Of course, but we agreed that only the best passive system can only match a maximum 50% recovery. Like so many things, it has to be a compromise.
Old style spur socket
- Thread starter Kevdiy123
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Of course, but we agreed that only the best passive system can only match a maximum 50% recovery. Like so many things, it has to be a compromise.
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Exactly - so unless one has a more complex/expensive heat recovery system (rather than a 'passive' heat exchanger) one simply has to accept that ANY method of extracting moist/smelly air will inevitably result in a substantial loss ('wastage') of heat, thereby rendering it questionable as to whether the effort/cost of making house more 'air tight' ('draft-free') is really justified.
Previous generations (indeed, the situation I experienced as a youngster) did not generally live in smelly and moisture-ridden homes, because they 'opened windows'. However, they tended to accept much cooler homes, so were not wasting large amounts of 'heating energy' because of the open windows.
Kind Regards, John
I would question the 'substantial' in your reply above. The house I live in had central heating added long before there was any attempt to make the place air tight and draft free, plus double glazed, insulated and properly ventilated. Heating bills were quite horrendous back then, before all this work was in place and despite the CH it still wasn't comfortable as it is now in cold weather.
I was likewise raised in a cold home as a youngster, but I don't ever remember a need to open windows in the cold weather, rather they would be frozen shut.
Well, the loss of heated air has to be as 'substantial' as is the extraction of moist/malodorous air.
Approved Doc F suggests minimum extraction rates of 30-60 L/s (108 - 216 m³/h) for a kitchen, and I think that a typical small house has a volume of around 280 m³. That means that, at the upper end of that guidance range, about 77% of the heated air in the house would be extracted in 1 hour.
How 'substantial' one would regard that is obviously an individual view/judgement.
Kind Regards, John
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It cannot suck the entire volume out, air flow simply doesn't work like that, rather it works by a process dilution of the general air and entire extraction close to the intake - which is why proper design is essential. I notice no change of air temperature, not my boiler running any more than normal, with my cooker hood working flat out for an hour.
If the volume extracted (to the outside) were equal to the entire volume of the house, then it can only have 'come from' within the house, or some substantial opening into the outside world.
One can obviously prevent heated air being sucked from the rest of the house by closing an air-tight kitchen or bathroom door and opening window(s) - but one would then probably not need the extractor, anyway, and the air temp of the room would fall to whatever the outside temp was.
Kind Regards, John
Try pouring for instance a 1/4 of a glass of orange into a glass, now place it under a running tap. How many measures of the volume of that glass would you need to run into the glass, before all of the orange is washed out? The more obvious less wasteful of water way, will be to top the glass up to fully dissolve it, tip it out, then refill, then tip the entire volumn of the glass out.
An extract fan works in a similar way, it draws in air which 'dissolves' a small proportion of the warm air volume - it will not suddenly suck all of the air out and replace it with a fresh lot of cold air.
That's all true, but I don't think it's a very appropriate analogy.
Provided there is adequate ventilation (somewhere) It will 'suck out' as much air as it is designed to suck out - so if it is designed to suck out 216 m³ per hour, it will suck out 216 m³ per hour.
If you starved the extractor of an air source (by making the room air-tight', with no ventilation), it would suck out nothing.
Kind Regards, John
Best example I could think of at the time. Swap the orange juice for a glass of hot water.
Yes, but the dilution principle always applies, so if a room is 216m³ and the fan can suck 216m³ per hour, it still will not replace all of the stale air with fresh. Only by evacuating the room completely, followed by refilling, can you avoid the dilution.
So where does the replacement heat come from?
One only has to hold a hand by the exhaust to feel the heat/warmth being disposed of.
Using a temterature sensor at the extract/exhaust position is a common method of measuring the space temperature.
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As has already been asked, where do you think the 216 m³ of air which is 'pumped out' has come from?
Kind Regards, John
The heating system does have to produce some extra heat to compensate for the heated air which is sucked out, of course it does. What I am trying to explain rather poorly - is that a 216 m³ per hour fan, will not suck the entire heated air content of a 216 m³ room or building, unless the room is an air tunnel. Air flow, like a liquid flow take the quickest, short route from where it comes in to where it goes out or is extracted.
Which would be rather pointless if placed at the exhaust of a cooker hood
As in my reply to Sunray - it is drawn in from outside, but as I keep repeating, the fresh air doesn't ever completely replace the heated indoor air, it replaces only a fraction of it. You are not reheating the entire 216 m³ of air in the room every hour, only a small proportion of it, because of the principle of dilution.
I agree that some of the air which is extracted will be 'cold' (or 'coldish'), either cold air which has come from outside and is extracted before being appreciably heated or, at least 'coldish' because it has mixed with warm air within the house (what you call 'dilution') - so, yes, if (for convenience) the house volume is also 216 m, the entirety of the heated air in the house will not be 'replaced with cold outside air' every hour.
However, I think that the real-world situation is much closer to that extreme than you seem t think. If, as will not be uncommon, the source of the incoming outside air is relatively remote (within the house) from the point of extraction, then when you switch on the extractor, it will 'pump out' a substantial proportion of the heated air in the hose before it even starts 'seeing' the cold air which has been sucked into the house behind it - i.e. mixing/'dilution' takes time, and a lot of the heated air may have been extracted before much of that has happened.
After all, when you first switch on the extractor, you expect much of the (heated) malodorous/moist air in the kitchen or bathroom to be extracted first, before any cold incoming air (often from distant parts of the house) gets to the extractor to be extracted, don't you?
As I've said, if one makes the room 'air-tight' (as far as the rest of the house is concerned and then create ventilation within the room (e.g. by opening a window), then the only heated air extracted will be that in that room (regardless of extractor flow rate)- but, as I've said, you then might well not need the extractor at all!
Kind Regards, John
At last!
Homes are far from being air tight - much tighter than they once were, but they still have lots of leaks. So really all an extract fan does is encourage local air to be extracted from a more concentrated point. I would suggest your 'substantial' is a massive exaggeration of the true situation. The small air 'leaks' are essential - imagine what life would be like in a fully air tight home.
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