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Baseboard hot water heat constantly needs purged.

I have a baseboard hot water furnace system, Repco brand, installed about 35 years ago. It uses an older style expansion tank mounted in the joists above the furnace. The furnace has a circulator pump.
Almost once a month it seems i loose heat in the system. The cause is that there is air in the system, so the water doesn't circulate. I have run water through the system to remove the air, and that corrects the issue for a while, but then the problem reoccurs in a month or so. Each time the expansion tank seems to be full and i drain about half of it. An auto-air bleeder was installed on one radiator upstairs, but that doesn't seem to help. Does anyone have any ideas what the issue here is ? I kind of suspect the expansion tank, because i think it shouldn't be filling up and maybe has a leak, and has no pressure. I thought the tank should be pressured.

I also should mention, this furnace heats the 2nd floor of a duplex, The HW leaves the furnace travels up to a baseboard inthe apartment, then down to the first floor to a baseboad in the hall/entrance, back up to the apartment for the remaining baseboards, and finally back down to the furnace in the basement. Some one else said this wasn't the correct way to install this, but this furnace worked fine the first 30 years, it's just the past 5 that it seems to be acting up, and now recently it has been once a month almost.

Any Ideas would be helpful.
thanks John.

Re: Baseboard hot water heat constantly needs purged.


There are several things you can do---it will take a while, but the condition you describe is fixable.

You should be aware that there are 2 major ways of handling air in a HW heating system---the first is what you have---an air REPLACEMENT system, where the water circulating thru the pipes eventually pushes the air up into the expansion tank, where it collects at the top half of the tank.

This half-tank of air is needed as an expansion medium, since water expands approx. 5% when it is heated----the air in the ET acts as a "coiled spring" to prevent excess pressure from building up & damaging the pipes.

A rust spot or hole could develop in the tank itself, but you would see a spot where water was leaking out, or there would be dripping at the base of the ET.

In reading your post, I don't think you're following the correct procedure in draining the ET----this type of tank installed in the ceiling floor joists, eventually gets "water-logged", that is, all the air is eventually absorbed by the water---thus when the boiler comes on there is no where for the excess water to go (expanded water) & thus the pressure relief valve opens & spills water onto the basement floor.

The proper way to "drain the tank" is to 1) shut off the switch to the boiler; 2) CLOSE the valve that connects the boiler water to the expansion tank; 3) open the drain valve to the garden hose on the ET & let ALL THE WATER DRAIN FROM THE TANK---you'll know when all the water is out when no more water comes out of the garden hose that is used to drain the tank---at that point you can close the valve that opens the drain hose, and open the valve that allows water up from the boiler to the expansion tank.

You will hear water flowing in the pipe to the ET, until the water pressure reaches the pre-assigned 12 psi---at that point you won't hear any more water flowing in the pipes---at this point the expansion tank is "properly charged" (half full of water & half-full of air).

If you still have problems hearing air in the pipes when you turn on the heating system, you'll have to PURGE the pipes of air, using the purge valves near the boiler---the content of your post seems to indicate that you know how to do this.

Another problem I had was your mention of the "auto vent" on one of the upstairs radiators---I strongly recommend you go to Home Depot, or a plumbing supply house & buy a MANUAL AIR VENT (cost = $2) & not use the auto vent---auto vents let air into the pipes & are not recommended with the system you have--a manual air vent is a 1" X 1/2" chrome object that has to be opened & closed with a flat-head screwdriver---any air venting at the rad should be done with the system OFF.

The other disturbing note was the description of the up & down piping that feeds an upstairs rad, then plunges down to feed a hallway convector, the UP AGAIN to feed the upstairs rads---this type of piping arrangement is a classic opportunity for the pipes to trap air in such a piping arrangement---a common remedy to this piping mixup is to install two zone valves---one for upstairs piping & one for downstairs piping; this would allow separate t-stats up & down---it should be changed before it drives you nuts.

Therefore, for now, concentrate on doing a proper draining and "recharging" of the expansion tank & if you still have air problems, perhaps to purge the air out of the pipes.

You should be aware that there is another air elimination system that was invented to avoid much of the hassle of what you're going thru---this is the Extrol pre-charged expansion tank---this is a different way to expel the air out of the system & would require that the Extrol tank (it looks like a 20 lb propane tank seen on gas grilles)---be installed on a horizontal section of the SUPPLY pipe as comes out of the boiler top---the old ET is disconnected & the tube to the boiler is shut off & soldered.

The new Extrol system prevents any air from entering the piping & any air that develops is expelled by an air scoop & a manual air vent on the highest rads in the system (2nd floor).

