Water treatment for old hot water system?

jesmed1

Another question about our two Weil-McLain WGO-5's heating cast iron radiators in a 100-yr-old converted gravity system in a 4-unit condo building. My previous project was installing an Airtrol fitting on our plain steel compression tank for better air management, and that seems to be working well. Then I installed Ecobee 3 Lite smart thermostats with remote sensors, so I can now see temperature vs. time graphs for all 4 units.

The system is working well and the Ecobees are doing a good job of averaging the temperature between the unit with the thermostat and the adjacent unit with only the sensor in it. But the temperature vs. time graphs are confirming what I've suspected, that there are some erratic temperature variations that suggest that, from one boiler cycle to another, sometimes one unit gets more heat, and sometimes the other unit gets more heat, without any rhyme or reason.

It's as if there is some restriction in the hot water flow that shifts around at random, sometimes allowing some radiators to get hotter, and soImetimes making them colder by restricting flow somewhat. We've been religious about bleeding radiators to get the air out, but I did have to introduce some air when I installed the Airtrol, and I didn't ask the other owners to bleed again after the AIrtrol installation, so it's possible there is still some air in some radiators.

But I'm also thinking there's probably sludge in some of them which might be causing these seemingly random fluctuations in how much heat each unit gets during a given call for heat. I know for sure that there's one upstairs radiator at the end of a main that has chronic problems with not heating sufficiently. Before I got here, bleeding had been neglected, and in troubleshooting that radiator we found a whole lot of trapped air in it. But even after throrough bleeding, it still doesn't heat as well as it should. I suspect it's full of sludge, as I can feel the return pipe from it in the basement, and it never gets hot.

So I'm wondering if I should try to inject some water treatment like Rectorseal 8-way. I'd want to use stuff that isn't going to harm old boiler and valve seals, and that can be left in the water indefinitely. I also don't want to de-pressurize the system and then have to add oxygenated make-up water to repressurize.

So my questions are:

(1) should I try to add water treatment?
(2) what treatment should I use, and
(3) is there a way to inject the treatment liquid without depressurizing the system?

For example, I was wondering if I could get a new #30 expansion tank, depressurize the bladder, add about a gallon of the treatment fluid to the water side of the tank, hook it up to the boiler drain valve via a simple threaded pipe assembly with a ball valve on it, then pressurize the air side of the tank to slightly higher than boiler pressure, open the boiler drain valve, and inject the fluid into the (cold) boiler by continuing to pressurize the expansion tank with a compressor until all the treatment fluid has been forced into the boiler. Our plain steel compression tank is big enough that I could add a gallon of fluid to the system with very little change to the system pressure, so I'd only need to pressurize the expansion tank to about 20 psi to force all the fluid into the boiler. Then drain a gallon of water out of the compression tank to compensate.

Thoughts?

EBEBRATT-Ed

A closed system seldom gets much corrosion unless water is constantly being added. When you cut in to replace the expansion tank what did the inside of the piping look like?

doubtful if sludge is the problem

jesmed1

EBEBRATT-Ed said:

A closed system seldom gets much corrosion unless water is constantly being added. When you cut in to replace the expansion tank what did the inside of the piping look like?

doubtful if sludge is the problem

The only piping I cut out was 1/2" copper from the boiler to the expansion tank, and that was probably installed new when the boiler was installed about 25 years ago, so it didn't really tell me anything. All the near-boiler piping is copper from when the boilers were installed. I've never cracked open any of the original steel pipes in the rest of the system.

Radiator bleed water is typically clear, but I think that's mainly because the boiler techs have been draining 20 gallons out of the steel expansion tank every year and refilling it with fresh water. That was until this year when I found out that wasn't a good thing, and I installed the Airtrol and put a note on the tank asking the techs not to drain it.

That still leaves me with at least one problem radiator at the end of a main which I know isn't getting sufficient flow. Maybe because the circulator is making the flow short-circuit through other radiators teeing off the main in the same place. I'd feel better if I could figure out how to get more flow through it. I don't have regular access to it because it's in a different condo unit. The owner tells me she has bled it and hasn't found any air in it recently, but a few years ago it had a massive amount of air that we finally got out of it after a lot of effort.