nibot | (no subject)

Check out some books from the library on passive solar heating. The best and most energy efficient way to utilise water cooling would probably be to circulate it to the cool spaces underground (below the "permafrost"!)and then up into the household. Water retains its temperature longer than air, and the earth longer than water.

waterfalls in every room!

You can pump cold water through radiators, but they won't radiate cold. You'll have to force air through the fins to circulate it around.

Also, beware that humidity will condense from the radiators and pipes, so you'll have to find some way to deal with that. Otherwise, you'll ruin your floors and walls.

but will they effectively "absorb radiant heat"?

It will absorb radiant heat that radiates to it. However, radiation is straight-line propogation so you're not going to absorb much of the ambient heat in the room. The reason why heaters work is that they can radiate heat in all directions into the room.

And radiation is probably not your biggest heat worry in the summer. Conduction will be, and to remove that you'll certainly need to be moving air across your cooling element.

You've nailed it on the radiation absorption issue - its all about line of sight surface area. Fins don't help, neither do rows of tubes.

Heaters work by running at a higher delta T from the room than is possible with cooling.

Radiation is, surprisingly, your biggest heat worry in the summer (unless you have a very leaky old house). It's all about sunlight - coming through the windows is a huge load, and the roof surface commonly will be 30-50F hotter than ambient. That high temp pumps heat through the roof (OK, you got me, that is conductive heat transfer, but it is due to radiation on the roof!).

Human comfort is impacted greatly by the radiant temperature around you, but modern systems (modern meaning as envisioned by Mr. Carrier around the 1910's - ugh) address this by over conditioning the air temperature so the air conductively warms or cools exposed surfaces. Some squishy numbers are people transfer around 45% of their waste heat radiatively to the surroundings, 20% evaporatively, and 35% convectively.

surpheon would be the perfect person to ask this question to. (He designs HVAC systems for green buildings.)

I've been summoned :) Although my last two posts are anonymous... Hmmm, perhaps I should log in and take blame for my meanderings.

Are you available for more internet consultations?

We have a huge attic and are interested in insulating it. The last picture on this page is of the attic:

http://www.pas.rochester.edu/~tobin/co-op/prospective_houses/960_south_plymouth/

We live in Rochester, NY. How would we go about finding out how much insulation we would have to add? The landlord says it will only be worthwhile if we extend those beams to make room for a greater thickness of insulation.

You managed to hit me in the moving/house buying chaos, but things have finally settled back down! Technically, how much insulation to add is pretty easy to figure out.

Doing the math, heat loss is linearly related to the U value, and the U-value is the inverse of the R-value. Roughly, give or take maybe 10% depending on your exact roof construction, the benifits of adding batt insulation in terms of the total assembly R value are:

R-0, current, 4.5R => 0.22 U-value
R-13, 3.5" batts => 16.7R => 0.06
R-19, 6" batts => 22R => 0.045
R-30, 9.5" batts => 31.5R => 0.0317

Heat loss is proportional to the U value (other factors such as the difference in temperature between the sides and the area are assumed equal in this comparison), so even a little insulation gets you the big savings jump. You can think of your current roof as loosing 22% of your heat, while throwing in R-13 batts means you only lose 6% of your heat (assuming you don't turn up the temperature now that its insulated!), and R-30 would cut that 6% in half to 3%. So any insulation helps you out quite a bit.

However, installation labor is probably 2/3rds to 3/4ths of the cost. And hanging R-30 batts is about the same effort as R-13 batts (unless you have to extend the rafters...). That's probably where the 'worth it' is coming in. As long as you are paying the high, pretty fixed labor install cost, you may as well pay the small incremental cost to install to the recommended value.

The textbook answer to how much insulation is needed looks to the much-chewed-over society standard, set with due assumptions of construction cost versus life-cycle savings. The ASHRAE guidelines for Rochester call out minimum insulation (not final assembly) R values of:
R-15 for above deck insulation (usually hard foam that is a continuous sheet under the roofing, without air gaps or thermal bridges)
R-19 for metal buildings
R-38 for attic and other installations

Note that one option may be to throw batts between the rafters and then cover that with hard foam insulation for additional r-value (although this may be an obscenely expensive way to go). Also, remember to vent between the insulation and the roof deck properly. I have an attic full of insulation that I have to tear down and reinstall with the proper air gap (http://www.cor-a-vent.com/PDF/S4002P.pdf) the winter :(.

