Thermostatic radiator valves

ttekushan_3

Most radiator valves for single pipe have a bleed hole in them to allow water to exit the rad when the system shuts off.

You are absolutely correct about the way those valves work, and they cause problems assuming your boiler's pressure is set too high and never shuts off.

So you're mind's made up. Good luck with that. Glad we could help (?).

John H_3

The difference between a one pipe and two pipe valve

I want to give an existing one pipe residential radiator some sort of automatic control. My thought was to replace the manual radiator valve with a thermostatic radiator valve. Armstrong recommends proving an integral air vent with the valve. I am having trouble understanding why this would be required. Why do I need the air vent with the valve when the radiator had a separate air vent already existing? Can I use the thermostatic valve that will just cut the steam supply off? This will allow the radiator to modulate even when the boiler is still firing.

ttekushan_3

because

> I want to give an existing one pipe residential

> radiator some sort of automatic control. My

> thought was to replace the manual radiator valve

> with a thermostatic radiator valve. Armstrong

> recommends proving an integral air vent with the

> valve. I am having trouble understanding why

> this would be required. Why do I need the air

> vent with the valve when the radiator had a

> separate air vent already existing? Can I use

> the thermostatic valve that will just cut the

> steam supply off? This will allow the radiator

> to modulate even when the boiler is still firing.



single-pipe steam valves don't work well at all as modulating devices. At the point you have meaningful closure of the valve, condensate from the radiator will block the entry of steam. This causes the radiator to run cold, and sometimes gurgle or hammer.

The air vent controls the inlet of steam by controlling the rate at which the steam can purge the air from the radiator. Steam and air do not mix. So the vent is the logical place to modulate the temperature.

Also, remember you can't downsize the inlet valve so a thermostatic valve at the base of the radiator would be an expensive affair.

Go with the air vent type valve for your single pipe heating system. I'll work fine.

-Terry

Also note that if the radiator is able to heat with the valve closed at the air vent, your system pressure is way to high for a single pipe set-up. Lower it and see what happens.

ttekushan_3

because

single-pipe steam valves don't work well at all as modulating devices. At the point you have meaningful closure of the valve, condensate from the radiator will block the entry of steam. This causes the radiator to run cold, and sometimes gurgle or hammer.

The air vent controls the inlet of steam by controlling the rate at which the steam can purge the air from the radiator. Steam and air do not mix. So the vent is the logical place to modulate the temperature.

Also, remember you can't downsize the inlet valve so a thermostatic valve at the base of the radiator would be an expensive affair.

Go with the air vent type valve for your single pipe heating system. It'll work fine.

-Terry

Also note that if the radiator is able to heat with the valve closed at the air vent, your system pressure is way to high for a single pipe set-up. Lower it and see what happens.

Mike T., Swampeast MO

With one-pipe steam the TRV body replaces the vent. Such valve body is completely different than is used for two-pipe steam or hot water where the TRV body replaces the existing hand valve. To avoid very likely problems, make certain that the TRV body used for the one-pipe system has an integral vacuum breaker.

The TRV actuator however is the same regardless of the type of TRV valve body used.

John H_3

The only reason I say that the the radiator will still heat is because the purpose of the air vent is to purge the air originally. Once all of the air is purged and the radiator is full of steam it will continuously heat until the boiler cycles off via a thermostat in a central location. It does not allow the radiator to modulate in the meantime. In other words it can turn up but cannot turn down until the boiler cycles off. In essence, it gives you slightly better control than using a standard air vent but not complete temperature control. Is this correct?

I don't understand how using a TRV (without the integral air vent) instead of a hand valve is different that modulating it or even shutting it off using the hand valve? This is the way I always did it growing up and I never hear banging when I reopened the hand valve.

ALH_4

why risk it

It would sure be a lot easier just to replace the vent with a TRV, and not risk creating problems with your system.

Chris_82

MONEY

John H_3

My mind isn't made up

My mind isn't made up, that's the problem. I keep being told not to do that but still no explanation of why the TRV creates a problem that a hand valve doesn't. If the bleed addresses the problem for the hand valve why does it not for the TRV? Does it not have a bleed valve? I've learned a lot so for from the responses and I thank you for that but I still haven't gotten the critical explanation.

Mike T., Swampeast MO

Here's a photo of the radiator side of a wide open Danfoss 3/4" TRV of the type that replaces a hand valve.

Condensation from my breath...

The arrow points to the actual opening--visible because I was holding the valve above a 300W bulb. The black area above the opening is the washer-like disc that provides the seal. Overall travel (from full closed to full open) is slightly less than 1/8".

The other side of the valve, while initially full outside diameter of the pipe has a "ledge" the width of standard steel pipe and then tapers quite rapidly to an opening right at 3/8" across.

So...everything must pass through an area about about 3/8" diameter by a maximum of a bit less than an 1/8" long. While larger TRVs are available, these opening sizes are very similar if not identical. 1/2" TRV bodies have slightly a slightly smaller opening.

Back to the photo. Notice how the actual opening only begins half way up the overall opening?

With one-pipe steam, it's my understanding that once the air is purged and the rad is filled with steam that such steam begins to condense--as it condenses such condensate flows back out the radiator and is replaced by new steam (provided the boiler is running).

If you used this valve for one-pipe steam it sure looks to me like the condensate would have to back up somewhat before it could even begin to drain--then it would be fighting the steam to get out the same small opening that the steam has to enter. Guess which one wins? I suspect the winner will let you know in an audible way...

[added] I didn't even mention the condensate you'll get before the rad is full of steam when such rad was cold. How could that water possibly drain back to the boiler with such a valve?

Mike T., Swampeast MO

I don't have a hand one-pipe steam valve handy to show a similar view. But I know what they look like. BOTH openings are the full size of the pipe and the body of the valve itself is a bulbous sphere to give even more area where the steam and water are passing by in opposite directions.

It's that bulbous sphere that differentiates one-pipe steam hand valves from hot water valves. While the openings are still full size, the hot water valve body is a cylinder.

DWood

1 ps

t.p.tunstall

lookin the library. it explains tjis issue very well

John H_3

Great Explanation

That's a great explanation, pictures and all. That's dedication. I was just curious because I grew up with a wood furnace that didn't cycle so I always used the hand valve to turn down the heat. It seemed to work pretty well but then again I never paid that much attention. It sounds like the orifice size for a hand valve is larger and allows more of an area for the condensate to pass through. Thanks. Looks like I will be using the Danfoss TRV.

Steverino

maybe this will help?

borrowed from this website's library: On one-pipe-steam, I learned that TRVs don't work well with angle-pattern steam vents. You're not controlling the steam on these jobs; you're controlling the release of air from the radiators. The TRV goes between the vent and the radiator, and since the radiator starts with an angle-pattern vent, it's tempting to use the same type of vent with the TRV. Trouble is, the condensate won't drain from the angle-pattern vent. You have to use a straight-pattern vent instead.



I also learned that TRVs for one-pipe steam need vacuum breakers to work properly. The steam pushes the air through the TRV and out the vent. The room comes up to temperature, and the TRV closes. At this point, no more air can escape from the vent. The steam condenses and leaves a partial vacuum in its place. Since the TRV is closed, air can't get back into the radiator through the air vent. The partial vacuum in the radiator sucks steam from the mains and risers, and the room overheats. It took a while for TRV manufacturers to figure this one out.