Pressure in a hydronic heating system

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We have a four story plus cellar 1840's 2 family house with a hydronic heating system with large radiators that was probably installed in the 1920's. The gas boiler is about 20 years old. The system has both a conventional expansion tank (that has not been emptied in years) and a Filltrol diaphragm expansion tank in series.
Every place I looked on the web (including the Filltrol site) says that the system pressure should be set at 12 psi when cold and should be 20 psi when hot and circulating. That is exactly where it is, but the radiators on the 4th floor seem to have some air left in them. When bled, water does not come out of the valve and the top of the radiators are not very hot (the bottom halves are).
Should the pressure be higher in a 4 story house that in a shorter one? And if so what should it be? And how do I adjust it? The Filltrol manual does not give instructions other than to say that it is adjusted via the intake air valve and should be adjusted when the tank is empty.
So (if I really do need to increase the pressure) do I add air (and how much) or remove air from the Filltrol expansion tank. And do I have to completely drain the system to do it? Or just let out a few gallons of water? Or not concern myself with letting any water out.
A link to a good website will suffice. As I said, my Google search was unproductive.
--
Peace,
BobJ



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http://heatinghelp.com/ Tons of info there if you can not find what you need on the site they have a forum where you can post questions. My feeling is 20 psi should get to the fourth floor. 1 psi will roughly lift water 2.31' so 20 psi should get you 46.2 feet-- there are other varibles that may come into play too that may shorten that estimate. If you search google using combos of verticle lift, psi, water, head-- you will find some more of what you are looking for.
Steve

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You dont even know if your gauge is accurate, Add water to a COLD boiler of 70f while bleeding air out of the top floor while boiler is running till water comes out the radiator. Your issue is not enough Altitude or Pressure. Dont mess with anything else. Then see how it works and monitor pressure cold and hot, it may only take a Lb or 2 of water to get heat to the top. After a few hours bleed radiators again. It may take a few tries over a few days.
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Thanks, I'll try that.
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Peace,
BobJ



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Thank you for the info. I did not see your response before I responded to a later post reasking the question that you already answered. Since the water only has to rise about 35 above the circulator pump outlet, I should be fine.
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BobJ



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Not correct, on two counts.
First, the water needs to rise 35' (if indeed that's correct -- see my other post in this thread) above the inlet where [pressurized] replenishment water enters. The position of the circulator is irrelevant.
Second, you need enough pressure at that point to lift the water to the highest radiator when the system is *cold*. The dynamic pressure (when the system is operating) is immaterial. You must have enough *static* pressure to get the water all the way up to the top, and 12 psi is not enough to lift water 35 feet, let alone the 38 to 40 that I suspect is really the case.
Measure the height carefully. Don't forget to include the depth of the floor joists: you have four sets of floor joists between the basement and the rads on the top floor. That's at least 2 1/2 feet, and maybe as much as four.
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Doug Miller (alphageek at milmac dot com)
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You do not say whether you are getting enough heat in the top floor. If the rooms up there are comfortable, then there is nothing wrong with having the radiators half full, and if I were you I would not touch it. If you need more heat up there then first try bleeding air from the radiators. The system should have an inlet valve that adds water automatically when the pressure drops below 12 psi. -- H
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wrote:

The 4th floor is occupied by our tenants. They tend not to complain, but did alert us to the radiator problem at the beginning of the winter when it was not cold. I did note an electric space heater in a room when I last bled their radiators. In any case, I would want it to work right.

I have bled the radiators twice this winter. In both cases only air came out few a few seconds. Water never came out of these radiators no matter how long the valve was left open.

Yes, I understand this. My question is whether 12 psi cold pressure (20 psi hot with circulator pump on) is enough for a system with large radiators that are about 35 feet above the level of the boiler.
--
Peace,
BobJ





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Marilyn & Bob wrote:

Short answer: NO! Cold pressure must be enough to maintain the head at 35 Feet. Anytime the pressure drops (cold or hot) below that required at the highest elevation, air *will* be sucked into the system. On the next startup, that air will block flow to the highest rads, even if pressure rises.
You may need more expansion tank capacity to reduce the swing between cold and hot pressure.
In any case, you need higher system pressure.
Jim
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If you haven't actually measured, I think you should. This sounds a bit low to me, for rads on the top floor of an older 4-story building.
You need to measure from the bottom of the boiler to the top of the rad.
Consider also that older buildings frequently have 9- or even 10-foot ceilings, and floor joists were often 2x10 true (instead of 2x10 nominal = 1.5 x 9.25 true).
The 12 psi recommendation is based on having the boiler in the basement of a *two* story building, where the head probably won't exceed 20 feet. I wouldn't be at all surprised if the head in your building is closer to 40 feet than it is to 35.

