2 radiators on hot H20 get cold a day or two after bleeding now; Help?

Call_Me_Al

I've got 2 radiators in a hot water

system that have worked very well for years, yet they both just

stopped working this winter. I have purged the system a few times,

and everything works when I do that, but 2 days later, the radiators

have gone cold again. I don't see any signs of leaks anywhere, esp

the ceiling below. Could the problem be the shutoff valves? I'm

worried that if I try to replace the shutoff valves and the old

piping breaks, I will be in for a BIG job tearing apart a whole

ceiling below, and replacing a lot of pipe. Can anyone tell me what

they would do?

STEVEusaPA

I don't think....

It's the shutoff valves, especially if you haven't touched them. It's more likely a combination of a few things--water logged/failed expansion tank (depending on type), low pressure, or minor leak.

Are you sure you bled it properly? I mean, keep the pressure up, bleed in the right order, make sure the steel expansion tank isn't water logged (or the bladder tank hasn't failed).

I'd start there first.

Tell me more about your system. Are the 2 rads on the same zone as the rest? Are they on their own zone? How many zones? Circs or zone valves, etc. How many stories is your house?

Was their ANY work done to the water side of your system before this happened?

nugs

Radiators

If they heat up after you purge them then the valves are open and not the problem

A water logged expansion tank will cause the relief valve to blow off but it will not effect the radiators or circulation in the system unless in doing so the water feed valve fails to replace the water and your boiler pressure drops below around 5lbs. 



make sure you have between 12 and 18 lbs pressure on the system

Check the high limit, if it goes much over 210 you will make steam and steam puts air into the system

Check your circulator and make sure it is working

icesailor

Conditions:

And under the right adverse conditions, here is a situation where the circulator on the supply, "pumping away" will cause air in the system that ends up in the upper floor radiators.

But no one will believe that.

The system pressure is wromg, the pressure gauge on the boiler must not read correctly, and the Extrol tank is broken. If it is a regular expansion type tank, it is waterlogged or way too small. And the supply temperature is too high.

Mark Eatherton

Color me a skeptic with an open mind Chris...

Just exactly HOW will pumping away from the PONPC allow air into the system?



Never seen it in my 35+ years of spelunking around boiler rooms.



ME

Call_Me_Al

More information

The boiler pressure is holding at 12 psi and I have a bladder type expantion tank.  I do not see any signs of boiler  relief discharge on the floor, and the boiler is cycling between 160 and 180 degrees F.    At first I did not bleed the rads in any particular order because these were the only two not working.  The second time I purged the radiators, I purged the first floor then the second floor.   I just went around the room sequencially.   Its hard to see all the take offs from the supply and return mains to determine the exact order.  This is an old house and 20 different plumbers have seviced it over the years.  I did find a capped radiator in the basement on the same side of the house of the problem radiators.  I did not think that would cause the problem I am having.  

icesailor

Pumping Away and Air:

Cavitation from the impellor and cavitation around the impeller. And the water doesn't need to be all that hot. Plenty of friction from the spinning impeller going no where.

The same scenario as with ejector "Jet" water pumps. If the pressure drops, and the flow is restricted, friction causes cavitation/steaming and the longer it runs, the more air it makes that goes into the system.

I saw it again the other day. Someone wanted to know if an in-line 3 piece circulator would work better than the 007 he was going to have to replace for the third time. I looked it over and decided that the problem was that the system pressure wasn't high enough. I raised the pressure and it came without purging but a lot of air. The relief valve was dripping on the floor. He kept the fill valve off. And the Extrol tank was shot. All pumps depend on a flow of liquid to keep the pump cool.

Its kind of like your lazy brother in law and the tow boat. If the boat is heavy, and you have a high speed prop on it, when you try to jerk the BIL up on the skis and you wind up the motor too fast, the prop may cavitate and over rev the engine. Back off the throttle and let the prop get back its bite. You don't want to over rev the motor or you will blow it. Put on a "Barge Prop" and try to pull him up, and you may over stress the engine from the excessive load.

Put a stethoscope on the pump body of an over pumped zone. If you hear a "guush, guush" noise, it's cavitation.

Mark Eatherton

Cavitation...

In all of my years, I have seen one pump that cavitated, and it was a 25 H.P. pump. It sounded like fire crackers going off inside the volute. Can't say as I have ever seen a residential sized pump cavitate, but I suppose it is entirely possible, if the circulator is over sized, and the piping is undersized.



The guush guush noise you refer to would indicate to me a system that still had air in it, or was at least partially air bound. Once the impeller gets a good "bite" on the water, unless there are additional bubbles still floating around in the system, it should keep its bite, and do a good job of moving the fluid around. This shows the need for a good air elimination system, and I must admit that I am now a big fan of these more expensive microbubble resorbers. My time is more expensive than the time spent waiting for air to get out of these systems under normal circulation conditions. It SAVES the consumer (and me) time and money. They (scoops and MBR's) eventually get the O2 content to the same level, but the MBR's do it in MUCH faster time.



As it pertains to the potential of cavitation, exposing the impeller to a negative pressure, i.e. pumping towards the PONPC, would increase the possibility of creating cavitation. Having the pump set up so it pumps away from the PONPC pretty much eliminates that particular cause, except where the piping is significantly under sized, or other system components (spring checks etc) are creating such a large drop in pressure that cavitation raises its ugly head.



Your boating analogies are not really applicable to a properly set up closed loop system. I have had a boat that did cavitate due to to steep a prop pitch angle, and I had to be careful not to over power the motor to avoid slippage. Scary feeling for sure. Am going to replace it with a lesser pitch prop.



Chris, I realize that you can't see the PONPC concept in your minds eye, but people a whole lot smarter than I or you established these principles long before we graced the face of this fine Earth. I know that your empirical field experience tells you that even with the pump on the return side, you (and I and millions of others) have seen these systems work just fine for many years, and if it ain't broke, why fix it?



The reason that what you see in the field continues to work is because the pumps that were employed (series 100, and or Taco 110) are a FLAT curved pump. They CAN"T generate much pressure differential. Its not in their design. Its the newer wet rotored HIGH head pups that cause problems when misapplied as it pertains to the PONPC.



When I make a recommendation that the pump be moved, it is based on my 35+ years of empirical field experience, whereby moving the pump or moving the PONPC eliminated recurring air problems in closed loop heating systems. And it is backed up by the thousands of other contractors who have had the same experiences in the field with problematic systems. If it weren't a problem, people wouldn't be showing up here asking for help. If it ain't broke, why fix it. If it is broke (continual air problems) the fix is to move the pump so it pumps away from the PONPC, or, if possible, move the PONPC to the inlet of the pump so that it has NO option but to create positive pressure. And it doesn't matter whether the air is being "managed" or eliminated.



A while back, somone posted a graphic of a closed loop system showing the effects of having the pump before or after the PONPC. You said you didn't care WHO made that drawing up, that it was WRONG. What you probably didn't know is that Gil Carlson, the pioneering God Father of hydronics generated that drawing, and it wasn't based on theoretics, it was based on a actual test system he developed to demonstrate his theory to people who can't see the concept in their minds eye. It is real. It is nature, and it can't be changed.



Are there systems out there that are less than ideal that work?



Oh yeah. Like you, I've seen working systems that theoretically should NOT work, but have been doing so for many years. If it ain't broke, don't fix it. If it is broke, fix it right and be done with it and move on to the next problem. When building from new/scratch, do it right, or don't bother doing it at all, unless you are really into doing things twice, the second time for free.



Peace, out.



ME