I have recently had a combination boiler fitted to my new build house. It appears the circular hole cut through the bricks and mortar for the flue pipe was cut way too big. The circle is probably 2 to 3 inches bigger than the flue and I can see outdoors through it! Is this normal and acceptable or should I challenge the plumber about it? Is it his job to make it good and what is the correct way to do it?

a thimble must be cemented into a tight hole in the chimney..

Probably not into a chimney packy. Anyway, it is the installers responsibility to vent the unit according to the I&O manual. In boiler manuals the venting section is the largest sections of the manuals. If not done correctly it is what can kill people.

Are you saying that rain, insects and other stuff can come inside the home due to the space around the flue?

It's not the chimney. I have a combination boiler and its vented out through the front wall of my house downstairs. The oiler is in my kitchen and the flue is white plastic and goes through the wall construction to the outside but it was drilled too big.

OK, then it absolutely must be sealed 100% tight.

If it is his intention to leave it that way, it would make me question every thing else that was done on the installation. The unit requires a pipe for combustion air and one for exhaust. Sometimes they use what is called a concentric vent, which is basically a pipe within a pipe. So, there would be a single penetration of the wall to the outside, with a special termination on the outside. His work should be impressive in it's neatness, and reflect the work of a craftsman. If everything looks nice and this is the only problem, give him the benefit of the doubt, and ask him to correct it. If all his work appears shoddy....it's not a good sign.

There is usually a larger rubber collar that fits over the pipe that covers the opening around the hole, normally one inside and one outside.

Here's a couple of ways it is done. One photo is a wood stove but it is a similar idea:









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