cast iron radiators

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Umm, no, we're talking about a guy with hot water radiators and a heating problem. You bring up extraneous stuff, like a gravity system, which has, well, exactly nothing to do with the OP's system with a circulator. A gravity system moves much more slowly and the heat transfer takes place over a longer period of time - there's that pesky conductance thing coming into play again. And the slower movement also allows greater heat transfer (conduction) so the supply and return will have a greater difference in temperature...which increases convection inside the radiator.
You are equating two different systems. They do not operate the same. They are not equal. So why are you bringing them up other than to obfuscate?

The OP measured the water temperature in his system. 10F deltaT between supply and return...for the whole system. Exactly how much difference do you think there will be between a particular radiator's supply and return water temperature?

Laminar being totally insignificant in this case.

Curious that you don't mention conductance, yet mention the time in contact. That's conductance, right? You do know the thermal conductivity of iron vs. water, right?
What exactly is bugging you? The thermal constants or the hierarchy of heat transfer efficiency? I would change them to suit your ideas if I could, but I can't. Sorry.
R
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On Sat, 30 Oct 2010 10:43:52 -0700 (PDT), RicodJour

See below.

Essentially true, but the circ pump and fluid dynamics probably don't allow maximum efficiency when radiator inlet and outlet are on a straight line at the bottom. Depends on whether flow pressure differential overcomes the convection currents. The pump pressure/flow destroys "pure" convection even if the system was designed for gravity convection. Even in gravity systems convection can get screwed by design of piping. But convection is still working in any hot water system. It's unstoppable. Be interesting to measure different configurations.

Steam and water are entirely different animals mainly because of steams latent heat, You're using one measurement term that attempts to simplify a complex system. You don't mention the specific heat of water, which is transported to the upper radiator surfaces by convection much faster than could occur by cast iron conduction. If what you said were true radiators would be solid cast iron except for the lower hot water passage.
Hot water radiators are designed with open upper passages to allow free circulation of convection currents, not so they could be vented at the top. What do you have against convection?

Where did you get the idea that steam doesn't convect heat? Heat given up by steam on a radiator surface causes more steam to be convected to the same surface. Seems you think convection only occurs with air. Nobody said the heat doesn't move by conduction to get to the outer surface of the radiator. But there's plenty of convective heat transfer going on inside.
--Vic
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On 10/29/2010 10:00 AM, Jules Richardson wrote:

As I understand this the new system is what I would call an open loop, the return drains back into the tank. If I have this wrong then...

No, it will make no difference. Since this is pumped and the effect of rising heated water is small indeed.
This all seems like the old boiler was better than the new. That is where your efficiency is. The rest is distribution, see below.
If you want to know if the flow is sufficient, feel the first radiator and see if it is a good bit warmer than the last. If it is then tending to the circulation would help.
Pumping a closed loop will have greater flow than an open loop (for the same pump). You lose the pressure of the return (head of falling water) in an open loop and have to start from zero. That can be very significant if you have a substantial rise in the system and will be a factor in any rise. No doubt why you needed a stronger pump. With a closed loop there is no head (from pumping up) to overcome, just that caused by resistance in the lines and radiator.
I don't know boilers, but I know something about solar and the principals are the same for the distribution.
In general you can look up flow rates at various heads for any pump. They vary widely.
Don't know about the backwards valve, may be a factor.
Jeff
(It should,

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I think it might be more accurate to say that the medium that carries the heat is what rises, air, water or other.
--
Tegger

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On 10/28/2010 6:25 PM RicodJour spake thus:

Something important to keep in mind, and not immediately obvious.
In fact, now that I think about it, I may have been guilty of believing that heat rises. Never really thought about it until now.
So why does warm air rise? Because it's less dense than cool air, that's all. Not because heat rises: heat radiates in all directions.
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And it's also important to remember the hierarchy of heat transfer - conduction, convection, radiation. As far as flowing hot water through a radiator, the input and output locations are not all that critical. Think of it this way - try pouring some hot water really slowly onto cold water in a pot and see if you can get the different water temperatures to stay separate. Any flow or turbulence just accelerates the mixing and stabilization of temperature.
R
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It seems to me...
A heat-exchanger is a heat-exchanger, regardless of media on either side, and regardless of the nomenclature and exact physical appearance of the exchanger unit.
Therefore, the basic principle is the same for all vertical-flow heat exchangers: Heat rises, cold falls. If a hot medium enters the TOP of the heat exchanger, it will transfer its heat to the (presumably) colder medium on the other side. As it gives up heat, it will fall, since fresh hot medium is entering the exchanger behind that which has already entered and been cooled via radiation.
I'm not quite sure exactly what you'd get if you tried to push the medium backwards (uphill) besides impaired heat-transfer, but I'd bet it would make more work for any mechanical pump that would be in the system, since the thermosyphon effect would be working against the pump.
--
Tegger

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Probably the pump is still to small if boiler temp is much higher than it used to be and radiators dont heat, the installer it seems wanted to save a few dollars more for himself. What ive usualy seen is bottom in and out. Are you sure which way its pumping and that the boiler isnt reversed also, meaning simply flipping the pump 180 would make it all right.
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Joe wrote:

It should make no difference on efficiency. It would enhance gravity circulation the way it is now. I use gravity on my wood boiler. I'll be warm as toast when the power fails.
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Wow a lot of points have been brought up but I'm only going to cover some of them. Yeah I know heat loss from the house is important but the point of this thread was really about why this system is so much more wasteful than the last. Things like heat loss from the house need to be addressed but they were present on the old system too.
I forgot to mention on the old system my boiler lines through the crawlspace were not even insulated. I insulated them after getting the new system. So even after drastically reducing the heat going upstairs and insulating the pipes I'm still less efficient.
Answers: My wood burns fine. The pump is installed correctly. My outside lines are dry. Everyone with the boiler co and the several dealers I've talked to say my pump (Taco-009)should be plenty big for the job (except the installing contractor who says even the smaller Taco-007 pump which didnt work, should work fine).
So I've learned the flow direction doesnt matter for efficiency, that's good. I've learned (or some say) that a pressurized system with a blower is more efficient, though I've read some conflicting information on the blower. I've come to the conclusion that this newer boiler is just a big fat wasteful pig compared to the old one, even though it was ancient. Good conclusion?
I should reverse the zone valve but I dont think thats going to affect my bill much. Its possible to install a blower on my boiler-does it make any sense to do so? It seems just another waste of time and a couple hundred dollars trying to stem the blood loss from what will still be a big, fat, wasteful pig of a boiler.
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