speed motor could easily be adjusted to an acceptable level that meets both noise and efficiency requirements.

Most speed controllable fans seem to go down to 10%. If you're shopping for cheap EC (electronically commutated == high efficiency, long life) ebm papst fans, there seem to be various available some are standard parts (i.e. ebm publish the data sheets), and some seem to be customer-specials (may be a bit more tricky to control).

I don't fully understand the ebm naming, but many seem to come in 4 different variants:

ending in "xyz / blahblah P" are PWM controlled (easy with arduino etc.) 0%, 10%-to-100% (i.e. probably) 0-9% not possible.

ending in "xyz / blahblah I" are controlled by a 0..5v (or sometimes 0..10v) signal (this is also possible with an arduino with a few extra components - many sample designs findable on the net) 10% speed minimum same as PWM.

ending in "xyz / blahblah T" are controlled by a thermistor in the air flow (either built-in, or user-supplied), dunno how far you can "turn these down", might vary by type. Can control these from an arduino too, but might require a bit more fiddling.

Most of the EC fans the market aren't speed controllable and just have a 2 supply wires (sometimes you can reduce them down to 70% speed by changing the supply voltage, but this is a PITA, and probably not worth the hassle - I think 70% to 100% is probably not very well suited to domestic loads, and a wide control range is probably better suited.

There seem to be a few cheap used "P" fans floating around from Sun servers, and also quite a few cheap "T" fans from Lucent kit (new and used).

Generally as far as I can tell, axial fans are generally the most efficient for low resistance set-ups. Centrifugal fans are generally the most efficient for medium and high resistance set-ups, OK for low-resistance too, but a bit less efficient and a less convenient shape. Mixed flow are a sort of hybrid between the axial and centrifugal - most efficient at intermediate pressures.

They do DC and AC supply ones.


Another possible option which I looked at were 12v DC "bilge" fans for boats. These are quite cheap, and you could control these fairly easily with something like an Arduino + motor controller, but most of them probably use "brush commutated" motors, which won't last as long, and aren't as efficient.

The larger standard computer case fans are also possibilities (14 cm etc.), and are quite cheap, but tend to be a bit less efficient, and need very low duct+heat-exchanger resistances. You can install two (or more) fans in series tho' (2 fans in series will roughly double the static pressure at a given air flow).

[Piwoslaw]

Originally Posted by Student 07

I have some concerns about combining the HRV with a HP (which would be a hacked dehumidifier or AC unit): most HRV are rated at 150-350CFM while an Air Source Heat pump requires a lot of air. My 4 1/2 ton unit flows about 3500CFM.

This is a small heat pump, it doesn't need a 16kW compressor! Since you'll have relatively slow airflow, then make the HP's heat exchangers large. Also, you won't be moving a terribly large amount of heat, plus the temperature difference is small, so you could probably get away with a compressor from a fridge or freezer. OK, not a 30W tiny fridge compressor, but a 150-250W compressor from a larger unit might suffice.

Going slightly off topic, in that drawing of mine I suggested that the intake path start with an underground heat exchanger to preheat the fresh incoming air, which should also lower the temperature differential between the two airstreams coming out of the recuperator.
Since building an underground HX isn't always possible, how about a solar collector instead? Imagine that the recuperator's fresh air enters through a vent in the wall. Now place a solar box in front of the vent (south facing insulated box with glass on top). If the sun is shining, then the intake air gets free heat, else it is the same as without the solar box. Similar to a solar air warmer for ASHP I once proposed.

[kostas]

Piwoslaw I am currently planning to use the terminal part of my propane heater's exhausting pipe as a pre-heater for my HRV (you can see some early pics here). When the heating is on the incoming air gains about 8 degrees Celsius, which is not that bad at all.
Connecting a solar panel is also a good idea, but you have to add extra fans due to pressure drop inside it. Keep in mind that a well constructed air panel easily arrives at high temperatures. If the incoming air temperature is higher than the out coming stale air it gets cooled by the exchanger. To avoid this a thermostat is needed in order to stop the stale air circuit and allow only the hot air to pump into the house.

[Piwoslaw]

^^ Yeah, I forgot to mention the thermostat by-pass. Also, The solar box would be removed altogether during warmer months.

