cast iron radiators

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I have those old, vintage cast iron radiators in my house on a wood boiler. They are not marked for direction of flow as far as I can see but I always assumed the hot should come in the bottom and as I recall from the early days of owning the house, thats how it was. But when someone installed a new boiler on my existing pipes some years ago, they reversed the flow-hot goes in top, flows out the bottom. I immediately realized an increase in wood consumption. Not being a heating contractor myself, I only figured out today that is what happened and I wonder how much money I've thrown away because of this contractor mistake. So I'm trying to figure out in real numbers how much less efficient these radiators are when flow is reversed. Any ideas how I can find out??
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I don't believe it works like that. If the hot comes in at the top or bottom, it usually exits at the bottom. I don't know what your initial installation was like, but if it was as you say, it wasn't typical.
http://chestofbooks.com/home-improvement/repairs/Mechanics-Household/Radiator-Connection.html From the bottom of that page: "The position of the valve on these radiators is of little consequence. The valve is intended merely to interrupt the flow of the water and may occupy a place on either end of the radiator with the same result." And the pictures show all sorts of different piping setups, some with the valve high and some with it low.
If you've purged the air out of your radiators I would suspect the boiler installation itself. Try posting your question and pictures of your boiler setup on www.heatinghelp.com That's a top notch site and the people have a lot of experience.
R
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Not much was typical in here when I bought the place-but it worked pretty well. My current setup now looks exactly like figure 39 on that page you linked. The flow used to go the other direction with my old boiler.

None of my radiators have valves, it is wired as one continuous loop in the house. I did install one shut off in the line both before and after the first and last radiator in the loop because I was having issues with the zone valve too. It works but is installed backwards with flow arrow pointing against the flow direction so it doesnt shut off completely sometimes (one of the many reasons I dont trust the guy...).

I have rarely gotten any air out of this non-pressurized system. I used to get a lot in the old pressurized one.
Maybe the flow direction isnt an issue, I'm just suspicious of the contractor because he caused me a lot of other headaches, and it seems to make more sense for the flow to go the other way, but I'm no heating expert and could be wrong. I actually called another contractor today and he said it does make a difference, but he wasnt the top guy so maybe he was mistaken too. I'll see what else comes up and check out that other forum too, thanks
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How does the water circulate?

If the contractor was no good, then there could be problems in all sorts of areas. It'll be a process of elimination to locate the problem(s).
If you post pictures on heatinghelp, get a consensus from the guys on there, and you have four local guys who disagree - the four local guys are wrong. Just so's ya know. The web site creator/owner, Dan Holohan, is THE steam heating guru, and he's attracted a lot of talented and knowledgeable guys to his forum.
Why do you feel the flow should go the other way? What's your reasoning?
R
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feel how hot the radiator is at the top and at the bottom...
if it is about the same, then it's fine..
if the top is much hotter then the bottom, then it might be significant.
having the hot water enter the bottom and exit the top just makes the heat a little more even across the radiator, but if the water is flowing fast enough, it won't make much difference either way.
If the radiator is hot over its full height, it is working.
Mark
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Not sure what you mean exactly, but it circulates...it seems a bit slower than with the old system though, even though they (even the boiler manufacturer) say the pump should be more than adequate. Other than that I dont know how to answer. In fact when they installed it initially it was so slow as to be completely ineffective. The contractor said I had clogged lines or radiators (despite the fact it worked fine with the old boiler), so I spent days and $$$ installing flush points and flushing it out ad nauseum, which did absolutely no good...it only started working halfway decent after he finally agreed to install a bigger pump because the boiler makers told him to.

Just seems hot water coming in the bottom will heat the bottom, and will naturally rise and heat the rest all the way up and then exit the top. You are using the hot water to circulate itself. Seems more efficient than forcing hot water to sink via pump action....but clearly there are a lot of variables and I dont know what they all are.
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It does sound like a circulation problem. When you find the answer post it back here, okay? It'll help other people out down the road.

