Boiler pressure i keep having to fill the boiler every 20mins as the pressure keeps dropping.

There should be a water pressure reducing valve in the make up water line. This valve or regulator should keep the boiler pressure at about 10- 12 pounds. It sounds like that valve is letting more pressure to the boiler than it should. There is also an expansion tank that works to keep the boiler pressure at 10 pounds. The rubber seal inside can go bad and the tank will fill with water.

It sounds like your thermostat could be the problem. Have you noticed the boiler cutting off at all, or does it run continuously until it goes out on low water?

You stated, "when boiler is on", so I am wondering how you get the boiler on. If your boiler is controlled by a thermostat, you need to check it for a cold draft possibly from behind the thermostat in the wall where the wires come through. The thermostat should never be on an outside wall. The thermostat could be defective.

An aquastat can also control your boiler. If you have an aquastat, check the setting, typically 175 - 180 degrees. You should also have a "high limit" which should be set at 210 - 220. Some people run that high side limit higher; not a good idea - defeats the purpose. If the aquastat is set too high, the boiler will operate too hot and the problem you have will occur.

"pressure rises and pressure relief valve opens" This could mean that your pressure switch is prematurely venting. Check boiler temperature to see exactly at what temperature the T&P valve purges. If it blows off below the high side setting, either adjust or replace. The T&P valves are usually not adjustable. That blow off temperature is stamped on the head of the valve. So, if it discharges too soon, replace it.

Have you actually seen evidence thgat the T&P valve is opening? You may have a bad bladder in your expansion tank and your system may have air in it. Your boiler should have 12 psi when cold and 20 - 22 psi when operating. Check your boiler pressure guage.

"this eventually drops below the minimum operating pressure" Technically, there is set point for a NO MINIMUM OPERATING PRESSURE. That is a function of the amount of water in the boiler. If the T&P is blowing off, it will vent water from the boiler and the boiler pressure will go down. There is a minimum operating TEMPERATURE.

Your boiler could be low on water. You will have to manually fill the boiler. Be careful of thermal shock to the boiler. Feel the piping, and fill only when the circulator is running. Make sure the water is not going directly into the boiler. The supply water should go into the circulating supply side which feeds the radiators, radiant floor, or base board circuits.

All this will adversely affect your hot water. The heat exchanger for the potable hot water may have scale build up in it. Using the hot water keeps the boiler running, and a scaled heat exchanger will give you cooler hot water and the boiler keeps running. If this is a problem, you will need to replace the heat exchanger. A better idea is to not use the boiler for your domestic hot water. Why run the boiler in the summer?

When water is heated it expands. To deal with this all boilers are fitted with a small expansion vessel which is like a reservoir which holds the excess water.

This expansion vessel is like a balloon in a metal pot. When the water goes in the metal pot the baloon squashes a bit to make room for the extra volume of water. When it cools the balloon pushes the water back into the heating system.

What has happened is your expansion vessel no longer works. When the water heats up there is nowhere for it to go so the pressure jumps up to 4 bar. The Worcester boiler lets high pressure water escape (most boiler allow this to go through the safety valve at say 3.5 bar).

When the water cools the pressure drops very low as the volume of water contracts.

The problem is your expansion vessel - normally a little red painted metal cylinder inside the boiler.

Your boiler has an expansion vessel that is like a bladder full of air. Yours has lost it's air pressure. Check your manual for this vessel and where the valve is to re-inflate it and what pressure to inflate to. It should have a simple tyre valve, pump it back up (usually to about 1 bar after releasing the water pressure) and should be all fixed. If you can't re-inflate the bladder is holed and the expansion vessel will need replacing.

what kind of heater combi? if combi you may be pressurising boiler to high ie over 1 bar boiler may be blowing off through release valve check discharge pipe

if your boiler is a combi re-pressurise locate filling loop and refere to manual

you may need to hook a hose to the drain and to a spigot depending on your water pressure in your home and back feed the unit to desired pressur,, does t have an auto matic feed valve or just a shut off valve,,

on the pipework below there may be a silver braided hose with one or two valves on the end. turn the valve(s) so the handle is inline with the pipe this will allow water from the mains supply into the heating circuit , remember to keep an eye on the pressure gauge. fill the system to 1bar cold then shut down the valve(s). turn on the boiler and it should be ok, if you lose pressure again then you may have a leak somewhere which needs to be found.

http://www.worcesterbosch.com/Go there see if you can get some help.

Hi tiger650,

I'm Harvey the Master Plumber.

Your problem is: The Alpha SY24 boiler has stopped working. The pressure gauge has dropped, you need to know how to re pressurise the system once the radiators have been bled

Every hydronic heating system is required to have a presure regulator. Most guys call it a feeder valve. Every one I've installed or worked on had one. It's job is to keep the water pressure constant by adding water when you release some air without over filling the system. Sounds like it's not working. Some types can be serviced, others cannont so you may have to replace it. If you're not sure what to do it might be prudent to check the Repair Directory for a repairman.

Feel free contact me again!
Please give me a rating here at fixya.com before you sign off
Thank you,
Harvey your Master Plumber