50-ohm power resistors

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r column, of hotter, less dense air above your heat source which creates a larger pressure difference, and a higher flow rate across the heat source, than you would get without the chimney.

Fins without a chimney still work. You still get convection. The air close to the fins gets hot and rises away from the fins - convection - and colde r air flow into to cool the fins.

The chimney just plays with the pre-existing convective flow. It's a differ ence in degree rather than kind.

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Bill Sloman, Sydney
Reply to
bill.sloman
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Heat, though, moves by conduction (insulators move less than conductors) and by convection (moving air removes heat by heat capacity, irrespective of its heat conductivity). If you make good air contact with fins, conduction doesn't determine the efficacy of your convective heat removal.

Filling your house walls with anything that does NOT convect, is a general win. Adobe houses, with no air-fill in the walls, are said to be quite comfortable.

Reply to
whit3rd

If only you had said any of that.

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Rick C
Reply to
rickman

Yes, no fins at all also works, just not as well just like the fins don't work as well as a chimney. But none of this will help dissipate

700 watts without a huge temperature rise. That's why they blow air across heat sinks with fans. I suppose an adequately huge hunk of aluminum would handle the job if you don't mind a large temperature rise across the heat sink.

Exactly. The chimney greatly increases the air flow removing the heat more effectively. But 700 watts in a package smaller than a shoebox will be pushing the temperatures way up without a fan. I guess the heat sink fins "work" for smaller values of work.

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Rick C
Reply to
rickman

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onger column, of hotter, less dense air above your heat source which create s a larger pressure difference, and a higher flow rate across the heat sour ce, than you would get without the chimney.

ger column of hot air, and thus a larger pressure differential, but the air has to flow through the chimney, which creates drag.

chimney - you only have a fixed amount of heat to be dissipated, and the mo re air you spread it across, the cooler the air column.

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t of heat. Because it's at half the temperature you need twice as long a ch imney to get the same pressure difference to drive the flow, and a root two larger diameter chimney for that pressure difference to generate twice the volume flow rate through the chimney.

I'm a bit surprised that anybody would have thought that I should have need ed to. It's as obvious as it is boring. The single word "optimisation" shou ld have triggered your version of the same routine.

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Bill Sloman, Sydney
Reply to
bill.sloman

Well-- that's more of a filtering effect.

Dense walls (adobe, brick, concrete, etc.) are always said to "retain heat well". This is a bug sold as a feature. Don't you love marketing?..

It's doesn't work out so well when the average outside air temperature goes high or low. I've lived in an old brick house before: the walls had a thermal time constant of about 18 hours, so when the weather suddenly turns hot, you get about a day's respite while the walls remain cool. Then it's sweltering for at least two days after that (one for the hot day following, one for the day after cool weather returns, where the walls are still hot).

It's even worse in winter (up here, where winter is a thing), because there's never a temperate day. You're just burning the furnace non-stop, and never making any progress.

And you have to burn the furnace non-stop, otherwise it takes >4 hours to warm up the place when you get home.

In a desert environment (where adobe originated), the average temperature is probably just fine, it's the swings that kill you (sweltering direct sunlight, to near-frosty nights).

Adobe and brick also have a lot of porosity, so it's not entirely fair o say they aren't air-filled. Still, they're in the 1 W/(m*K) range, so you need to use ridiculously thick walls to get any useful insulation value.

I've heard of adobe construction where the outer wall is built over hay bales. Seems a rather perishable choice, should any water leak in there... but again, in a desert environment, that'll probably never happen. That makes a thermal CLC filter, with a high impedance (low conductivity)! So the indoor temperature is extremely stable, /and/ central heating/cooling is effective (though it still has to run 24/7).

The ideal home insulation is light (so it heats and cools rapidly), and low conductivity. It's hard to beat wood and drywall with fiberglass or foam packing: the materials are light, so you can turn off the HVAC while you're away, to save cost (i.e., reduce the average interior-to-exterior temperature drop, which is the figure costing you money). And turn it back on and have things come to equilibrium within a reasonable time frame (an hour or so).

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
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Reply to
Tim Williams

Straw bales in the UK, not normally known for being dry :)

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And, from a less disinterested source:

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I'd be concerned about vermin; straw is a food source.

Reply to
Tom Gardner

It is provided that the result isn't rising damp...

High thermal inertia works OK the prevent rapid fluctuations.

It isn't too bad in the UK either where thick walls can slow the ingress or egress of heat sufficiently to counter the occasional few days of extreme temperature event. It is a bit odd when the house ends up cooler than the outside in a warm summer though.

A bit more insulation in the right places helps a lot. Especially loft insulation which is easy to do and highly effective.

That and designs that shade the house walls in daytime. Cacti have the right idea about self shading structures.

A farmer come eco warrior round the corner from me built his own home from straw bales covered in daub (mud/cattle dung). It works surprisingly well even in our cold damp climate - much to everyone's surprise it didn't rot away! It has impressive insulation properties.

ISTR the walls are at least two bales thick for stability and it is a hexagonal design a bit like an oversized summerhouse.

A thin outer skin to isolate the chimney core of the house from the exterior temperature is probably optimal. It is surprising how much the insulated foil stuff intended to go behind radiators will do to prevent heat ingress through a roof space (eg in a garage).

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Regards, 
Martin Brown
Reply to
Martin Brown

So the windows probably have a 5-degree field of view.

Oh, joy. A very dark, smelly, soon-to-be rat-infested allergy extravaganza. Glad he's living in it and not I!

Cheers

Phil Hobbs

Reply to
Phil Hobbs

I know someone with a straw bale house.. been up for ~5 years, so far so good.

Straw is the left over stalk after the wheat is harvested. (it might be some other grain.) Typically it's not food. (though mice like to make homes in it.) Hay is cut and dried grass.. it is food.

George H.

Reply to
George Herold

I haven't been inside but judging from the outside I'd say about a 30 degree field of view from the windows. The straw has been chamfered.

It has been there about 5 years now and is allegedly quite cosy.

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Regards, 
Martin Brown
Reply to
Martin Brown

I expect the rats agree. Probably tolerable once the dung smell dies away a bit, for low values of 'tolerable'.

As Carew said in his "Survey of Cornwall" (in Elizabeth I's time),

" Of all manner of vermin, Cornish houses are most pestered with rats, a brood very hurtful for devouring of meat, clothes and writings by day ; and alike cumbersome through their crying and rattling, while they dance their gallop galliards in the roof at night ".

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

It is a working heuristic that you are seldom more than 5m away from a rat in a major city (usually vertically). In the countryside where I live you are more likely to see them nearby only if there is food.

There isn't much of food value in straw and the outer coating of daub seems to be sufficiently inert and strong as a composite material.

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Regards, 
Martin Brown
Reply to
Martin Brown

It's certainly not a bug in the desert, where the daily temperature differential can be 100F.

Reply to
krw

How about a 90W and a gallon of transformer oil, like the old Heathkit Cantenna dummy load?

A MFJ version sells for $64.95 with the oil.

A MFJ version sells for $45.00 without the oil.

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Never piss off an Engineer! 

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Reply to
Michael A. Terrell

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