Entirely correct (as is what follows) -- but incomplete.
The missing part is that 1 foot of head = 0.4335 psi. Thus 35 feet of head is a bit over 15 psi, and 40 feet of head is 17.3 psi.

Exactly.
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Doug Miller (alphageek at milmac dot com)
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OK. It's about 36' to the bottom of the top radiator about 39' to the top where the bleeder valve is. So about 17 psi. Now the question is how do I get to that? We have a Filltrol diaphragm expansion tank and standard expansion tank. I have not drained that standard tank in many years. m Ransley suggested bleeding the radiator with the water cold (but the circulator pump on), is that a place to start? I've always bled it with the system on full blast.
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BobJ




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Marilyn & Bob wrote:

Your boiler *may* have an automatic fill valve, like: http://www.cashacme.com/cbl.php
If so, the regulator section is adjustable.
Drain the expansion tank first. Get the pressure (cold) up where it should be and then bleed.
Jim
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Jim, See my response to Doug Miller.

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Shoot, *I* would like to see your response to me! :-) This post showed up, but I haven't seen anything else...
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Yep. I think I'd go 18.

This has *nothing* to do with the expansion tank. The tank's only purpose is to give the water some place to go when it expands due to being heated. Its purpose is not to provide the initial pressure in the system.

No.

That's fine when all you need to remove is a little bubble here and there. Your problem, though, is that the system isn't full. You need to get it full. And that means you shut it down completely. Boiler off. Circulator(s) off. Everything off, and water cold. The fastest and easiest way of getting the water cold is to run the circulator(s) while the boiler is off -- the radiators will remove the heat from the water fairly quickly. Once the water is cold, shut the circulators off.
Now, on to filling the system. It should be connected to the building's domestic water supply through a pressure-reducing fill valve. That valve is probably set at somewhere around 12 to 15 psi. You need to adjust it upward to somewhere around 17 to 18 psi. (If the pressure setting is fixed and cannot be adjusted, you need to replace it with a valve that can be adjusted.) One at a time, open the bleed valves at the tops of the upper-floor radiators until water comes out. Once you get water out of the bleed valve on every radiator, the system is full. Turn it back on.
You'll probably still have some air in it. If your automatic air vents (you do have some, right?) don't take care of it, then you'll need to do more bleeding, this time with the system operational. Be careful you don't scald yourself.
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Doug Miller (alphageek at milmac dot com)
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wrote:

Yes, it does, see below.
The tank's only purpose is

Doug & Jim,
Thanks again for your help.
I think I now understand our communication problem about the expansion tank. The Fill-trol expansion tank see: http://www.amtrol.com/pdf/filltrol.pdf also serves a pressure regulator. I have the one combines with the air purger. Yes, I know that the second (standard type) expansion tank that is parallel in the system is irrelevant here. So, based on your advice, it looks like to adjust this, I have to get an air pump and raise the air pressure in the tank to about 18 lbs. from the current 12 lbs. The only thing that I still need to understand is what is meant by the caution that "The Fill-trol must be free of water when changing the air charge." It's not clear to me how to achieve this "free of water" condition. I can e-mail the manufacturer to find out what do if it is not obvious to either of you.
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BobJ



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Aha! Yes, that explains a lot. Clever mechanism, isn't it?

Correct.
If you shut off the water feeding the FillTrol, then shut off and drain the boiler, that ought to empty the tank. Then you can adjust the tank pressure. Close the boiler drain, and open the shutoff.
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Doug Miller (alphageek at milmac dot com)
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wrote:

OK. Since it means draining the system, it can wait until the spring. (It takes at least five or six hours to drain and refill the system. Huge radiators, and since we have no floor drain (sewer lateral is above cellar floor height), draining has through a hose. Thanks again for you advice.
--
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BobJ



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You don't have to drain the whole system -- not unless you don't have any shutoff valves anywhere, that is. There should be shutoffs ahead of, and behind, the circulator. Close those, so that the water in the rads and the pipes doesn't drain back. Then drain the boiler.
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Doug Miller (alphageek at milmac dot com)
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Marilyn & Bob wrote:

Adjust the FillTrol regulator to give the required system pressure when Cold. This has nothing to do with the air pressure in the bladder; this adjusts the pressure of city water connection to boiler.
Do NOT drain the boiler. Admitting fresh water to the boiler only results in increased corrosion.
If it is possible to drain the old exp tank without draining the whole system, that may help. Do it *before* adjusting the FillTrol.
Jim
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