Questions about one-pipe systems, thermostatic valves, and condensation

JimJoyce

Hi steam folks,

I've got some naive questions that I tried to answer by reading this great wall, but I'm not sure I've figured it out just yet.

So I know that Dan Foss makes a thermostatic valve, and that this valve can make one-pipe systems a bit more customizable when it comes to room temperature. What I'm wondering, though, is how it interacts with the radiator. I haven't bought one yet and I'm just trying to learn more about it prior.

I understand--I think--that the valve closes when the radiator is filled with steam. But how does the valve actually help us control temperature? Is it by letting more air stay in the radiator so that it cannot completely fill with steam, which in the end equals less heat? In that case, would I be manually unscrewing some part of the valve to keep it just a little open?

Further, I wonder about the condensate factor. If the thermostatic valve closes, how does condensate trickle out? I probably misunderstand this, but doesn't condensate trickle out somewhat continuously as opposed to all at once? If this gradual condensate release were so, I feel like a trap would block that flow and cause trouble.

Finally, do these traps, if they allow more air to stay in the radiator, increase the rate of corrosion?

To anyone who has time to respond to my questions, thank you big time.

Jim

Steamhead

You have the idea. If the air can't get out, the steam can't get in, so limiting the air that leaves the radiator limits how much heat the radiator gives off.

The expansion ratio of water to steam is 1700 times. That means when you boil a certain quantity of water, its volume increases 1700 times when it turns to steam, and it shrinks 1700 times when it condenses back to water. So the amount of water in there is very small, and it doesn't need much room to drain out.

The expansion ratio is also why steam doesn't need a pump to move thru a system. The expansion that occurs when water turns to steam creates the slight ounce pressures that fill the pipes and radiators with steam.

hvacfreak2

The vacuum breaker feature of the Danfoss allows air to re-enter the radiator during the off cycle allowing condensate to drain.

s3.supplyhouse.com/product_files/013G0140-Submittal.pdf

JUGHNE

Jimjoyce, just so you are clear on what you are looking for, this is an air venting control. Not a steam inlet valve.

As explained above it controls the air leaving the rad. It is adjustable for that room temp. It only will cut that rad heating capacity down. Don't put one near the wall thermostat.

Also your boiler must cycle on and off at low pressure, less than 2 PSI. The off cycle will let the condensation drain back. You want a control that includes a vacuum breaker and the proper air vent.
Don't worry about corrosion.

aircooled81

Does a frequent cycling of the boiler prevent the radiator from eventually pushing all the air out, and be at full heat?
As some what mentioned, by original poster (valve closes, condensate trickle), is there a valve that can also be installed in line with the supply to the radiator that limits steam entry, but allows for condensate drainage?

JimJoyce

Steamhead, HVACfreak, JUGHNE, thanks very much for these pointers, it's clearing things up for me. Aircooled81, I wish I knew clear answers to your questions!

JUGHNE

Constant steam pressure on the rad will continue to compress the air and steam will travel further into the sections....overheating the rad.

So the pressure has to drop to let the radiators inhale a full lung of non compressed air.
But your TRV will probably be closed at that time and isolate its vent from the radiator. So a vacuum breaker between TRV and rad will open and allow air back in. Ready for the next heat cycle.

As far as a controlling/modulating inlet valve on the single pipe radiator, I have never seen any info here or anywhere else.
Every where they are mentioned, it is stated to leave completely open or completely closed. If open just a crack the steam will slip into the rad and condense to water that cannot drain out the small opening and probably water hammer when meeting the small stream of steam.
Many older valves that you think are completely off tight will still allow steam to seep in. At best that rad will fill with water that is trapped. So often the concern of valves not closing is over rated.
Best to make sure they are fully open with full passage allowed thru the valve.....IMHO.

Jamie Hall

The only truly reliable way to close off a steam line -- one pipe or two pipe -- is with a full port ball or plug valve rated for steam. Then, to handle any possible condensate, you must have a drip to a wet return on both sides of the valve. So it can be done (note that one can close the valve on a two pipe radiator, as any condensate can still escape through the trap and not cause trouble). But, on one pipe steam, why bother? Closing the vent does almost as well. Yes, a bit of steam will get in, but the radiator will be pretty well off, and if the valve is open the condensate will happily go back where it belongs.

There will be a small amount of corrosion from the condensate and general dampness in the radiator, but nothing to worry about.