To clear one point of confusion. All modern boilers are 'condensing boilers', which means that the 'products of combustion' ('fumes') which consist of water vapour and carbon dioxide, pass through a secondary heat exchanger to extract maximum heat. Traditional (obsolete) gas central heating boilers were about 60 - 65% efficient, but also had a permanent pilot light running 24/7/365. Most condensing boilers (except in apartment blocks) are on traditional open vent central heating systems, with an airing cupboard, hot water tank and a cold water feed tank and expansion tank in the loft.
Some condensing boilers are also 'Combi' boilers, which originally were meant for use in apartments where there is nowhere to fit a feed tank or expansion tank. There is no hot water storage tank either. The system is sealed and has a pressure vessel inside the boiler with water on one side if a diaphragm and air on the other. When commissioned, a bypass valve is turned on which pressurises the sealed system to 1.5 - 2 Bar - not much about the 1 Bar or so (about 27Ft) than in an open vented system with a tank, so no dramas.
One problem with combi boilers is that minute leaks on radiator valves which may not be noticeable, will cause the pressure in the system to drop over time to a level which causes the boiler to shut off. There's a dial on the boiler which shows what the pressure should be and what it actually is. If it's too low, there is a bypass valve on two flexible pipes covered with stainless steel mesh. To re-pressurise the system, the valve needs opening while watching he dial till the pointer reaches the correct pressure, then the bypass valve should be turned off. The User's instruction explain how to do that. Some boilers need a little key to operate the valve, which often get lost but can be obtained.
Combi boilers don't have a hot water storage cylinder and only heat water when a tap is turned on. They hot water temperature is constant winter and summer, but the flow rate in winter when the incoming water is much colder, is slowed down to keep the outlet temperature constant. Pitifully slow if you like a bath, but acceptable if you are happy just with a shower.
If a condensing boiler fails when the outside temperature is below zero, it will almost certainly be due to the condensate drain pipe having frozen up. Most are only 22 mm diameter plastic pipe so will soon freeze up with a constant trickle of warm condensate running through.
I spent my whole career (40 years) with British Gas and in Sept 2010, Gordon Brown offered grants of £400 to householders who replaced old conventional boilers with condensing ones. To get the grant, I had to suffer the indignity of watching inept installers doing their best to incompetently install the new boiler. Within couple of days the boiler kept shutting off. Looking into it, I noted that the condensate drain was sloping the wrong way and teed into the sink water pipe as was the washing machine. This meant that water from the washing machine and sink backed up the condensate pipe and shut the boiler off. I altered the slope of the condensate pipe and all was well till winter. Outside the house there 35mm diam plastic sink waste was exposed for 600mm. The steady flow of warm condensate into the waste pipe quickly froze until the waste pipe blocked solid with ice. I therefore insulated the sink waste and boxed it in. No problems since.
This nonsense is what happens when traditional craft apprenticeships and City & Guild exams are replaced with NVQ -' Not Very Qualified'. Little skill, little experience and o pride in the job at all.
The first pic shows the condensate drain sloping the wrong way towards the boiler. The second after I'd corrected it by sloping it at about 40 degrees. Not the verdi gris (green) corrosion on the pipework installed fifteen years previously when Persimmon built the house. That's from the 'self-cleaning flux' used by the 'plumbers' - flux for those who are too lazy to clean copper pipe and fittings, hen too lazy to clean the flux off after the joint had been soldered. I was taught to be clean at my mother's knee. They should be' doing time' - not 'doing plumbing'.
Hope that might clear up a few points.
(Who is a gas enngineer) so hopefull will get it up and running for the next wee while. I'm sure the condensate trap is still full of krap and maybe some of the drain pipe. I hope he is able to sort it as I am too scared to open it all up and take it out.
