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32 posts


#223536 5-Oct-2017 00:52
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I have read many great posts about HRV (Type) ventilation systems.. and i understand that bringing in warm air from the ceiling space to any significant degree has been debunked by consumer groups etc.. But i have a question around a way to use that heat that we know is actually up there in the ceiling space..


I would like opinions on how people think this idea would work..


Using the same principles as a heat exchanger.. e.g. with cool incoming air.. passing over warmer outgoing air.. how do you ventilation experts think this idea would work.. 


I live in a 70 year old bungalow on the Westcoast and we can at times even in the winter have long periods of sunshine.. even though the outside temperature is still quite low.. But having done lots of work over the winter months up in my ceiling space.. I know for a fact that it still gets really hot up there.. and I have also noticed that coming into the house at times during the day.. I can feel warm air coming in from the open access door to the ceiling..


So my idea is this: How about instead of putting in a (positive pressure) ventilation system that draws air from the roof space.. because - even with effective filters - it is blooding horrible up there.. pink batts, rat droppings etc.. I was thinking to bring in fresh air from under the eaves then run uninsulated ducting along the length of the roof at the highest point of the roof apex.. eg. just under the ridge line.. (possibly even running two or three lengths of ducting) if this area is still really hot, would this have the effect of warming the incoming air??


In much the same way as those DIY heat panels you can place on an outside north facing wall work??


Then duct that air into several rooms.. I realise that this air will only be hot during certain periods of the day.. But thats fine.. as to be honest it never gets that hot down here that one would not want a little extra warm air coming into the house.. 


In principle do you think that would work.. ?? 


FYI.. A bit of extra info.. Since moving into the house - just this year - we have put polythene liner on the dirt under house.. Fitted Expol polystyrene insulation under the wooden floors, and added R3.6 Pink Batts in the ceiling.. and to be honest it is pretty easy to heat.. even though we (temporarily) are only using oil filled heaters for heating.. We will be installing a Woodsman burner before next winter.. 


I have also started to double glaze the old wooden windows.. using acrylic.. I will post later an instructable on this.. but when I looked into the crazy prices I saw people on here paying and quotes I got, I was shocked.. I have so far only done the south side bedrooms.. but have noticed a huge difference in those rooms.. particularly with retaining heat in the evening.. often without any heating on.. It has cost me about $200 so far.. and based on the windows left to do.. I think will still be less than a $1,000 for the entire house.. and a using a VERY simple method. 


Your thoughts on my ventilation idea will be most appreciated.. 



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  #1877548 5-Oct-2017 01:29

The main problems with positive pressure ventilation systems are mostly caused by commission based salespeople, selling them to owners of unsuitable properties. And running advertising that implies that you can use them as a full heating system. They also run at night blowing cold air into the rooms (you might as well just open a window).


Consider getting or making a cheap positive pressure ventilation system. But use a differential thermostat to control it instead of the factory controller. Measure both the roof temp and temp in your rooms near the ceiling. And only switch the fan on if the roof temp is 5 or 10deg hotter than your room temp. As for filtering the air - add a HEPA filter.


I can't see your heat exchanger idea working that well. As you will need to measure the outside temp, roof temp and room temp. To make sure that it will only run when it is able to add heat to the rooms. And any kind of heat exchanger always has a certain resistance to transferring heat. If this is too high, your system will only be useful when the roof is extremely hot. But when that is the case you probably won't need much room heating.


I would love to install such a system in my own house - except it has skillion ceilings. So no roof space.

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  #1877550 5-Oct-2017 02:30
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I have an HRV PPV and use it as described above... only to cool when cooler air is above in summer, and warm when the roof warms so much quicker in winter. We also added a heat transfer component which takes the warmth from the fireplace in the living area, and drops it into the rooms once I initiate the process in the later evening. I find this to be a most effective system to heat (and cool) our home, but do need to manually turn it off many nights after the house has reached the desired temp and the roof temp continues to plummet.

In summer we rarely need to do anything as the HRV controller basically switches to minimum once the room temp is reached and simple maintains this in the evening until the heat is gone from the roof too.


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  #1877597 5-Oct-2017 08:26
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Ventilation systems that use air to air heat exchangers to transfer heat between exhaust air and incoming cool air from outside  already exist and are in widespread use.  They work both ways.  In winter, incoming air is warmed, in summer incoming air is cooled.  We installed one in a 1950s house in Nelson and we were very happy with it.


