Losing heat through extractor

[robinbanks]

As a general comment (far from the first time) there is obviously a 'conflict' between requirements for insulation ('heat retention') of homes and requirements for ventilation

Indeed, and that's been a string driver for the development of heat recovering ventilation systems. Everyone I know who has lived in a house with such a system would be reluctant to live in one without

I have never understood the heat recovery claims, because I make it, at best, 50%. All they can recover without heat pumps, is to recover half of the difference of the incoming air temperature, versus the outgoing. Outgoing 20C, incoming 10C, at best the fresh air warmed to 15C.

Take a look at how a shark, which is warm blooded, stays alive in cold water. The sea will certainly quickly bring the blood flowing through the gills and extremities down to sea temperature so it potentially represents a fatal source of heat loss for the shark. Usefully however, the blood flowing to the gills etc follows the same path but the opposite direction as the blood returning from the gills

When the flows are opposed a temperature gradient establishes, rather than having a transport that is uniformly the average temperature of the extremities

The top gradient is slightly advanced on the bottom one, so some heat is still lost to the environment at the blue end but the more efficient the heat transfer can be made the more these gradients align. The same is true for HRV

Whether domestic appliances are that sophisticated is another question.

They are more sophisticated than even that (independent flow of control through each side of the exchanger, management of summer heat buildup, frosting, condensation), but that's the core principle

 

[Harry Bloomfield]

A counter current heat exchanger can do a lot better than that. Whether domestic appliances are that sophisticated is another question.

OK, then explain how, because I do not understand how they can physically recover more than 50% of the temperature.

 

[robinbanks]

OK, then explain how, because I do not understand how they can physically recover more than 50% of the temperature.

Perhaps putting some numbers on it will help

As the fluid (blood in a shark, air in an HRV HX) proceeds along the top pipe, heat is transferred into the fluid coming back along the bottom pipe, in the opposite direction. The pipes are thermally connected but their contents do not mix. There is persistently a small temperature difference between the pipes all along their length, and the heat flow from one pipe to the other is at the same rate all along the length.

Let's say the fluid starts at 20 and the environment is at 5 (these numbers are for ease of math). The fluid starts out at 20 and reaches the environment interface (gills, end of the HX) at say 6 degrees. It contacts the external medium (sea water, world air) at 6 degrees and is cooled more or less to 5 degrees, so it starts its journey back at 5 degrees and manages to warm up to 19 degrees. Only 1 degree has been lost to the environment so the efficiency of heat recovery here is 95%

This only works for counter current. If the two flows were in the same direction then they would equalise at 50%, like you say:

The transfer starts off with the hottest and coldest in contact and heat transfer rate is high, but diminishes as the temperature difference between the two fluids diminishes, til they settle out at 50% of the difference each. Sharks and HRV don't work like this

 

[Harry Bloomfield]

This only works for counter current. If the two flows were in the same direction then they would equalise at 50%, like you say

Thanks, for the explanation, I think I get it now.

 

[ebee]

I must say I have found all of this very interesting.
Heat transfer systems, I have wired the odd one so I know the very basics but the opposite gradient thingy and sharks was fascinating and has added to my knowledge.
In nature we can often learn a lot from the animal and the plant kingdoms

 

[aesmith]

Tuna as well.

 

[JohnW2]

Indeed, and that's been a string driver for the development of heat recovering ventilation systems. Everyone I know who has lived in a house with such a system would be reluctant to live in one without

That's clear one way of addressing the issue. As with all these things, any 'active' heat recovery system will presumably have 'negative', as well as positive impacts on energy usage (hence also, to dome extent, also financials environmental effects), but I presume that the net effect is 'positive'.

I imagine that another (perhaps 'additional') theoretical approach would be to reduce the ventilation requirements ('air changes') by re-circulating and 'purifying' heated air within a building (rather than pumping it out of the building) - but I don't know to what extent that has been investigated, let alone implemented. Don't passenger aircraft largely do that?

Take a look at how a shark, which is warm blooded, stays alive in cold water.

I'm pretty sure you will find that most sharks (like most fish in general) are 'cold-blooded', but that a few types (like White Sharks) are partially 'warm-blooded' - to the extent that they can maintain their body temp at something like 10°-20° above that of the surrounding water (but that still leaves them 'pretty cold' if they are in near-zero temp water!).

Kind Regards, John

 

[robinbanks]

And the warm blooded ones (if we're getting picky I ought to have used that in place of which, though I'm sure you read through it) operate a countercurrent heat exchange

Passive heat recovering ventilation exists if you want to look into it. Overall a MVHR consumes a very low number of watts compared to the heat watts it recovers, so you're expending a low amount of energy to avoid having to expend a large amount of energy, netting positive.
When looking at running costs of HRV you need to factor in other benefits too, not just preserving warmth, unless eg your trickle vents have particulate filters installed.

 

[JohnW2]

And the warm blooded ones (if we're getting picky I ought to have used that in place of which, though I'm sure you read through it) operate a countercurrent heat exchange

Sure - and really no different from any other 'heat exchanging;' process.

Passive heat recovering ventilation exists if you want to look into it.

I'm sure it does. It's not difficult to think of some measures which would result in the ('passive') retention/recovery of at least some of the heat that would otherwise be 'pumped out of the building'.

Overall a MVHR consumes a very low number of watts compared to the heat watts it recovers, so you're expending a low amount of energy to avoid having to expend a large amount of energy, netting positive.

Sure, as I said, that's what I was assuming. A full examination of the environments impact would, of course, have to also take into account the sourcing (and distribution) of materials, manufacture, distribution and installation of the equipment but, again, I doubt that would have an appreciable impact over a reasonable amount of ins-service time.

When looking at running costs of HRV you need to factor in other benefits too, not just preserving warmth, unless eg your trickle vents have particulate filters installed.

Yes, as I said, one other approach (or part of the approach) would presumably involve attempts to 'purify' re-circulated heated air, rather than 'pumping it out'.

Kind Regards, John