Hot air rises - not heat. The efficiency of heat transfer, in descending order, conduction, convection, radiation. Hot water transfers heat to cast iron just fine, and the amount of difference between the supply coming in at the top or the bottom is negligible. That is not what is causing your problem.
The reason I asked what circulates your water was due to you saying the system was 'non-pressured'. The circulator (pump) pressurizes the water. It's not a lot of pressure, but there is pressure. It also depends how your guy filled your system. I don't recall you saying where the boiler was and how many floors you have, but there's pressure involved there as well.
R
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My old system was truly pressurized, this one is open-the boiler maker calls in non-pressurized but of course there is a pump.
if the reversed flow isnt an issue I dont really know that I have a problem anymore, except the zone valve is in the line backwards. They told me I would use less wood with this boiler than my old one, but that hasnt been true. Maybe they just lied.
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Joe wrote: ...

...
I'd guess the latter...a pressurized boiler will be more efficient than a non-pressurized. I'd wager there's the difference in that you had a better boiler to start with...
As rico says, the direction of flow in a radiator is of very little practical difference as it is simply a heat transfer to the same surface area only changing which is the inlet doesn't change the area nor the delta-T nor the heat transfer coefficient.
What has changed is that you likely had higher temperatures w/ the pressurized boiler and therefore, higher thermal efficiency.
--
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On 10/28/2010 9:45 PM, Joe wrote:

I am betting it is one of those belching outside wood boilers where you are supposed to pile in a bunch of wood and close the door?
Using a non=pressurized system is really a misapplication because you have ferrous materials in your system. Unless you have a heat exchanger between the boiler loop and your house loop you are going to get lots of corrosion because you will always have a source of oxygen because the boiler is open to the atmosphere. I had that argument with a number of suppliers at shows and was told "this will work...and a heat exchanger is expensive..."
A friend wanted one of those a few years back. We spent time looking and most of those units basically consist of the least amount of stuff necessary to accomplish having an outside wood boiler. Little thought is put into using standard practices because that costs more money.
The big box quality units really killed it for everyone in my area because you can no longer install them because of the smoke. They don't even use fire brick or a forced draft so the wood is sitting right on cold steel and when your house calls for heat it opens a damper and smolders for a really long time belching acrid smoke.

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On Fri, 29 Oct 2010 08:47:27 -0400, George wrote:

I'm certainly curious as to what corrosion inhibitor has been added to the system. Wandering around with a temperature probe and measuring metal temperature at various points might be enough to show where there are problem spots.
As for backwards rads, I would have thought there'd be some efficiency loss because you're no longer allowing convection to transfer heat to the room - but I'm not sure that this would be significant. (It should, presumably, be easy to rectify if all rads are installed like that)
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On Oct 29, 10:00am, Jules Richardson

A hand would suffice, but the idea is right.

How does the direction of water flow have anything to do with the radiator's convection? The delta T that dpb mentioned is the temperature differential. That coupled with the surface area of the radiator are the only factors in a correctly functioning radiator. There is not a lot that can go wrong with a radiator - primary culprit is air preventing proper operation, and that's why they have bleeder valves/vents. If the radiators have been bled and the valves are open, then the problem is with the boiler.
R
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The boiler is indeed a newer outdoor boiler with the fancied up sheet metal housing, with just a damper. My old pressurized boiler, and it was probably 25 years old (it finally rusted through), was basically just a barrel with very thick spray on insulation. It also had a blower draft. It had a much smaller firebox. It was inside an outbuilding where I stored my wood. Although the boiler was insulated the door wasnt, so some heat came out and helped raise the temperature of the building and firewood a few degrees, though its such a drafty old building it was still very cold in there. But maybe it warmed the firewood a bit. I wanted the new boiler installed in the building but they wouldnt do it, it's against code I guess. I just assumed this fancy and expensive new unit would still be more efficient even outside, and they kinda chortled and told me it would be dramatically better (of course) but that just hasnt been my experience. But these other differences are worth mentioning. Also there is about 15 feet more outside line than there was with the old boiler, so that adds some loss.
Another thing that just occured to me is that now with things reversed the first radiator in the loop is under a large, fairly drafty window. Its also at the base of the stairs so you would THINK that more heat goes up that stairs than before. BUt after I got this new boiler I stopped using the upstairs in winter because I was burning so much more wood just heating the downstairs. I hang curtains at the lower end of the stairway to prevent all of my heat going up there. I have to let some up because there is plumbing up there. But before I left that stairwell open and used the upstairs all winter.
I dont sense any problem "spots" in the system. I have thermometers where the lines enter and leave the attached garage on their way to/ from the house. The outgoing is about 10 degrees cooler (today, on a fairly warm day, 40's).
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You could simply choke down on the radiator at the base of the stairs radiation efficiency, thereby sending more heat further down the line. Restricting air flow around that radiator (a cover) or even insulating part of it, would work. You're the only one that can determine if installing a smaller replacement radiator would make sense.

