Open a drain-off valve on the Vaillant boiler or central heating system with a hose attached and let the pressure drop. Removable side panels and push-fit flue system – For improved accessibility, ease of installation and greater installation flexibility Vaillant boilers are sometimes undersized when fitted and are not large enough to cope with a large house’s central heating system. When opening the drain point do you need to open all valves up on the rads and then open the bleed valves or can you just literally open the drain point and draw off 10 litres and then shut off the drain?

Thank you so much everyone for your responses. I've read through the FAQ and it makes sense on the whole. I just have a couple of questions: Next attach a hose to a drain off somewhere on the heating system and open the valve, leave it open until the water stops coming out..... The pressure on a Vaillant boiler should be between 1 and 1.5 bar when the central heating is off and the radiators are cold. Correct Vaillant boiler pressure High-quality heat exchanger - Stainless steel heat exchanger designed for long life and easy maintenance

Leave the drain off open and pop the front cover off the boiler. Now don't touch anything except for the red expansion vessel down the right hand side, looks kind of like a metal hot water bottle!! PUMP, HIGH-EFFICIENCY (IF THIS IS THE REPLACEMENT PART THEN PART NUMBER 0010030691 - CABLE IS ALSO REQUIRED) The pressure relief valve could also be letting by. Or, the filling loop on the Vaillant boiler could be slightly open or damaged. Do I need to turn my boiler off to repressurise?

This means the expansion vessel on the Vaillant boiler might not be big enough and can cause problems with the pressure. If you need to repressurise your Vaillant boiler once or twice a year I wouldn’t worry about it as it’s such a small leak that it’s barely noticeable. If you have another brand of boiler and not a Vaillant, there?s a chance that recharging the expansion vessel may not fix the problem because the vessel has split internally. If this is the case, you would need to replace the expansion vessel, not just recharge it. This almost never happens on Vaillant boilers though. Faulty Filling Loop Letting ByA filling loop will be built in on the bottom of your boiler on most modern and small combi boilers, but there could also be an external filling loop on the pipework. If it is below 1 bar, you should top the pressure up. If you have to keep topping the pressure up, you have a problem that needs to be fixed. If you need to repressurise your Vaillant boiler often you have a problem that you need to get fixed.

Total system solution available from one manufacturer – Intelligent controls and accessories available Our boiler seems to run with little pressure so it may have been low for quite a while during the summer. The hot water on reflection wasn't as hot as it is once the pressure is topped up, which makes me think it has been like it a while. I've not been topping it up at all during the summer but last winter I had to top it up fairly frequently when we were using the heating - every 3-4 days maybe. I thought it was a slow leak. If the pressure drops, it means the water is getting out which should not be happening. Low pressure on a Vaillant boiler

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Recognised Grundfos high-efficiency pump - Modulating high-torque pump uses low energy to deliver a balanced output to meet the boiler demand, helping to reduce energy consumption I then just refilled the boiler, vented it and tested the system. It?s a nice easy job that shouldn?t take more than an hour. What Else Can Cause This Problem? Split Expansion Vessel To find out which it is, you can tie a carrier bag to the end of the copper blow-off pipe outside. Boiler blow-off pipe If your Vaillant boiler is losing pressure overnight you will have a leak somewhere on your heating system or the pressure relief valve is letting the water out. Having the boiler pressure too high (above 1.5 when the system is cold), means you will need to let some pressure (water) out of the system.

This is usually below the boiler but there could be an external flexible filling loop anywhere on the system, sometimes hidden away in cupboards or behind boxing. If the bag is dry after your Vaillant boiler loses pressure overnight you must have a leak on the central heating system which will need to be found and fixed. If the expansion vessel diaphragm is faulty when the air valve is engaged then water should/will come out? You should see a schraeder valve on it - don't let the name panic you it looks just like the ones that you pump your car tyres up withCompatible with aroTHERM air-to-water heat pump hybrid system – Making the perfect entry into renewable systems Having been reading various threads here and from watching the boiler this morning I think I have gotten close to the problem.