Ventilation or air heat exchange -- passive or low energy

[Grinnin]

I'm interested in fresh air intakes and/or heat exchange ventilation that is passive and either adjustable or programmable.

The house I'm building will be different from the building I live in but pretty similar in a lot of ways.  I have lived in this building since I built it in '01.  I know that this building works OK and I want to improve on it for my future house.

The current building leaks air.  It's supposed to hold various petrol vehicles so leaky is OK.  I heat with wood and leaky works pretty well for that too; I burn a bit over 2 cord a year at a cost of about $4 for fuel for the chainsaw, so the leaks aren't that expensive either.

For the house I want NOT TIGHT but tighter.  More control.  Reducing conductive and radiant heat losses are important to me.  Keeping air exchanges is also important.

The big squirrel-cage blowers on manufactured heat exchangers are WAY too big for the 1,000sf house I'm building.  I CAN easily run 2 or 3 small fans all the time.  I currently run a brushless fan that ventilates 3 areas of this building, including hydrogen from the battery box.  (I'm off-grid with a very small electrical system.)

I am interested in something like this convective counter-flow heat exchanger:  http://sustainabilityworkshop.autodesk.com/project-gallery/passive-heat-recovering-ventilation-system.   These have been in use in GB for a few-to-several years.  Everything I read about them seems to be written by people who really want to believe they work.  While I also believe that convections work, I'm not certain it'll work in all situations.  Would a wood stove draw cold air down what's supposed to be the hot-air exit?

When I say "adjustable or programmable", I mean either a manual knob or an electronic control.  I built the analog differential thermostat that runs the pump on my solar hot water system and may replace the analog unit with an Arduino computer for fun and to account for more factors.

Passive heat exchangers?
Low-power heat exchangers?
Seasonally adjustable air intakes? 

Tell me about your experience please.

(I just scuttled my 100' earth-tempered air intake but this first post is already too long.)

Thanks.

[UK4X4]

I live in the UK right now and have a drafty old 120 year house.

I have 2 points, UK houses have cavity walls, two layers of solid bricks with a vented 2" wide center gap.
Ground floors are timber suspended over bare earth with vents in the walls.

They are drafty enough to not have issues with venting !

The design on the website .....yep hot air rises and yep a curved cowl promotes lift, I have one on my chimney for the wood burner.

The issue with the design is your letting all the heat out and sucking air in through the ground floor, there's no way the conterflow will do anything.

Even if air did get drawn in,,,,,as it heats it wants to rise and go back up and out.

If it was mechanicly drawn down, I could see it sort of working,,,,,but cost wise I don't see any benifet at all

we've been back in this house only a month after 12 years and are comming into winter now, I'm looking at ways to draft proof the house not add more !

[MushCreek]

House ventilation is a relatively new area, as houses have become too tight in recent years. The usual path is to use an HRV or ERV, which are heat exchangers that condition incoming fresh air to more closely match the inside temperature.

[Grinnin]

The issue with the design is your letting all the heat out and sucking air in through the ground floor, there's no way the conterflow will do anything.

Even if air did get drawn in,,,,,as it heats it wants to rise and go back up and out.

The sketches in that link shows the exhaust air leaving the room at ceiling level and the incoming air at floor level.  In convections as in siphons, the height difference is everything.

[Grinnin]

House ventilation is a relatively new area, as houses have become too tight in recent years. The usual path is to use an HRV or ERV, which are heat exchangers that condition incoming fresh air to more closely match the inside temperature.

All the manufactured HRV units I've found use over a hundred watts.  For the mfr, they can make a few units to cover different CFM and simply use more electricity to make it work.

For a small house it's clear to me that a few kWh can be saved each week by using a better system.  I first saw a HRV in a house on a Green Building Tour in the '90s in the midwest US.  That house was huge and the info sheet confirmed that it used lots of electricity.  I never figured out how anyone thought it was "green".  It may have been "healthy" in some ways.

I think the mfrs are content to make now what worked then.  A large fan means it'll work even if it has to work against natural convection or different air pressures.  The mfr doesn't have to know anything about the house, climate, or local conditions; add a big enough fan and it'll work.

What I have now is efficient, warm, and has plenty of fresh air.  I hope to improve on it.  Using an extra kWh per week is not an improvement if the same result can be accomplished without.

I hoped that other owner/builders had worked on the same puzzle.

Thanks.

[UK4X4]

The issue with the design is your letting all the heat out and sucking air in through the ground floor, there's no way the conterflow will do anything.

Even if air did get drawn in,,,,,as it heats it wants to rise and go back up and out.


The sketches in that link shows the exhaust air leaving the room at ceiling level and the incoming air at floor level.  In convections as in siphons, the height difference is everything..

My comment was referencing UK construction and that it would be easier for air to migrate from below the floor as you let all the warm air in the room escape up the chimney.

Pulling cold air down a tube from outside would need power, any heating from the escaping air would just mean the air in the inlet tube gets hot and goes back up and out, meanwhile air is drawn in at ground level through doors floors and windows taking the easiest route.

I work in the oilfield working with fiber optic distributed temperature measurements, where we measure and log fluid and gas flows.

through vertical and horizontal tubing.

