Watching the pressure drop during a long heating cycle.

Hap_Hazzard

So, we had a cold morning today, so I fired up my boiler for the first time this season so I could check for leaks. The house was about 65 degrees, so I set the thermostat to 70 and let it run for about a half hour, keeping an eye on the pressure gauge.

Once the main vents closed, the pressure shot up to 0.5 psi and stayed there for a while. Then, as the radiators filled, the pressure gradually decreased. By the end of the cycle, the pressure had dropped to about 0.05. 

This surprised me. I guess I’ve never sat and watched it this long before. I had assumed that, after reaching 0.5 psi, the pressure would stay about the same or maybe increase as the radiator vents closed, but, apparently, as the radiators fill with steam, they start condensing it faster than the boiler can generate it. Am I right about this? And if that’s the case, would I be correct in concluding that the EDR of the radiation (including piping) is a little higher than the output of the boiler?

bburd

It sounds like your boiler capacity and radiation are perfectly matched. Did the  house heat evenly?

EBEBRATT-Ed

could be a little under but as long as you have pressure (some at the far end) it will be fine. Just need enough to distribute the steam evenly

Hap_Hazzard

bburd said:

It sounds like your boiler capacity and radiation are perfectly matched. Did the  house heat evenly?

Pretty much. All the radiators got hot, but I’m not sure if they all got hot at the same time. I was more focused on the boiler room. 

bburd

You can fine tune the distribution by adjusting the radiator vents. Your system should run efficiently and well.

Hap_Hazzard

bburd said:

You can fine tune the distribution by adjusting the radiator vents. Your system should run efficiently and well.

Yeah, I used the Maid-o-Mist interchangeable vent port method. 

109A_5

Hello @Hap_Hazzard,

Hap_Hazzard said:

Once the main vents closed, the pressure shot up to 0.5 psi and stayed there for a while. Then, as the radiators filled, the pressure gradually decreased. By the end of the cycle, the pressure had dropped to about 0.05. 

This surprised me. I guess I’ve never sat and watched it this long before. I had assumed that, after reaching 0.5 psi, the pressure would stay about the same or maybe increase as the radiator vents closed

Seems normal to me, when the Main vent closes less overall venting, higher pressure. The higher pressure persists until the Horizontal Run-Outs and the Risers warmed up. (this is why pipe insulation is so important to minimize the reheating of the pipes every cycle). I'm betting your radiator vents did not close before the thermostat was satisfied (or you shut the test down early) since you did not see the pressure rise near the end of the cycle. If you want to see the pressure rise near the end of the cycle you probably need a recovery from a deeper setback or the outdoor temperatures need to be much closer to design day temperatures. Especially if your boiler and EDR are well matched.

ChrisJ

To me, it sounds like your radiator venting is lacking
The reason being, it's building up to 1/2 a pound to get the steam into the radiators. Once they start condensing the pressure drops to something more reasonable.

Under the same conditions here's what I've observed many times. This is what the system runs at with everything cold. I started recording as the main vents were closing.

https://youtu.be/zyxLbXq61-U




Now, if the radiators are fairly warm or even hot like when it's in the single digits out I have seen it climb up a little and then drop down, repeatedly. Up to 0.75" and back down to 0.5" for example.

The reason yours is getting to 1/2 a pound is your holding the steam back more than you need to. Let the system breath.

Hap_Hazzard

ChrisJ said:

To me, it sounds like your radiator venting is lacking The reason being, it's building up to 1/2 a pound to get the steam into the radiators. Once they start condensing the pressure drops to something more reasonable. Under the same conditions here's what I've observed many times. This is what the system runs at with everything cold. I started recording as the main vents were closing. https://youtu.be/zyxLbXq61-U Now, if the radiators are fairly warm or even hot like when it's in the single digits out I have seen it climb up a little and then drop down, repeatedly. Up to 0.75" and back down to 0.5" for example. The reason yours is getting to 1/2 a pound is your holding the steam back more than you need to. Let the system breath.

I’m using all the available vent sizes, from #4 to D, so I can’t increase the venting without screwing up the balance. 

ChrisJ

Hap_Hazzard said:

ChrisJ said:

To me, it sounds like your radiator venting is lacking
The reason being, it's building up to 1/2 a pound to get the steam into the radiators. Once they start condensing the pressure drops to something more reasonable.

Under the same conditions here's what I've observed many times. This is what the system runs at with everything cold. I started recording as the main vents were closing.

https://youtu.be/zyxLbXq61-U




Now, if the radiators are fairly warm or even hot like when it's in the single digits out I have seen it climb up a little and then drop down, repeatedly. Up to 0.75" and back down to 0.5" for example.

The reason yours is getting to 1/2 a pound is your holding the steam back more than you need to. Let the system breath.

I’m using all the available vent sizes, from #4 to D, so I can’t increase the venting without screwing up the balance. 


That's good to hear.
So you're not running all Hoffman 40's or Gorton 4's like some.

I suppose it is what it is. your system wants that little bump to get things moving and then it drops down.

Do you hear any noise from any vents during that or do they all remain fairly quiet?

Hap_Hazzard

They’re Maid-o-Mist, bimetal type vents, so they’re silent unless they get condensation in them, but that won’t happen until it gets colder.