Sure you can. It's what air conditioning does, only AC uses a fan over the radiator and Freon instead of water.

On a somewhat related thread, I thought of using the cold water line for the toilet in my bathroom to dehumidify the room. Basically, I'd just extend the water line using 30 feet of copper tubing with a drip pan underneath. I have more or less decided not to do it since the water would warm up quickly unless the toilet was flushed a lot and the system probably wouldn't remove much water.

The key difference I'm getting at is that supposedly a radiator radiates heat (eg, infrared?). I want to know whether it can absorb radiant heat. I have no doubt that it can act as a heat exchanger. In other words, if a radiator can heat via radiation (as opposed to convection/conduction), then can it cool in the same way?

What's the difference? Radiating heat and absorbing cold are the same idea just in different directions, right?

I just realized that you must be talking about an indoor steam-driven radiator, as opposed to the one in a car. They're both heat exchangers but the indoor type is more of a radiator in that it works primarily through radiation, not convection, as you said. I imagine you could get one to work as a cooling device but the problem would be surface area/convection. Without much surface area (car radiators have tons) or any forced air, you'd probably just be cooling the immediate area around the radiator.

The tinkerer's ugly but functional solution would of course be to replace the steam radiator with a car radiator or something similar.

I imagine the reason for freon instead of water is that you can get a much greater temperature differential between the room and freon than between the room and water. I think it's also the case that you can normally get water to a greater temperature differential above the room temperature than below it. If the room is about 70 degrees, then you can get the water almost 140 degrees warmer, and only about 35 degrees colder. You can get it about 170 degrees warmer than a 40 degree room, and only about 60 degrees colder than a 95 degree room, so Newton's law of heating/cooling would suggest that the conduction work will be about twice as effective at heating as at cooling.

And actually, you could probably do even better for heating, because ultrahot steam will still flow through a radiator, while ice won't.

Universities and other large institutions use a chilled water system which basically runs water through something like a car radiator and has a fan blow through the radiator to chill the air. During the winter cold water is exchanged for hot. This allows for an efficient central heater/chiller and cheap one moving part (fan) local units. A thermostat controls the fan and therefore the temperature.

I always thought that the reason for freon was that it had a convenient boiling point - on the 'hot' side it's a gas, and on the 'cold' side it's a liquid (for human-friendly definitions of hot and cold). Hence, you can effectively 'pump' the latent heat of vapourisation around - typically a fairly large number. Freon being non-flammable is, of course, a rather nice bonus. I suspect that this is the reason it is used instead of the variety of alcohols which have a similar boiling point.

Yup, its about boiling and condensation. Freon is neat because it boils and condenses at about the temperatures we want when compressed to sane levels. And it is non-toxic, non-reactive and non-flammable (butane is also used as a refrigerant at times, heck water can be used as a refrigerant).

A change in state (liquid to vapor) absorbs or releases a huge amount of energy. Remember shivering when you get out of the shower? Water on your skin is going to vapor and sucking heat out of you. Here's the neat bit, the temperature at which it occurs can be altered by changing the pressure of the working fluid. So, you use a compressor to put freon vapor under pressure until its phase change point, in this case the point at which it condenses into a fluid and releases a buncha btus and turns into a liquid (boiling in reverse), is about 120F. Allow the freon to condense on a metal tube which gets hot, and blow outside air over the tube to keep it cool enough to keep condensing freon. Heat is rejected to the 'cool,' up to 110F outside air, being blown over the tube.

Now, for the cool part (pun intended). When you compressed one side to make the freon change phase at a high temperature, you sucked another chamber to a very low pressure. So low, that if you squirted a little bit of the now liquid freon into it, it would boil (sucking up btus) at a temperature of about 40F. Air conditioners use a throttling valve to squirt liquid refrigerant onto a surface on the very low pressure side, where it immediately boils off - sucking heat out of the surface and the air flowing over the other side. This creates a vapor, which you squish through the compressor (its a vapor, so you can use a simple turbine fan type compressor - bad things can an do happen if any fluid refrigerant hits the compressor) until its pressure is so high it condenses at 120F on the hot side, rejecting heat and the cycle continues.

