Hot water running from vent pipe into header tank

I've just had my old oil-fired boiler (a Worcester Danesmoor 12/14 - 48000 BTU) replaced by a new one (a Grant Multipass 70/90 - set at 80000 BTU) by the guy who normally does my boiler servicing. He's OFTEC registered, and has always seemed knowledgable and reliable, but I now have some concerns.

My house was converted from a 2-bed bungalow to a three-bed house by the previous owner, via an extension on the back and a loft conversion - so it's now about twice its original size. The existing boiler had been retained, but was over 15 years old, and I assume undersized for the enlarged house - hence the larger replacement boiler. Also, all the pipes to the rads are 8mm microbore, though there are no manifolds in the system. It's all controlled via a "Honeywell Y Plan" system - actually an ACL version.

My first concern is that after he'd unpacked the boiler, he noticed that it was labelled "Sealed System" and was therefore the wrong type. However, he then said that he could still use it (basically by removing the expansion tank and bypassing the pressure system). Is this OK, or a bodge?

My next concern is that the spec shows that the input and output pipes should be 28mm - mine are 22mm. Also, although he drained the old system (obviously!), he didn't flush it as the water coming through was pretty clean.

Next up is the real problem. The system gurgles a lot, and I have had to do a lot of radiator bleeding. Then today I realised that, with the system running, the bleed valves on the upstairs radiators were actually sucking air in rather than blowing it out. A trip up to the loft revelaed that hot water was constantly pouring from the vent pipe into the header tank.

Because the new boiler had an integral pump, he disconnected the existing pump in the loft. Now I suspect this is where the problem lies. The output pipe goes up to the loft, and the vent pipe runs off it, and then the main pipe goes on to the motorised diverter valve. The existing pump had been fitted to the other (non-boiler) side of the vent pipe. Now, by using the internal pump, the pump is on the boiler side of the vent pipe. I'm by no means an expert on central heating systems (but I'm learning fast!), but it seems to me that pumped water facing a choice between passing up a 22mm vent pipe or going round the system would probably prefer the former. Most of the rads are working - apart from two new ones he installed. However, hot water still goes through the vent pipe even when on hot water only.

By reducing the pump speed from the no 3 setting to the No 2, the flow of hot water through the vent pipe is still present, but reduced. This seems to support my feeling that the pump should be the other side of the vent pipe. However, if this is true, then for him to have consciously created this situation is causing me to wonder whether he knows what he's doing. And hence I'm starting to get concerned about the other points mentioned above too.

One other detail - he had to replace the motorised valve as the existing one was seized.

Finally, the installation guide for the old boiler says that there is no requirement for a system bypass - and there appears to be none fitted. The guide for the new boiler doesn't mention this, one way or the other. All the rads have TRVs - should there be at least one without to provide the bypass function?

Sorry for such a long (first) post - but I would really appreciate some expert comments on this to either put my mind at rest, or to prepare my arguments for when he returns to complete the boiler commissioning and documentation.

Many thanks.

The pump on most sealed system boilers is on the return side, so this will cause negative pressures around most of the system when the pump is running.

The flow pipe from the boiler (outflow from boiler) should go directly to the 3-port valve, but in your case it will have the cold fill & vent just before it. You'll have to transfer them to the return side, last connection before the boiler return.

Why not convert it to a sealed system and get the benefits of what you've paid for, plus you'll lose the expansion tank?

As for the 22mm pipework, it is not right, but neither is it a show stopper. For outputs greater than 60,000 btu/h the velocity in the pipes will exceed that recommended by British Standards, so it will be noisier. BUT if you have microbore you'll probably struggle to shift more than 60,000 btu/h through the boiler and your loading is probably more like 50,000. So set the output to minimum at commissioning time (70,000) and don't worry.

Finally, give the installer a chance to put things right.

Many thanks for the speedy reply. The sequence is indeed 22mm vent, then 15mm feed from the bottom of the header tank, then the 22mm output pipe goes on to the 3-port valve. Your reply (inevitably, I guess!) raises a few other questions.

1. Should a qualified and experienced installer not have realised this in the first place?

2. Is it safe to have the vent on the return side? It's just that I assumed the vent was there in case of a blockage somewhere in the system, thus providing a safety outlet.

3. Re the sealed system - he did say this was an option, but the downside is that, as a result of the loft conversion, all the existing CH pipes are now buried under the floor. He pointed out that, as the sealed system would be pressurised, it's possible it would expose weak points in the existing (15 year old plus) microbore pipework, which would be a nightmare to replace if it were to spring a leak or two. How big a risk do you think there is of this happening? That's the $64,000 question I guess!

The installer's a nice guy - it's point 1 above that concerns me, and has dented my confidence in him - but hopefully it's unfounded. It's always good to have second opinions in these situations.

One other strange thing. As I mentioned, he also installed two new radiators. There were two existing (i.e. output and return) 15mm pipes just after the 3-port valve, which were blanked off, but had two 8mm pipes running directly to a rad in the hall downstairs. He removed the blanks and ran 15mm pipes off to the new rads (about a 20m run), going down to 8mm just before the rads. The existing rad still works fine, but almost no heat gets through to the new rads, even after hours of running. There's no air in these rads when bled. Does this point to an airlock in the new pipework? These rads are actually now the lowest in the house, but only by about 6".

Sorry to ask more questions - but I'm learning fast!

Thanks again for your reply.

Trying to deal with your points as follows:-

1. Yes, though there are 2 sides to every story, so lets not bury the poor guy in his absence.

2. IMPORTANT the boiler MUST have the pressure relief valve piped to a safe place outside; forgot this one earlier but as the boiler will have isolating valves there must be a way of discharging excess energy in case valves are shut and boiler interlocks fail (or it becomes a bomb ) In this case you can have the vent on the rtn but I repeat it is better you convert to a sealed system.

3. If it fails at about 1.5 bar then it hasn't long to live anyway. A false argument, it's no more likely to leak now than in 1 day/week/month. The lack of corrosion will help the pipework last longer as well.
BTW there must be connections between main pipes & microbore, normally by manifolds. What do you have?

4. I'd say the previous installer tried to make a system boiler work in open vented arrangement. He made a couple of mistakes (alledgedly). Judge him by what he does to put it right.

5. Can't comment on rads performance based on your description, though the flowrate depends on the pressure difference (head) between the flow and return. If they're now effectively on the same leg there'll be very little head, and no proper flow. If you think it's air try discharging each supply pipe into a bucket to displace any bubbles.

Thanks again "Meldrew's Mate". The plot thickens - I took the top panel off the boiler and checked which pipe the pump was connected to. The pump is at the top of the boiler, and is connected to the output (flow) pipe rather than the return pipe. Hmmm.

Re the manifolds or lack of - all the pipework feeding the rads is buried under the loft conversion - bar one. This is the hall rad I referred to earlier. With this one, there's a pair of 15mm pipes, with the 8mm pipes branching off them at 90 degrees via "step down" 15-8mm T-pieces. I assumed that the rest of the system would be the same, but as I can't get access to it, I guess other rads may be connected via manifolds.

Thanks again for your very helpful replies. I'm going to phone the engineer tomorrow and ask him to bring back the internal expansion vessel he removed and convert to a sealed system.

The engineer returned yesterday, and he converted it to a sealed system. I must admit that it was a very unnerving experience hearing the 15 year old pipes creaking upstairs as he temporarily put the pressure up to 2.5 bar, at which point the pressure relief valve in the boiler operated. 24 hours later, the pressure is steady at around 1 bar, there are no leaks or airlocks, and all the rads are working. A very happy customer!

Thanks again for your help and advice here.