No Hot Water from Combi Boiler

One of the most commonly asked questions that I get asked is: why there is no hot water in my taps? To be able to diagnose this problem we need to establish the type of heating system that is installed at the property in question. Generally there will be many options to choose from such us: instantaneous electric water heaters (electric), instantaneous gas water heaters, storage water heaters, electric showers, gas fired combination boilers, gas fired traditional boilers and hot water cylinder, unvented hot water cylinders and variations on above.

This series of articles will deal with the most common systems in UK starting with combination boilers also known as ‘combi’ boilers. Other systems we will discuss in further articles are:

– Unvented hot water cylinders

– Traditional hot water cylinders (vented)

No hot water from combination boiler.

The first thing to check in case there is no hot water on combination boiler is to see if it works in central heating mode. To do that you will need to make sure that your wall mounted thermostat is calling for heat. You also may have a programmable thermostat. In that case make sure that it is set to constant on central heating mode. Be aware that on some programmable digital thermostats may prevent boiler from firing up if the batteries are low. They may be strong enough to support LCD display but not strong enough to switch thermostat. It is one of those very simple problems that may be very hard to diagnose. Remember – check your batteries in your programmable thermostat. If boiler fires up in central heating mode but doesn’t fire up in hot water mode then we can suspect that a diverter valve is stuck on heating mode. Professional help will be required.

If combination boiler does not work in both hot water and heating modes there are some steps that can be performed by the owner before calling a heating engineer to eliminate most obvious possible causes. Combination boilers require on average 1 bar pressure in central heating system to function satisfactorily. Look for a pressure gauge on the front of the boiler or sometimes on central heating pipe work. It will look similar to the one on a picture below.

Most boiler makes will have markers in green or red pointer (as on the picture) showing required system pressure. If system pressure is low then that may be the reason for boiler not firing up in both domestic hot water and heating modes. By low we understand below marked field on the gauge. Most boilers will not fire up if pressure drops below 0.5 bar. If low pressure has been detected next step is to top up system pressure. Majority of boilers on the market would have been fitted with a filling loop that comprises of no.2 valves and a flexible metallic connector. Good plumbing practice requires that filling loop should be detached and left by the boiler with no.2 valves capped of to avoid accidental over pressurising the system and leaks. In reality however 90% of installation have filling loops attached. If yours is not attached and present and new flexible hose will have to be purchased and attached. Let’s assume we have our filling loop in place. It should look similar to the picture below:

Macintosh HD:Users:simonc:Desktop:filling loop.jpg

Some filling loops will have a small tap as on the picture above. Some may have 2 taps or a flat screwdriver point. On single tap ones it is enough to turn one tap on. On filling loops with 2 taps you will have to turn both on. On filling loops without taps you may need a small flat screwdriver to turn them on and off. It may look similar to this one.

Be aware that some combination boilers and not fitted with filling loops. The best example is Worcester Bosh CDi range of boilers that require a special plastic key to top them up. This may be a major hassle if the key had been lost! I will not detail how to top those boilers up as there is a very detailed guide on how to do that on Worcester Bosch web site. Have a look here:

How to repressurise Worcester Bosch CDi combi boilers?

Once you know where your filling loop is and how to use it turn the boiler off and fill up the system to around 1.5 bar pressure. On some boilers (Vaillant Eco Tec Pro) there is only green and red field without digits showing pressure.
Top up to 2/3 of the green.
System topped up?
Hot water back on? Great!
No hot water? Read on…

With your system at correct pressure check if boiler controls are set properly. There should be hot water thermostat on the boiler. It may be the case that it is turned all the way down. If it is set it half way up and try again. It may look something like this (No.2 controls on the left, one for hot water and for heating):

System pressurised? Controls in on position? Hopefully your hot water is back on.
If not suspect a more serious issue and call a qualified heating engineer to help you with your hot water problem.

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