The 1st site below on page 2 illustrates the "conventional expansion tank" and the valves & vents connected---click onto the + icon for a larger picture---note that most of these tanks have a little hex nut at the base of the drain valve to allow air into the top of the tank so that the tank will drain more quickly---some have an air vent at the top of the tank exterior for the same purpose, as in the diagram.

The 2nd site shows what a manual "coin-operated" air vent looks like---a B&G #4v--they're all standard 1/8" npt screw-on fittings---if you want to replace the present one, wrap the threads of the 4V in white compression tape, have a bucket ready, unscrew the auto-vent & block the stream of water with your thumb---then place the 4V into the hole & screw in.

The 2nd site also shows a red "diaphragm" (expansion) tank, discussed above (most of them are colored gray), which is widely used these days.($30 + $10 air scoop plus labor).

Please post back.


Re: Baseboard hot water heat constantly needs purged.

Thank you so much for the info !!!,

I am sure i am NOT recharging the ET properly. I'll do it the way you suggested and also re-purge the system, again. See if taht gets me through the next couple days.

I will also call up a HVAC guy and see about, first gettting rid of the old ET and putting in a Bladder style ET, and also have him replace the upstairs bleeders with all manual ones.

Re: Baseboard hot water heat constantly needs purged.

After looking at the diagrams a bit closer. I noticed that the expansion tank , and the bladder style tank are both mounted on the output side of the boiler. I know in my set up that the ET is on the input side of the furnace before the circulator pump. Does this make any difference ? what about if i changed to a diaphram type ?


Re: Baseboard hot water heat constantly needs purged.


Ah yes, you're a perceptive man---it's true that heating engineers recommend that the circulator should be installed so that it "pumps away" from the ET---this is to avoid the occasional negative vacuum that occurs in some looped heating pipe systems when the circulator is pumping into the ET---something more problematic with the diapraghm tanks than the older steel tanks.

However, in practice, it is usually not an issue in residential heating systems where the piping loops & run lengths are relatively small.

Many boiler mfgrs still ship their boilers with the circulator already mounted at the "in" return side of the boiler & it rarely creates a problem.

Anoher issue I forgot to mention, which I think you should check is the PSI pressure on the boiler gauge---the psi reading should BE AT A MINIMUM OF 12 PSI on your boiler gauge---not only that, but when you open the bleed valves on the 2nd floor, you SHOULD GET WATER coming out---sometimes air will precede the water, but IF YOU DON'T GET WATER from the 2nd floor rads, that means that the boiler pressure is insufficient to push the water up to the 2nd floor radiators---this will create air pockets at the tops of the 2nd floor rads & is a common cause of air in the system.

The formula is Height of water = 2.31 X boiler psi.

Thus, for example you have a 5 psi reading on the boiler gauge H = 2.31 X 5 = 12 ft.---if your 2nd floor is higher than 12 ft. from the boiler, the water won't get up to the 2nd floor.

If you have 12 psi on your boiler gauge, H = 12 psi X 2.31 = 28'---the boiler will push the water in the pipes up 28', which is probably high enough to supply the 2nd floor rads.

There's a way to easily increase slightly the water pressure, if needed.

Please check to see if you get water out of the 2nd floor bleed valves when you open them & post back.

Re: Baseboard hot water heat constantly needs purged.

I'm sorry i didn't quite follow part of your reply.
if i change my expansion tank to a diaphram tank. Do i want it to be diaphram tank , circulator pump , furnace ? or should it be
circulation pump, furnace, then diaphram tank ? the Current set up is ET, pump, furnace.

The pressure at the furnace says between 15-18 , so that sounds like it should be good as the 2nd floor rads are at about 20 feet.

Re: Baseboard hot water heat constantly needs purged.


The standard way of installing a bladder ET is to install it on the first horizontal section of output supply pipe coming out of the boiler--the sequence would then be: supply pipe out of boiler ----> ET--->pump--->supply pipe to radiator loop---> return back to boiler.

See Amtrol diagram at site below, "Typical installation of residential models".

There are several variations that are acceptable, as long as the pump's discharge end is not pumping directly into the ET---for example the pump & old ET on your system are already in place, so it may be convenient to install the new ET on the existing copper tube to the old tank---that way nothing has to be unsoldered & moved around---the old ET is just disconnected.

That sounds good for the psi reading on the boiler, but please check to see if you get water from the 2nd floor rads, just to be sure---sometimes boiler gauges register incorrect readings.


Re: Baseboard hot water heat constantly needs purged.

OK, Great ! sounds like i can just swap out my current expansion tank and have a newer diaphram tank installed in about the same location, which is good because i have more room on that side of the furnace.

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