A limitation of the system you describe is at night your roof will cool down and then the system will be drawing in cold air.  You would have to switch it off overnight in winter - arguably when fresh (warm) air is needed most.  You would also want to switch if off in summer (roof too hot), and during cold days in winter (roof too cold).


Another challenge is that most ducting is designed to resist changes in temperature.  The heat in the ceiling space is convective and I don't think air ducting in the roof hip would pick up much heat.


The best way I can think of to make your system work (thinking out loud) is to circulate the ceiling space air through an air-air heat exchanger (when the temperature differential suits) and use this to heat the incoming fresh air.


When the temperature differential doesn't suit this operation switch to a second air-air heat exchanger that uses outgoing air from the house to heat incoming fresh air.


The drawback is you need either: -


- two air-air heat exchangers or;


- remotely actuated duct valves and two fans.


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  #1877611 5-Oct-2017 08:59
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I think the air will be moving too fast to pick up any heat. You'd need a proper heat exchanger, and in summer you'd need to bypass it.


I have a PPV system. I stopped it getting ceiling space air because it smelled, another old house here, I get air from the eaves. Works fine. I run it on a timer, so in winter it runs during the warm daylight hours, in summer when it's hot I run it early and late. I just have to work out how to disable the thermostat built into the thing... probably have to take it apart.


I think you'd be better off getting a proper heat recovery ventilation system like the ones Cleanaire makes.

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  #1877659 5-Oct-2017 09:49
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I think you'd be better off getting a proper heat recovery ventilation system like the ones Cleanaire makes.



I can recommend smart-vents systems based on our (Nelson) experience, but obviously you want to do some research on the various local installers.


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  #1878212 5-Oct-2017 23:43

The OP basicly wants something that is not for sale as a ready made product or system. It would be incredibly expensive to hack up a heat exchanger unit just to try and get out of filtering the air. And you would still need to either filter both airstreams going into a heat exchanger (any kind) or go with a heat exchanger with a replaceable core (like the Mitsubishi Lossnay units)


And since the sole purpose of doing all of this is to reduce heating costs, you also need to consider how long it would take to recoup the money you would spend on such a system from reduced heating costs. Best option would be to probably forget about what you are trying to achieve, and bring forward the installation of the woodburner.


Once you have the woodburner installed, you would most likely need a heat transfer system anyway. Might as well just build a heat transfer system. Way cheaper as no need for either filters or heat exchangers.

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  #1878274 6-Oct-2017 08:36
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Hi. Thanks for all the great replies guys. and they came much faster than I anticipated.. don't some of you guys sleep.. 1:30 am 2:30 am.. and I thought I was the only one being kept awake by my dilemma.


Yes I guess I was trying to use something that as Aredwood says is not currently for sale.. I was just trying to find a way to utilize that warm air up in the roof space. And yes a heat transfer system was the next course to navigate.


I guess as a cheap ass.. I was trying to use that hot air.. and it wasn't so much that I was trying to get out of filtering the incoming air, I just felt if the air was coming in from outside.. and given we are outside of a big city.. on the edge of a lovely river.. :-) I was questioning if a filter - other than a very basic one to keep the bees and bugs out - was even necessary.


I will check out the other heat exchanger/heat recovery systems that others have mentioned..  


I must say I am somewhat staggered by the amounts that some people pay for PPV systems.. Given what they are.. a few vents, a small fan, a bit of ducting and a controller unit.. some of these companies are making some pretty good profits I'd say.. I had a PPV system installed in my house in Auckland... With one fan and 5 outlets for a little over $2k and I hear others paying up to double that.. I plan to install my own system in this house.. so will be looking for the components from one of the many wholesale suppliers around..  which I have estimated should cost less than a $1,000. I also have no issue using a non-auto system.. just turn it on and off when I think it is useful - in terms of warming the air. 


Again, many thanks for your feedback..  








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  #1878286 6-Oct-2017 08:56
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Your idea of using ducting as a heat exchanger won't work unless you have huge ducting runs which then need much bigger fans etc.


Heat exchange relies on surface area contacting the media for a period of time to work. If you have air moving at 2-3 m/s then you need more area to achieve efficient heat exchange. Ducting won't have enough of the incoming air thermally contacting the hot roof space for any period of time to have a significant effect. Given you are wasting your heat using a PPV system by pushing it out of the envelope rather than recirculating via a return air system you are effectively heating all your air from scratch.


You could use a proper heat exchanger but then you need an additional fan etc.

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