Advice needed for disconnected back boiler

PK27

Hi, I moved into a new house 6 months ago and last week I decided to light the fire. Chimney seems perfect, good draught coming down, and reasonably clean. I lit a very small fire to start off just to be safe. Within 2 minutes, smoke started to enter the sitting room. At the side of the fireplace is a removable wooden box where the pump for the back boiler used to be. At the moment, a lot of the new gas fired central heating pipes are running through this box to the downstairs rads and up the side of the chimney breast to upstairs. The old inch pipes from the back boiler can be clearly seen and are disconnected and dormant. The smoke was getting into the back of the hearth and coming out the side of the chimney breast into the wooden box. Not a huge amount, just a wisp but obviously it's a problem.
So after some investigation, I figured out what the problem was.....in the fireplace, there are the three cast iron "elements" that carried the heat off the fire and transferred it to the back boiler. Problem is, someone had a go at them with an angle grinder and cut the front off them so you can see inside. I assume they were starting to cut them away to open the fireplace visually and probably gave up as it would take a number of runs to cut them back bit by bit.
So now that they are "open" , when I look inside them I can see two holes in each one at the back, about 12mm diameter. I assume these are vents to prevent the cast iron exploding if the air was trapped inside. So with the "elements" as they were when the back boiler was in service the smoke couldn't access these breather holes as the front of the elements was obviously still intact, but now they've been cut open the smoke is being drawn in, and the smoke enters the sitting room and also there was a strong smell of smoke in the bedroom that the chimney rises through.
My question is, can I pack fire cement into the elements to seal them back up, they are about 8" deep, or should I grind them back as much as possible and then just fill the remainder with fire cement to seal the breather holes? Or should I stay well away and get an expert to look at it?
Thanks in advance and sorry for the long post just wanted to get the details clear in the hope one of you lovely people might know what I need to do!

Billy Bunting

plumbing and heating forum would be more productive

PK27

Billy Bunting wrote: »

plumbing and heating forum would be more productive

You're probably right cheers