Two years ago, I moved into a house equipped with cast-iron hot-water radiators. Cosmetically, they were in pretty bad shape, so I pulled them out of the house, scraped and sanded, and repainted them. They’re now approaching the end of their second winter since.
Of course, I had to drain the system to remove them, and had to refill it afterward. I expected that I’d have to bleed the air out of the system more than once after refilling them, but I’m a little surprised that two years later, I still have to bleed the top-floor radiators fairly regularly – about once a month.
I’m also surprised by the appearance and smell of the water that comes out when I vent them – it’s bluish, and it smells strongly of fuel oil. I doubt it’s a coincidence that my boiler runs on fuel oil.
Can anyone tell this misplaced Southerner whether these things are normal? Should I be alarmed by the water’s appearance & smell? I thought the heat-exchange part of the system was supposed to be relatively sealed – how might the water have become contaminated?
I can handle small plumbing repair jobs, and I’m willing to call in a plumber if necessary. But I’m not even sure if I really have a problem. The heat works fine, and I can imagine the bemused look on the plumber’s face when I tell him I want him to make my radiator water smell better.
I’ve never heard of putting oil into the radiator water before. I can only guess it was a misguided attempt by someone in the past to prevent rust in the system, or a really boneheaded blunder where someone thought they were filling an oil tank but opened up something like an expansion tank or an accumulator.
There’s no close connection between anything in the system that holds fuel oil and the circulating water, so I can’t think of any kind of mechanical failure that would put oil into the water. The oil is burned in the bottom of the boiler, which heats up the water in a piping coil above and/or around the flame - it’s not like the water pipes are running through the oil tank.
That could be a number of things. In the UK, on older systems, the heating water loop is often topped off by a header tank in the attic, open to the air, so more air can diffuse into the system. This is a corrosion issue, so you need to have a corrosion inhibitor in the circulating water.
Bleeding once a month is a bit frequent though even for a header tank system. It might indicate a problem. Header tank systems also have an overflow for pressure relief. If the system pressure gets too high, the water overflow continuously in the header tank, which mixes a lot of air into your circulating water, wastes heat, and can accelerate corrosion.
In a completely sealed system, there is air dissolved in the water right after you refill it. (That’s why fish work!) The gas comes out of the water when it is heated, but remains in circulation as bubbles, accumulating in high areas such as the tops of your top floor radiators, where you then bleed it out. That will lower the system pressure, which might lead to you adding make-up water, or an automatic system doing the same. The make-up water will also contain dissolved gas, so you’ll need a smaller, second bleed, and maybe even a third after that. It can take some time to get all the air out of the circulating water. Two years is too long though. More air is getting in somewhere. Are you having to frequently top up the circulating water? That will introduce more dissolved air, and also suggests you have a slow leak somewhere.
The oil contamination is just plain odd. No idea about that one, I’m afraid.
I have a gas-fired central heating system and I’ve noticed dark, stinky crud in the water when I’ve drained it - the pipes are copper, the radiators are steel - there’s some sort of galvanic corrosion thing going on (even with a corrosion inhibitor). The black sludge I tip out of the radiators when I remove them to paint the wall behind does smell sort of like mineral oil.
Most central heating systems will accumulate some sludge. The header tank system more than the closed system.
Most systems have an oily smell, or worse, when they are drained. Depends on the interval between draining.
It’s a good idea to flush out and disconnected radiators with lots of fresh water to remove any sludge in the bottom which otherwise will end up reducing the flow of water considerably. If you feel the radiators in a running system, those cold at the top need bleeding - air accumulated in the upper portion. Those cold or barely warm in the bottom could be sludged-up. Could also be the way the inlet and outlet ports are set or just cooling down as the system is not circulating after being shut down by the thermostat. If one is cold at the bottom, check it a few times before tearing it off the wall, may not be bunged up at all.
There should be a corrosion inhibitor in the system. As Matt has described, introducing fresh water into the system also brings with it dissolved air and the oxygen released when the water is heated is just looking for something to corrode.
Corrosion inhibitors can be fairly smelly but usually are not that obnoxious in dilution.
My old system needed bleeding fairly frequently (open header) but the new one (closed, pressurised ) never does. It does need topping up occasionally, about once every 18 months unless someone has been tinkering with it.
I had a gas-fired boiler up until two weeks ago, and the bled water was a dark grey, and very stinky - like Mangetout, I reckon this could smell a bit like oil. According to a plumber I spoke to about a former residence, the smell and sludge are something to do with corrosion at the header tank creating acidity, and corroding the inside of the pipes and rads.
If it were truly contaminated by oil, surely you could capture some in a glass, let it settle, and see if there’s a patina floating on it?
Random thoughts,pertinent perhaps.
Initial system fill and/or make-up water could be acidic.
There is an additive,corrosion inhibitor,to combat dissolved oxygen in make-up water,perhaps a source of odor?
Initial manufacturing or construction could have left traces of oils,especially in threading pipe where sulfur bearing cutting oil is used.
No advice, but several years before we moved into our house, the radiator system was changed from oil to electric. When I very occasionally bleed the radiators, the water is pristine. It looks drinkable. The blast of air that precedes the water, however, has a brief whiff of gas to it.
I don’t know if there’s any advantage for the OP in switching to electric to heat the water. There used to be an economic advantage in the area where I live (for various reasons), and maybe there still is, but I don’t know if this would hold for the OP. And I don’t know if there’s a mechanical or maintenance advantage.
Thanks for all the insight. The system has a closed header – at least that’s what I think the barrel-like tank attached to the system near the boiler is. It got drained at the same time as the rest of the system. I vaguely knew that it was there for pressure relief, but I wasn’t sure whether there was a two-way flow between that tank and the rest of the system or not. If there is, maybe the excess gas is entering solution as take-up from there? Was I not supposed to have drained it? Oops.
I was more concerned about the color & smell of the water, and really mentioned the excessive bleeding in case it was related (and because I was curious.) It sounds like others have observed similar conditions, so I’m going to file it away as something to talk to the plumber about next time I have one visit, and not something I need to address immediately.