I'm planning to bleed the radiators. How do I know if my boiler is a gravity filled or combi?

Question

I plan to bleed my radiators for the first time. I've read that If you have a gravity filled system the system will look after itself but if you have a combi boiler, the pressure will need to be checked after bleeding. How do I know what I have? The boiler is an "HB Smith G200 Series" that is about 30 yrs old. Thanks.

This boiler is in unbelievable shape and my inspector says it could last 50 years! No problems.... just one upstairs that is cold and another upstairs that is only partially hot. Thanks for all your answers. I think I know what I have now.

Answer 1

-Combi boiler usually heat the hot water also. So they heat the house & serve as the hot water tank.
- Gravity filled are just plan steam systems. All your boiler does is heat the radiators.

Combi systems PSI usely goes upto 30-40
Gravity systems PSI shouldn't go past 15

Answer 2

The real question is why. Do you hear gurgling? Are some of the radiators cold?

If you do, the first question is why isn't water getting back into the system? Water is supplied by a regulator and an expansion tank, sometimes combined together.

The regulator reduces the water pressure, which may be 50 to 80 pounds for the rest of the house, down to 10 or 15 for the heating system. After 30 years it may have rust inside that impedes it's filling action.

It's sometimes hard to tell by the gauges on the boiler, after 30 years at the same pressure, the gauge might be stuck there.

The expansion tank has an air baloon known as a bladder in it, and when the water heats up, it expands, but the pipes don't, so the water is forced into this tank and compresses the bladder. As the water cools down, the bladder forces the water back into the line.
After 30 years this bladder may break, and the air gets into the system and rises to the top, and blocks the water flow.
This tank has a valve on the end of it like a car tire, and you can stick a pressure gauge on it and see a few pounds. If water drips out it's bad.

Repairing these would require professional help and evaluation of the entire system.

A professional inspection is a good idea for all heating systems to clean the burner, check the air fuel mix, and check for carbon monoxide leaks anyway.

If you get through all this good, with the system cold and off, a bleeder valve should squirt water (or air) under pressure. After 30 years some might be a little stiff, and might need replacing. If you don't get a healthy squirt, we still got water supply problems mentioned above.

Hot water systems should have an air bleeder in them near the boiler, a little brass can about the size of your thumb, with a valve on top that should let the air out automatically, but after 30 years, they could be bad too.

We went through all this on my grandma's boiler, also about 30 years old, ended up getting a new one under a grant from social services, Free! I know this is more than you wanted to hear, but it all started with a little gurgle.

Answer 3

the most simple way of telling if you've gotta combi or gravity fed system is whether or not you have tanks in the loft and also weather or not you've got a hot water cylinder/tank.

If you've got two tanks in the loft (generally one small one and one larger one) then you'll also have a hot water cylinder, if this is the case then your system is classed as a conventional gravity fed and NOT a combi. If however your boiler heats the hot water as well as the central heating then this is a combi, the quickest way to tell is whether it's gotta pressure gauge on the front of it.