Pump Over/Pumping Over - What is it ?
Pumping over occurs when water is pumped over the open vent pipe and into the feed and expansion header tank. This cannot happen in sealed systems. It can be caused by an incorrectly placed circulator, too high circulator speed, insufficient static head or incorrectly sited system components. Conversely, seesawing is where water is pumped up the feed and expansion pipe. In either case, if there is enough thermal contraction then re-oxygenated water will be drawn into the primary circuit which will promote corrosion. The first likely evidence will be pinholes in radiators.
A correctly installed system will create positive pressure in as much of the installation as possible. The diagram below shows correct 'left to right' and 'right to left' installations. If the order in your system is different then we recommend you contact a qualified plumber.
B=Boiler
CF=Cold Feed pipe
F&E=Feed & Expansion pipe
F&EC=Feed & Expansion Cistern
HC=Heating Circulator
OV=Open Vent
11-Dec-2006, London Plumber DF (c) 2006
Boilers - Combi or Condensing, what's the difference ?
So what's the difference ? Often my customers are confused between the different types of central heating boilers, and I'm not surprised as there is actually quite an array of different types. Combi and condensing boilers are not the same thing, in fact here is a simplified list of boilers available:
Combi Boiler - Non-condensing (Conventional)
Combi Boiler - Condensing (High Efficiency)
System (Regular) - Non-condensing (Conventional)
System (Regular) - Condensing (High Efficiency)
A condensing boiler refers to the fact that the boiler has an additional heat exchanger, which causes the flue gases to exit the boiler at a lower temperature. As the temperature of the flue gases is reduced, and the fact that the flue gases contain water vapour, the water condenses to liquid water both within the additional heat exchanger and within the flue. You will also often see what is called a 'plume' coming out of the flue terminal(especially in cold weather), which is the water vapour in the flue gases that is condensing (and not smoke!). Older non-condensing (or conventional) boilers do not suffer from this phenomena, as the flue gases are much hotter when they leave the flue.
However a Combi (combination) boiler is one that runs the heating circuit (radiators) and also provides hot water that is heated at the time it is required. The hot water usually is connected direct to the cold water mains supply, and therefore is delivered at a relatively high pressure.
So a combi boiler refers to it's ability to provide both heating and hot water direct from the boiler. The alternative is a 'System' or 'Regular' boiler, which you need to have a hot water storage cylinder and traditionally a big cold water storage tank (cistern) often in the loft.
And a condensing boiler refers to it's high efficiency insofar that it contains an additional heat exchanger to remove more of the heat from the flue gases. In simple terms without this additional heat exchanger, a conventional boiler is literally throwing away heat that is locked into the water vapour in the flue gases.
The above is not intended to be a definitive guide, but as an explanation of the difference between the terms combi and condensing which for some reason confuse people. We are aware that there are other types of boilers, and systems not mentioned here, but they are beyond the scope of this explanation.
9-May-2006, London Plumber DF (c) 2006
