Modern day boilers are much more efficient. However because when they start up they use more gas (more of the heat is then transferred into your house so the net loss is much more efficient) they need to have a 22 mm pipe supplying the gas.
This may be a problem and the engineer might have to re run some of the gas pipe from the gas meter to the boiler. Modern boilers use much more of the heat created from the gas being burned. Therefore the flue gasses are considerably less hot than in the older models.
This in turn means that the flue gasses have much more noticeable moisture content contained within. In practical terms modern condensing boilers, plume. You can see this on a cold day boiler flues pumping out clouds of condensation. As well as this, a condensation pipe must be fitted to the boiler to take more unwanted water away. This pipe must be able to drain away.
It can be pumped if necessary. Modern day systems can be sealed or have small water tanks, there are pros and cons to both systems. But a sealed system must have a pressure release valve.
It is also very important to have the system flushed out before the new boiler is installed.
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