I have a two-story house with a Navien gas boiler and old cast-iron radiators. I heard that without flushing they get clogged up and heat output drops. How do you know when it's time? And how much does it cost?
The frequency depends on your system's condition - if the radiators heat normally and without noise, you can flush it every 5-7 years, but with cast iron and old piping I wouldn't drag it out and would do it every 3-4 years. My neighbors had a similar situation: they ignored the problem until the boiler started running under strain and almost broke down - they had to call someone urgently.
The main signal to flush is when radiators heat unevenly (as the guys above already pointed out), strange noises appear when you turn on the heating, or some radiators are completely cold while the ones next to them are hot. You can also check the water temperature at the boiler's inlet and outlet - if the difference is bigger than usual, it means the system is clogged.
The cost of flushing varies depending on the volume and method - hydropneumatic flushing is cheaper (maybe three or four thousand), but chemical flushing with special reagents will be more expensive. It's best to call a specialist who will inspect your specific system and give a recommendation - sometimes a simple flush is enough, sometimes you need something more serious.
Flushing your system really is necessary, especially if you've got old cast iron radiators - they tend to accumulate rust, scale, and all sorts of sediment inside. How do you know it's time? The main signs are radiators heating unevenly (top hot, bottom cold), or they're barely warm even when the boiler's working properly. You can also do a visual check: open the air vent on a radiator - if murky water comes out instead of clear, there's definitely stuff built up in there. If your system is new and sealed, you could skip it for five to seven years, but with your setup (old radiators, gas boiler), you should do a flush at least once every three to four years.
There are a few ways to do it. The simplest and gentlest is internal flushing with a special chemical that you pour straight into the system through the boiler. It's the cheapest option. Second option is hydroflushing (high-pressure water jet) - more expensive, but more effective if there's serious buildup. The most thorough way is to remove the radiators and flush each one separately, but that's more complicated and takes more time.
I can't say exactly what it costs, but roughly: chemical flushing in the system will run you a small amount plus labor if you call a technician. Hydroflushing is pricier. It all depends on your region and how complex your piping is. Better to call local service centers - they'll give you quotes. The main thing is, if you see clear signs, don't put it off, because later you might end up needing to replace radiators, and that's serious money.
Cast iron is definitely a problem - it rusts from the inside, and over time that rust clogs up the whole system. I'd go by a simple sign: if the radiators heat unevenly (top's warm but the bottom's cold) or the boiler starts acting weird (turning on and off a lot), that's your signal. Flushing will cost somewhere around a few thousand rubles, depends on how complicated your system is and what region you're in, but it's cheaper than replacing radiators or the boiler later because of a blockage.
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