[AC_Hacker]

Originally Posted by Student 07

...I have some concerns about combining the HRV with a HP (which would be a hacked dehumidifier or AC unit): most HRV are rated at 150-350CFM while an Air Source Heat pump requires a lot of air. My 4 1/2 ton unit flows about 3500CFM. If I design it to make the HRV more efficient (small gap between plates) then the HP won't get much air flow, and if the plates of the HRV are spaced farther apart to increase flow for the HP, then the HRV is less efficient... Which is the opposite of what a HP needs I suppose there would have to be a compromise between what they need. Does anybody know how much a small (say 1 ton) dehumidifier flows- CFM?
Every time I think about needing a HRV I come back to this, eventually I will have to make it.

I believe you need to scale the heat pump size way, way down.

I came across some European units that work as power-assisted HRVs and also help heat water. Their compressors were really tiny. As I recall, there was some information posted earlier in this thread about such a unit.

The smallest de-humidifier compressor I know of can be found in a 20 pint per day dehumidifier, and had a compressor of about 300 watts (about 3200 BTU).

Some of the tiny cube refrigerators have compressors about 125 watts, but their COP looks to be awful... But if you could find one cheap, it might be worth a try.

The little dehumidifiers are best for experimenting, because you already have the air-to-air HXs, and a useful cap tube. I doubt that you would want to use the fan though... you could do better with some of the fans that are currently being discussed.

An R22 unit is great because you can use propane, but then again, the R134a units use refrigerant you can get over the counter at auto parts stores. The R134a is not flammable, but it is a global warming gas.

If it were me, I'd go for the very smallest size de-humidifier compressor I could possibly find... see how it works out, you'll really learn a lot. Then scale it up if you need. But since you're only trying to salvage the heat in stale air, very little power is required... unless your house is quite large.

But all this HRV discussion is predicated on having a nearly hermetically sealed house, where mechanical ventilation is an obvious requirement.

-AC

[TimSmall]

Originally Posted by abogart

To me, it seems like this system would produce much more heat than an air-air HP located outside.

A has been said, the air flows involved in heat recovery are relatively much lower than is typically needed for the size of heat pump which will be able to heat a building. You will also get increased risk of icing on the evaporator size, since the air will already be at 100% RH. Both of these may limit the usefulness of this design in many situations, but you'll need to do some maths to see if they will be a problem in your situation.

Maybe it'd be of more benefit for summer cooling? You can also use evaporative cooling on the exhaust air before you feed it through the exchanger, that way you get the benefits of evaporative cooling (cost), without the problem of increased indoor humidity (which cancels a lot of the benefits).

Tim.

[TimSmall]

Originally Posted by AC_Hacker

I'd go for the very smallest size de-humidifier compressor I could possibly find... see how it works out, you'll really learn a lot. Then scale it up if you need.
-AC

Any idea what sort of COP the de-humidifiers achieve? Looking at this table:

http://ecorenovator.org/forum/conser...hanger-14.html

The efficiency of the best units seem to be about 4x those of the worst.


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. Having lived with a heat recovery ventilator in the past, and taking into consideration the way my house is used, I'm guessing that on average (during the heating season) the unit for my house is probably only going to run at an average of 50 m³/hr or so (a lot of the time there's no one in the house during the day, only a bit of humidity being generated by house plants, and maybe drying clothes in the "drying cupboard" which I'll place next to the HRV unit).

Playing with that recair calculator, if I build a unit to work at 300 m³/hr peak, but only run it at 100 m³/hr normally, then when it's running at the 100 m³/hr figure, it'll use something of the order of a quarter of the power to run the fans vs. the peak flow (which is a significant saving - maybe 60 watts), but the losses in the exhaust air will drop to about one eighth of the peak-flow amount (an even more significant saving - maybe 200 watts). The factor of 3 gain (from transferring only a third of the amount of air) is multiplied by the considerable improvement in the heat recovery core's efficiency which results from passing air through it more slowly.

So intelligent control software could be conservative about turning the fans up (if I get a bit of condensation on the windows occasionally, that's OK in many cases, so long as it doesn't sit around long enough for building materials to get damaged, and mould to start growing etc.).

[Fornax]

Quoting Exeric from another thread:

Retrofitting can be murder though if one gets too picky about things. Good luck!

To retrofit balanced ventilation with HRV to all the rooms in my house would be a nightmare. Over here all the walls are some sort of stone and all the floors/ceilings are concrete (Except the roof). In the near future you'll see some pictures.

Thursdaymorning I ordered the fans and some related parts. They could be shipped in less than 4 days so they arrived the next morning, that's timely indeed : -)
The package was short 1 of the 2 fans I ordered, which will be solved soonish.

Some pictures of the fans I'll be using, the Comair SOLO 44Y. Click the thumbnails.