Home Ventilation - Separating the Salient from the Sales

Forums › Home Workshop DIY › Home Ventilation - Separating the Salient from the Sales - Your thoughts?

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#827547 29-May-2013 09:37

joker97: 1) are there any other alternative ventilation systems that is cheaper than DVS?

2) ... in Dunedin?

3) any ventilation systems that recycles and reconditions rather than PPV? (would it be better?)

The university of Otago tested a couple of houses in Dunedin with respect to roof cavity heat recovery with DVS and "HRV" systems. They found that there were virtually no days in the year when running such a system would provide heat to the house.

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#827552 29-May-2013 09:47

timmmay:
TinyTim:
Niel: The heat recovery systems are a good idea to reduce wasted heat, but at most (if 100% efficient) will recover only half of the energy as the outgoing air will cool down halfway and the incoming air heats up halfway.

Aren't you describing a fairly unsophisticated heat exchanger design? A good heat exchanger will transfer most of the heat.

If you have incoming air at 0 degrees and outgoing air at 20 degrees, assuming equal volumes of air, they will come to equilibrium at 10 degrees. Without some kind of sophisticated device you can't easily get the outgoing air down to 0 degrees and the incoming air up to 20 degrees.

Only if the air is flowing in the same direction. In a proper heat exchanger the air flows in opposing directions. Look up penguins feet, an interesting example of a heat exchanger in an animal.

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#827554 29-May-2013 09:48

Skolink:
The university of Otago tested a couple of houses in Dunedin with respect to roof cavity heat recovery with DVS and "HRV" systems. They found that there were virtually no days in the year when running such a system would provide heat to the house.

I think its fairly common knowledge now that neither DVS nor HRV provide any sort of heat transfer. HRV may have had all sorts of claims around it but its been disproved for quite some time now hasnt it ?

If I've learned one thing from this thread its that everyone has a different idea and whatever people have installed it appears to have worked for them.

I think I'm going to stick with what I know and just install a DVS and be done with it :)

Cheers

timmmay

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#827555 29-May-2013 09:52

I still don't understand how if you have 1L of air at 0 degrees and 1L of air at 20 degrees that the incoming air can go over 10 degrees. Ideally you want the incoming air to take as much heat as possible from the outgoing air, but equilibrium would seem to be the key principle involved, in the absence of any active machinery. If you could get the incoming air up to 20 and the outgoing air down to zero with a passive device you'd solve everyone's heating issues.

I have a cheap ventilation system, and I know that on sunny days in Wellington the house is warmer with it on than off.

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#827557 29-May-2013 09:56

timmmay: I still don't understand how if you have 1L of air at 0 degrees and 1L of air at 20 degrees that the incoming air can go over 10 degrees. Ideally you want the incoming air to take as much heat as possible from the outgoing air, but equilibrium would seem to be the key principle involved, in the absence of any active machinery. If you could get the incoming air up to 20 and the outgoing air down to zero with a passive device you'd solve everyone's heating issues.

I have a cheap ventilation system, and I know that on sunny days in Wellington the house is warmer with it on than off.

Yes, the heat exchange systems work best when there is a BIG difference between inside and outside temp. So in Auckland for example, where most of the year it's not below 10-12 degrees, it's probably not worth it. But somewhere like Rotorua or CHCH where gets properly cold, definitely worth it.

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#827558 29-May-2013 09:56

Because the roof cavity has some heat to contribute?

Just a question ... pvc doors - are they reliable or will the handle break after 100 openings? Anyone has one?

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#827566 29-May-2013 10:22

timmmay: I still don't understand how if you have 1L of air at 0 degrees and 1L of air at 20 degrees that the incoming air can go over 10 degrees. Ideally you want the incoming air to take as much heat as possible from the outgoing air, but equilibrium would seem to be the key principle involved, in the absence of any active machinery. If you could get the incoming air up to 20 and the outgoing air down to zero with a passive device you'd solve everyone's heating issues.

The incoming air is heated by outgoing air going in the opposite direction. The outgoing air that is just about to leave the exchanger is warming incoming air that is at outside temperature. As the incoming air traverses the exchanger it gets warmer, and when it is just about to leave the exchanger it is already quite warm and is being heated by outgoing air that is just entering the exchanger so is at room temperature. As the outgoing air traverse the exchanger it is being cooled, until it exits and is being cooled by air at outside temperature.

See the graph in the link I posted earlier.

timmmay:
I have a cheap ventilation system, and I know that on sunny days in Wellington the house is warmer with it on than off.

We have an older house with sarking, in Wellington, and I measured the roof temperature on a sunny day one July at 10 deg. So for us ventilation doesn't make sense.

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#827584 29-May-2013 10:56

We have a very, cold , damp 1950's house with two cheap and nasty heatpumps (x Woolworths years ago)

The house has a major problem with mould in rear bedrooms, musty smell (in spite of open windows) all windows streaming water in winter. We installed one of the below two years ago and habe never looked back. All mould, musty smell and streaming windows stopped overnight. I was a sceptic but no more. Totally awesome and there are all sorts of other options that can be fitted to the absic system Look no further.

http://smartvent.rtrk.com.au/?scid=151765&kw=1061850&pub_cr_id=21278097258

mxpress

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#827601 29-May-2013 11:15

TinyTim:
timmmay: I still don't understand how if you have 1L of air at 0 degrees and 1L of air at 20 degrees that the incoming air can go over 10 degrees. Ideally you want the incoming air to take as much heat as possible from the outgoing air, but equilibrium would seem to be the key principle involved, in the absence of any active machinery. If you could get the incoming air up to 20 and the outgoing air down to zero with a passive device you'd solve everyone's heating issues.

