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Hot Water, Cast Rads - Bleeding Question

msink5 Member Posts: 2
I had an odd thing happen when I bled my rads tonight that I was hoping someone could help shed some light on. First, here is my setup:

- Residential, gas fired hot water boiler with B&G Series 100 pump
- Temp limit at 160, pressure limit at 30PSI
- Cast Iron rads
- Temp in house is fine - all rads hot

I realized tonight that I hadn’t bled the rads this season and set about the process. I always start on the second floor and as far from the boiler as possible (not sure this even matters). The first rad had about 30 seconds of air and then water flowed. The second hissed for about 15 seconds and then nothing - no more air and no water. This has never happened before. I even removed the bleed screw completely and got nothing.

I moved on to the other upstairs rads and had little or no air in them and then completed the downstairs with no issues.

Went back up to the problem rad and doubled checked it -it is hot all over but the top is slightly less warm. Still couldn’t get water to flow.

Went down and checked the boiler. At that time, it wasn’t running and the pump was also not running (not abnormal). Water temps were normal for the outside temp (110 degrees). However, I noticed the pressure seemed a little low for outdoor temp - it is at ~8PSI. It is normally at 10-12 PSI at my current temp. I also did a visual inspection for leaks anywhere that I could view. No water on the pipe that runs to that rad in the basement.

Is it possible that since the boiler and pump weren’t running, the water couldn’t get pushed out of one of the 2nd floor rads? Should I turn up the heat to force the boiler/pump on and try again? Can the bleed valves get clogged in such a way that air escapes but water doesn’t?

Thanks for the input!


  • bburd
    bburd Member Posts: 912
    edited December 2020
    The fact that you noted lower than normal system pressure after bleeding suggests that makeup water is not being added to the system as needed. 

    Most hot water heating systems have an automatic water feeder, known (somewhat confusingly) as a pressure reducing valve. It’s supposed to maintain the correct minimum pressure in the system when you bleed the radiators or pressure is lost by other means. If you have one, it  may be stuck closed.

    There is usually a manual shutoff valve before the automatic valve. The shutoff valve needs to be “on” for the automatic fill valve to work.

    Some older systems do not have an automatic fill valve, and require that water be added manually when bleeding.

    You will find this valve, or valves, at the point where your heating system connects to the house’s cold water supply.

  • msink5
    msink5 Member Posts: 2
    bburd - thanks for the input. I do have a reducing valve. It was replaced in 2005 as part of a system check when I bought the house. The shut off valve is also open.

    Is there a way to check that the Reducing valve is functioning? Also, the pressure has ticked up a bit as the boiler just cycled and it appears normal at 10PSI.

    Please note that I am not 100% certain on the PSI and temps. I watch the boiler pretty closely and my experience is that 30 degrees outside equates to about 10 PSI. 10 degrees outside starts pushing 20 PSI as the boiler has to work harder to cover the inside/outside temperature delta. However, this obviously isn’t scientific :) - just my observations.
  • bburd
    bburd Member Posts: 912
    Most pressure reducing valves have a lever on top that can be flipped open to quickly fill the system with water. You might try moving it and seeing what happens.

    A 10 psi range between cold and hot indicates a possibly waterlogged or failed  expansion tank. For a two-story house, the normal cold system pressure is about 12 psig. 

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,822
    edited December 2020
    Static pressure is what the boiler gauge measures. What does that mean? you might ask. Basically, static pressure is the pressure that the water exerts on itself. For example, if you were to fill a tub with water to a height of 28" and placed a gauge at the bottom of the tub the gauge would read 1 PSI. If you were to put a gauge at the bottom of a 1" diameter vertical pipe that had 28" of water in it, that would also measure 1 PSI on the gauge (Pounds per Square Inch).

    So if you have a column of water 28 feet high, the gauge at the bottom of the column would read 12 PSI. (12 inches in a foot) Are you starting to see it in your mind's eye? The more pressure there is on the water pressure gauge, the higher the water can go up a column. The pipes and radiators are the "columns" of water in your house. If you have only 8 PSI at the boiler gauge then the water can only be forced up as high as 18.3 feet high. If your radiator vent is 19 or 20 feet above the gauge, then 8 PSI at the gauge will only get the water partially up the radiator until the water has no more force to rise above the level of 18.3 feet. Can you see it in your mind's eye? The column of water is as high as 8 PSI can lift it.

    here is an illustration to help you see the water pressure at different levels in the system

    By having 12 PSI in the basement at the pressure gauge the second-floor radiators can have a little extra pressure to force the air out and fill them with water.

    I hope that clears up your query on why the radiator did not release all the air. It's not the radiator's fault. it can only work with the amount of water you give it.

    That is why many boiler gauges have two scales on the pressure dial. PSI and FEET. They are two scales for the same measurement.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,822
    edited December 2020
    Regarding your observation of the pressure increasing based on the outdoor temperature. As water is heated it also expands. To give you an example, 100 gallons of water at 60°F will become 105 gallons of water at 240°F. How is this so, if water boils at 212°F? Well, that is the temperature water boils at sea level with no additional pressure exerted on it. If you place the water in a closed vessel and raise the pressure the water will stay water at higher temperatures.

    Back to our discussion on the expansion of water. If you fill the boiler and all the radiators with 100 gallons of 60° water and heat the water to 240°, where do the extra 5 gallons of water go? That is the reason we put expansion tanks on our heating systems. To give the expanded water a place to go. On a larger system that tank needs to be larger, on smaller systems that tank can be smaller. The tank must have air in it to compress as the water expands. the compressed air will build up a little higher pressure when the water volume increases with higher temperatures. The air pressure will force the water out of the tank back into the system as the water shrinks with cooler temperature water.

    Can you see it in your mind's eye? Water growing in a tank filling up with that extra 5 gallons, compressing the air. The air pocket getting smaller. The burner turns off and the water cools. Then as the water cools the air pressure pushing back and the air pocket increasing in size as the water shrinks down to its original 60° size (5 gallons less).

    The colder it is outside the hotter the water is heated to keep the house warm. Look at the water temperature and compare it to the pressure. You will see that the pressure rise will correspond to the temperature rise.

    If the pressure increases too close to 30 PSI and the relief valve opens to release the overpressure condition, then the expansion tank does not have enough air in it to accommodate the expanding water. That will need to be addressed. If there is air in a radiator, that air can be compressed. If you bleed the radiator and remove all that additional air from the closed system, there must be enough air in the expansion tank to accommodate the expansion of all the water in the system.

    You can see it in your mind's eye if you look hard enough. The water is inside all those pipes and radiators and boiler passages. can you see it moving around in there? It's twisting and turning as it turns by elbows and tees while flowing thru the boiler heating up and getting pumped all around. Wow! What a ride that was.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics