Why does the farthest radiator gets hot then cools while the other radiators stay hot

SteamInWellesley

I have an one pipe steam system with 7 radiators all with VentRite adjustable vents. The radiators all heat up around the same time. The farthest radiator's air vent is set at 8 and all the other ones are set ranging 3-6. My steam pipes are only insulated around the boiler. The problem I have is that the farthest radiator is also located with the thermostat and would get hot with the other 6 radiators but then starts cooling down after it gets hot. So it takes a very long time for it to satisfy the heat setting while the other rooms are getting too hot. For example, the call for heat starts at 6:30AM the farthest radiator gets completely hot around 7:30AM but then starts cooling down around 8:15AM while the other radiators are still hot. My current work around is to stop the call for heat as the radiator is cooling down and restart it 30 minutes later. Any thoughts on why this is happening and solutions? Thank you in advance for your time and consideration.

acwagner

First, is this during a recovery from a temperature setback?

Also, does this happen every time or is it intermittent?

Jamie Hall

8 is pretty fast. How is this radiator piped from the main? I wonder if perhaps it is producing condensate sufficiently fast that the condensate, while not blocking the steam (I would think it might hammer), may be slowing it down -- particularly if the runout is long and not insulated.

AMservices

Just throwing this out there. But what if the air vent is sticking closed and the packing nut on the radiator valve is leaking?  
I think if the vent closes and air can be sucked into the radiator through the packing nut, that could cause the radiator to stop heating. 
Just as a test, I would try swapping that vent with another radiator and see if anything changes. 

SteamingatMohawk

You don't mention whether or not you have vents on the mains.

dopey27177

Typically radiators are placed under windows.
Cheap test. Cover the window area with a plastic drop cloth.
That will stop the cold air from dropping down on the radiator and cooling it quickly.

If that cures the problem install a drape to the floor which will stop the air infiltration from cooling the radiator.

Jake