Counter flow heat exchange is a dificult subject, but without mechanical movement of the inlet air, air will just get sucked back into the room from the nearest point, which won't be the tall thin chimney tube unless thats the only way for the pressures to balance. ie in a 100% sealed room with the only option being the dual chimney TUBE.

Modern horizontal exhaust on demand heaters use the counter flow heat exchanger, the route in and out of the combustion chamber is hermeticly sealed, and yes there's no need for a fan to induce the flow.

The temperature delta T is way higher in this case causing a good flow and pre heat of the incomming air.

So if you can hermeticly seal the said room and then heat the space, as heat rises and the pressure lowers in the room as it escapes air will be drawn down the inlet tube to equalise the pressure.

But in practicle terms, what building or room won't have an easier and faster route for replacement air to get filtered in, rather than get dragged down a long thin tube.

[Grinnin]

Pulling cold air down a tube from outside would need power, any heating from the escaping air would just mean the air in the inlet tube gets hot and goes back up and out, meanwhile air is drawn in at ground level through doors floors and windows taking the easiest route.

You may be right.

You base your argument on one premise that I don't accept:  that the incoming cold air will be heated to a delta T of zero.  I'm looking for a delta T reduction of about half -- from delta T of 30C to about 15C.

You say that the "easiest route" for incoming air is doors and windows, but that does not have to be the case.  Heated buildings create their own exhaust pressure and air will come in from the outside.  I just want control of the main path or paths.

As I said in the OP, it could be some method other than the heat exchanger from the GB folks.  It may be easier to achieve with earth-tempered intake or some other method.

[rick91351]

snip.......................................................................................
As I said in the OP, it could be some method other than the heat exchanger from the GB folks.  It may be easier to achieve with earth-tempered intake or some other method.

There is a high end home or cabin up here and they use an earth tempered intake and like it.  But I have nevered asked if they feel it was worth extra $$$.  Myself with the major heat coming from free wood other than the time and $  for gas, oil and chains.....  I really fail to see the need to get way out there.  However that said I still am interested in this.  Thanks for posting.

[Dave Sparks]

There is a high end home or cabin up here and they use an earth tempered intake and like it.  But I have nevered asked if they feel it was worth extra $$$.  Myself with the major heat coming from free wood other than the time and $  for gas, oil and chains.....  I really fail to see the need to get way out there.  However that said I still am interested in this.  Thanks for posting.

This is the whole point here that has changed with low cost 22+ SEER heat pumps and the mini-split is the star. Several of the houses I have been involved with offgrid have tried costing all the passive.  Passive is so much more expensive (now) and often has problems with overheating and bad ventilation. Passive can be difficult to analyze after the home is built.  Build a tight house for your area,  condition it with a heat pump,  and use supplemental heating when energy is scarce in winter.  A cracked window is not that difficult either.

[John Raabe]

My experience with earth tempering and passive air heating & cooling is that the heat transfer has not lived up to expectations.

Dave, are you setting up off-grid PV systems that will provide summer cooling via a mini-split HP? That seems like an interesting option for many climates.

[Grinnin]

Myself with the major heat coming from free wood other than the time and $  for gas, oil and chains.....  I really fail to see the need to get way out there.

I hope that adding fresh air that is not as cold will encourage me to bring in MORE fresh air.

When it's 0F outside, the area around the intake(s) will be less comfortable.  Warming that to 30F may not make a draught comfortable, but less uncomfortable.  While I'll want to put the intake in a less-used space, there is not much un-used space unless the air intake is through a closet.

Thanks Rick, Dave, and John.  I won't be using a heat pump here,  but am willing to hear other ideas.

[John Raabe]

Air intakes located in closets are a good idea. Bedrooms are where moisture issues typically show up (as condensation on windows). The clothes act to temper the colder air.

[MountainDon]

Allow me to comment on Dave's personal mini split setup.  It is very cool.      We've been there in warm weather and the house interior is quite comfy and all off grid.

[Dave Sparks]

My experience with earth tempering and passive air heating & cooling is that the heat transfer has not lived up to expectations.

Dave, are you setting up off-grid PV systems that will provide summer cooling via a mini-split HP? That seems like an interesting option for many climates.

I stopped specifying passive conditioning 7 years ago after I experimented with Sanyo on their mini-split for Offgrid.  They were bought out by Panasonic and were great folks to work with.  Since then I have over 30 offgrid homes  (mostly in climates like the Southwest ) that use the mini split for summer cooling and fall/Spring heating. The deep of winter is a challenge for solar and so wood or propane is used.  Up north and in the deep south they can work well if the solar is oversized.  This is not as big of a cost deal now with the lower solar prices.

Wild land fires are also a great reason for a mini-split.  For grid people, the second thing that often happens after the smoke is noticed is the power goes out.  A mini- split can run off a little Honda all night sipping power and keeping the smoke out.

They were a game changer in conditioning space!

[John Raabe]

A synergic solution of two technologies - both of which are increasing efficiency while lowering costs. When combined they provide a solution to a problem that couldn't have been solved only a few years ago. Bravo!