The incoming air is heated by outgoing air going in the opposite direction. The outgoing air that is just about to leave the exchanger is warming incoming air that is at outside temperature. As the incoming air traverses the exchanger it gets warmer, and when it is just about to leave the exchanger it is already quite warm and is being heated by outgoing air that is just entering the exchanger so is at room temperature. As the outgoing air traverse the exchanger it is being cooled, until it exits and is being cooled by air at outside temperature.

See the graph in the link I posted earlier.

timmmay:
I have a cheap ventilation system, and I know that on sunny days in Wellington the house is warmer with it on than off.

We have an older house with sarking, in Wellington, and I measured the roof temperature on a sunny day one July at 10 deg. So for us ventilation doesn't make sense.

I still don't understand why you can do better than equilibrium between incoming and outgoing air, given equal volumes. Sure if you had 1000L of air at 20 degrees and 1L at 0 degrees it's easy to get the 0 degree air up to 20 degrees, but with equal volumes that's not the case.

Roof temperature varies a lot by house, by day, by ambient temperature. In the middle of winter you might not get as much benefit, but on the shoulder seasons you get more benefit. The air in the ceiling cavity on a sunny day is usually warmer than the air outside, so you do better than just opening the windows.

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#827624 29-May-2013 11:38

mxpress: We have a very, cold , damp 1950's house with two cheap and nasty heatpumps (x Woolworths years ago)

The house has a major problem with mould in rear bedrooms, musty smell (in spite of open windows) all windows streaming water in winter. We installed one of the below two years ago and habe never looked back. All mould, musty smell and streaming windows stopped overnight. I was a sceptic but no more. Totally awesome and there are all sorts of other options that can be fitted to the absic system Look no further.

http://smartvent.rtrk.com.au/?scid=151765&kw=1061850&pub_cr_id=21278097258

What sort of price did this system cost?
Did you install yourself?

Opinions are my own and not the views of my employer.

Skolink

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#827646 29-May-2013 11:43

timmmay:
I still don't understand why you can do better than equilibrium between incoming and outgoing air, given equal volumes. Sure if you had 1000L of air at 20 degrees and 1L at 0 degrees it's easy to get the 0 degree air up to 20 degrees, but with equal volumes that's not the case.

The air isn't mixing, so it isn't the same as tipping 1L of say 80°C water in with 1L@20°C where you will get 2L @50°C.

The air is trying to achieve equilibrium, but at each point along the heat exchanger. I drew you a picture:

Also don't forget that if moisture is condensing from the stale air (on its way out), then you get energy recovered from the latent heat of evaporation (but of course the fresh air temperature can never exceed the temperature of the stale air, it just improves the efficiency).

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#827663 29-May-2013 11:57

Ahhhh... now it makes sense! Great diagram, thanks :)

TinyTim

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#827668 29-May-2013 12:08

Skolink:
timmmay:
I still don't understand why you can do better than equilibrium between incoming and outgoing air, given equal volumes. Sure if you had 1000L of air at 20 degrees and 1L at 0 degrees it's easy to get the 0 degree air up to 20 degrees, but with equal volumes that's not the case.

The air isn't mixing, so it isn't the same as tipping 1L of say 80°C water in with 1L@20°C where you will get 2L @50°C.

The air is trying to achieve equilibrium, but at each point along the heat exchanger. I drew you a picture:

Also don't forget that if moisture is condensing from the stale air (on its way out), then you get energy recovered from the latent heat of evaporation (but of course the fresh air temperature can never exceed the temperature of the stale air, it just improves the efficiency).

Good lord, that's the same as the example I was working on, right down to the inside and outside temperatures and the number of points along the way. But I was using a spreadsheet to average the temp at each point and it wasn't nearly as pretty.

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#827778 29-May-2013 15:21

I didn't read the whole thread but I self installed a SmartVent Synergy (balanced ventilation) into a 80's brick\ali house last year. The ventilation side of things works great, 1 bedroom in particular used to get significant condensation but that has reduced to 0 after installation.

The only aspect of it I'm not totally happy with is the heat transfer. Our primary heat source is in the lounge (wood burner), and that room can get up to 28 deg with it cranking. We have a reasonably long house and the further bedroom is 15m from the intake. Based on my research\calculations the R0.8 ducting has a heat loss of 1 deg for every 3m, and the Synergy Heat Exchanger is ~75% efficient (not quite the 'up to 90%' they advertise). I have a 6m run to the heat exchanger, then a 9m run to the furthest bedroom. This means the air temp going into the far bedroom is (28-2)*.75-3 = ~17degrees. In practice this isn't enough to warm up that room. The single biggest cause of heat loss in the run is the heat exchanger (~6deg for this example) vs 5deg for the ducting (which i could insulate further). There is no way to bypass the heat exchanger and the air directly.

TLDR; Great for ventilation, not great for heat transfer.

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#827869 29-May-2013 16:52

Hmm. Your heat exchanger efficiency calc should use the difference in the inlet temperatures, not just the temp of the inlet from the room.

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