Absolutely the best thing the OP can do. It's by far the biggest bang for his buck.

And as far as the pipes on the second floor, I'd consider using some thermostatically controlled heat tape on them, and just seal off the stairwell at the first floor level with a curtain/door/Ferengi force field. Just heat what needs to be heated.

How does the water get to the top of the radiator in the first place? Right, the pump. The location of the supply and return of a radiator has nothing to do with the heat transfer. You're assuming that your gut feeling outweighs the physics involved. Reread dpb's post about deltaT.
R
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wrote:

Since I have some experience with Naval/home boilers, and radiators, I'll toss in a couple cents. Joe, I don't have experience with wood burners, but I expect you know from the ash if it's burning right. No different than looking at coal clinkers I guess. So I'm assuming the wood is burning fine. If you're using more wood there's only 3 possibilities. Your new boiler either has poorer heat transfer than your old one, or the pump isn't pushing enough water, or you're losing too much heat up the stack, In any case, too many BTU's up the chimney. With a bigger firebox you might be loading too much wood in the there.
After I bought a house with a gas boiler I didn't like how much heat was going up the chimney. I throttled the gas supply to about half of what it was, and it reduced my gas usage quite a bit. Longer "on" cycles, but worth it. The pump doesn't use much juice.
If your new pump has lower capacity than the old one, and is out of spec for the system you won't get as much heat transfer at the water jacket or tubes. I don't know how they spec that for home boilers, but it's a factor, especially with your long piping runs. Restrictions - you mentioned a maybe backwards zone valve I think - will have an effect on heat transfer too. And check that your damper is operating properly. Anyway, the guys who installed your boiler should have some answers. Maybe. Some installers only know installation.
Harry, you got all the convection thermodynamics right. Every cast iron hot water radiator I've seen it probably doesn't matter enough to have the water inlet on top to justify the cosmetics of the long pipe. No doubt you get an edge though.
If you see one hanging on a wall with a top feed it's probably only because the piping runs made that the sensible way to do it. Not that I'm a "radiator engineer" but I've had some apart.and put them together.
Each section has the same size passage on top and bottom. The end sections have legs, and threads cut in the holes on one side to take a plug and nipple. And a vent hole tapped. All the inner sections are identical, and all sections are connected by push nipples. I think the plumbers put them together on site. One I replaced took 3 big guys to move it out of the house. I've only seen inlet/outlet on the bottom in homes, including mine. They always heat from bottom up, due to natural convection in the sections. Pretty obvious you're not circulating the coolest water back to the boiler. It would be interesting to measure the flow and convection for both top and bottom inlet, It would be more energy efficient if they were designed to pass all the water through from inlet to outlet. IOW, enter at one end of a section, exit only at the other end of the section. Then you'd be getting cooler water at the boiler and better heat transfer. But you'd always need an even number of sections to have in/out on the bottom, You'd have to have more castings. That's not how they did it. Hey, coal was cheap, so why bother. They work pretty good. I always liked hot water heat, but now that I've adjusted to forced air, I don't want to go back.
--Vic
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wrote:

Heat transfer equations are independent of flow direction for conduction between two materials, such as cast iron and water. Heat moves to cold. Conduction is by _far_ the most efficient form of heat transfer with the materials in question, to the point that convection and radiation are insignificant and can be totally ignored.