So, if you are bored enough to still be reading, freon works because its boiling/condensing temperature can be manipulated at reasonable pressures between the temperatures we want - 40F or so to cool the inside air, and 120F+ to reject heat outside on hot days. Freon also has nice properties regarding the number of btu's it sucks up and spits out in the phase change process.

Yup, and the water that runs through the fan coils is then cooled by big ass 'chillers' that cool water using a standard vapor compression cycle. As a side note, water is a miracle liquid for cooling. It's heat capacity is amazing, and its phase change is a fundamental part of every large system in the form of cooling towers. The chillers evaporate water directly to the air to dump heat at a rate of almost 1000 Btus/lb of water evaporated. 1000 Btus is enough to bring a quart and a half of water to 210F (note that going from 210F to a vapor at 213F would take over 6000 additional btus - phase changes take energy!).

The T-stat usually controls the valve on the coil rather than the fan because small variable speed fans are pricy, but thats a minor quibble. And cheaper variable speed fans are in the pipeline (which is cool, because reducing air flow rather than liquid flow through the coil would save more energy - pumping air is more turbulent and inefficient).

To make a refrigerator, you need a compressible fluid; that's why water wouldn't work.

Close, but actually water can work - if you are working at extreme enough pressures. Actually, water is used as a refrigerant in one device I know of that uses a chemical dessicant to create the pressure differentials used in the refrigerant cycle.

You do need a compressible working "fluid," but the 'working fluid' is always a vapor when it is compressed (HVAC engineers have some of the most screwed up lingo you'll ever hear - the go metric folks don't even waste their breath on us!). Compressing a fluid is not part of the refrigerant cycle, although sometimes a poorly operating system will get fluid into the compressor, a condition knowns as 'slugging' (due to the sound and destructive effects).

they used to run water over something like that(open to air I believe) in FL and from that devised the concept of refrigerators....so I'm sure if you could circulate it fast enough with good contact to air...without it picking up too much friction.....it works....(James Burke Connections).....really old desert houses used to have walls of water for cooling......personally I prefer hoses

On Stanford campus we did a building that runs water over the roof at night. Heat is radiated from the large surface area to the approx 0C sky at night (the atmospheric temperature - space is about 0K, but you have a bunch of air that increases the apparent radiant temp). The cooled water is stored in an insulated tank and used to absorb heat from the building during the day.

Desert houses use 'walls of water' as heat sinks. When it is 50F at night and 90F during the day, if you could just average those two temperatures you will be a comfy 70F all the day. A huge thermal mass of water can be easily set up just by filling barrels and clever things can be done to passively control the heat transfer (air flows and solar exposure/shading). Water is very heavy, easy to work with, and very good at soaking up/releasing heat.

As a matter of fact, radiators don't radiate much heat. They are mostly convective heaters. Or so says my HVAC engineering source.

That's what I was wondering—thanks.

Short answer, yes, but you probably won't get much cooling.

All 'radiators' in homes depend more on convective heat transfer than radiative, that is they create a thermal updraft that draws air over the hot surface to be heated. Radiation is usually at most a third (more likely about 10%) of their heat transfer.

The effectiveness of convectors is dependent on the difference between the air temperature and the surface temperature. In heating, a temperature of 140F to 160F is common, for a delta between room and 'radiator' surface temperature of 70-90F. In cooling, the lowest temp you can go is 60F-50F (in Oakland area) due to concerns about condensation, for a delta T of only 10-20F. The convective capacity is killed by the low delta T.

Radiant heating and cooling is all about surface area, and lots of it. Covering the ceiling is a common approach, and the last conference room I did used about 50% ceiling and 100% floor (tubing in a concrete slab) radiant surface, with active monitoring of space dewpoint to pre-empt condensation (this is probably a bit of overkill, but conference rooms are murder to do radiantly due to all the people who insist on spewing out humidity while in them).

Going back to the short answer, you won't get much cooling but you will indeed get some. With a very careful building design and low occupant expectations (particularly if occupants don't have to conform to a no-shorts-and-wear-a-tie dress code), you may get enough. You'd really need to maximize the radiative area though.

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they won't radiate cold

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insulation?

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