cast iron radiators

[Vic Smith]

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

[RicodJour]

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...?

'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 =3D 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

[LSMFT]

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.

[Jeff Thies]

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

[Joe]

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.

[RicodJour]

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

[RicodJour]

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

[Vic Smith]

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

[Vic Smith]

The figures I've see say 30% radiation, 70% convection. That's why your curtains move.

--Vic

[Jeff Thies]

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