No, you won't - at least not any measurable edge unless you're taking it to, I don't know, two or three decimal places...? http://physics.info/conduction / There's the equation for conduction and a list of thermal conductivities for various materials. This is a significant sentence: "Conductivities vary for material being greatest for metallic solids, lower for nonmetallic solids, very low for liquids, and extremely low for gases."
It's clear from the equation that flow direction is not a factor as there is no variable for it. It is also clear that cast iron has a _much_ higher coefficient of thermal conductivity than water. As soon as the water comes in contact with the colder cast iron, it will start giving up it's heat, and as the cast iron is much more efficient in thermal conduction, for all intents and purposes it will very rapidly achieve a uniform temperature.

Conduction.
As the water rapidly gives up its heat to the cast iron, the water flowing out is the coolest water. It's the lowest point (for you convection = conduction fans), and the outgoing water has spent time in the radiator and given up heat.

Old time steam radiators were not connected across the top of the sections, and they were/are converted to hot water radiators without major issues. Newer (the past 70 or 80 years) hot water radiators always are connected across the top. This is for reasons of bleeding the radiator.
The thermal conductivity coefficient of cast iron is around 100 times that of water. Convection in water, even artfully calculated and mixed turbulent water, won't come anywhere near that efficiency.
Another way to look at it - the effective surface area is the guiding factor in sizing a radiator. It's the effective surface area that will determine how fast the water/steam heat is transferred to the cast iron, how fast the radiator will radiate heat into the room, and how much air convection will be induced in the room (limited or improved by the radiator enclosure design).
People should make mental models of things to test their ideas. A steam radiator is also heated by a fluid - steam (air and steam are modeled as fluids in thermodynamics, and anything that flows is a fluid, so let's not quibble that it's a gas). In a two pipe steam system - one supply and one return - it's directly analogous to a hot water radiator (the radiator being a constant). In a _one_ pipe steam system the supply and return are the same line - the water vapor in steam has condensed and flows down the one pipe to return to the boiler, so once again, it's directly analogous to a hot water radiator. There's no airflow in a one pipe steam system (at least not after the system has come up to operating temperature and the vents have purged whatever air leaked into the preferably closed system), so how can there be convection without airflow? It doesn't matter where the steam hits the cast iron, it'll condense and give up it's heat, which is transferred _entirely_ though conduction. Yet even without convection the radiator gets really, really hot. How is that?
Right, because of conduction.
R
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On 10/30/2010 1:43 PM, RicodJour wrote:

Except that air is a poor conductor. Radiators largely work by radiating. The amount of energy radiated goes up by the 4th power of the temperature difference but is directly proportional to surface area. That is why radiators operate at a relatively high temperature. Radiant floor heating, because the radiating surface is so much larger can work on a much smaller temp difference. Heat lamps are much smaller because the temperature is much higher still.
I prefer radiant heat. YMMV.
Jeff

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Radiator efficiency can be tremendously affected by a radiator enclosure. If a radiator predominantly heats by radiation, how is it possible to _improve the efficiency_ of the radiator by obstructing the radiation to the room? Because of convection - a more efficient heat transfer mechanism.
In a vacuum, radiation is all that there is. Most radiators are in homes with a quite substantial amount of air surrounding them.
R
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wrote:

The figures I've see say 30% radiation, 70% convection. That's why your curtains move.
--Vic
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On 11/1/2010 2:58 PM, Vic Smith wrote:

It's been a while since I've seen a real radiator, but I can tell you that there is no breeze near the electric oil filled radiators I have.
In practice, taking IR temp readings, I don't see the same temperature curve from floor to ceiling that I would see with a forced air heater. It's much more even. That old hot air rising is limited from the radiators I've measured.
I have no argument that the environment and how it is placed will affect all this. I place my heaters toward the middle, hard to do with steam heat!
Jeff

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