Your correct Bolt, but you can't plumb the hot water into the central heating system as that's sealed, in this case it sounds like as mentioned the plumber has connected up to the old pipe work ( I do hope he's isolated the tank!) rather than run new, if there is a sink near the boiler that should be first in the circuit, think I'd be having him back to make amends.

As a matter of interest, I was under the impression a combi was what it said combination boiler it has a twin heat exchanger, one for hot water supply one for the heating

if so hes connected just one heat exchanger instead of both ??? correct me if wrong

Yes, the heating will be a closed loop separate from the hot water. The hot water outlet will be connected to a pipe that leads to the various hot water taps. In my house the first stop is the sink hot tap, so that gets hot water quickly. It takes a minute or so for hot water to reach my bath taps. That IMO is the preferred arrangement.

Why not ask the plumber what he did?

There are a few things you need to know about before you select a Combi as opposed to a system boiler. The hot water flow rate through the Combi will depend on mains water pressure. So if you are having a shower and someone flushed the toilet, the pressure reduces and your shower slows down. The better Combis cope well with this up to a point (Worcester etc). But if you live in a larger house with a larger family then a Combi is probably not the best option.

Also when you demand hot water the central heating is cut off, as the Combi gives hot water preference. A diverter inside the boiler will divert the cold water supply around the heat exchanger, this cuts off the supply to the radiators. So a large family, all bathing or showering one after the other for an hour, then your radiators will cool down. Also if the hot water system is empty to enable work on it you cannot use the Combi at all for heating or water so make sure it isolated eclectically and put out of use. Otherwise you risk serious damage and failure.

As for your situation it seems as if the plumber has taken the easiest route and simply re-connected to your existing system from the Combi. He should have altered the pipework to facilitate the shortest and most efficient run instead of taking the supply back to the original hot water connection. I found this when I moved into my present house. I could have had the plumber removed from the gas safe register for what he did, but I took pity. I removed a veritable 'christmas tree' form the loft and made direct connections with new pipework. Pure laziness not to in the first place. The outlet from a Combi is only 15mm pipe, so not difficult to run to the nearest efficient connection point. I would be inclined to ask him back to rectify matters and mention the gas safe register(formerly CORGI) to him. He has to be a member to install gas appliances and if they decide to investigate it could be bad for him. Good luck. Cheers Concrete

I think you are in desperate trouble if you mean these figures, which I assume will be in cubic feet :-) Even then, it seems quite high to me, but may not be unusual.

I have kept weekly records of all our utilities since 2012, for a fairly large bungalow with three bedrooms, two bathrooms and two occupants. Besides central heating we cook by gas, but a power shower reduces hot-water use somewhat. Our average annual gas use is 450-500 cubic feet. Nearly all of that is for heating, as during the summer we often use less than a cubic-foot a week.

I am 100% certain the figures I have quoted are correct. In 2018-2019 we used 1054 m3 which converts to approx 11,804 kWh. Our tariff was 3.359p/kWh which means we spent £396 approx on gas. Our current tariff is 2.512p/kWh thus we should be spend about £99 less this year if our consumption remains the same.

In the summer using gas for hot water and cooking we use about 2.5 m3 a week.

I would be absolutely delighted if we used only 450 - 500 ft3 a year. At our current tariff we would be spending about £4 a year on gas (500 ft3 = approx 14.2 m3 x 11.2 to covert to kWh = 160 kWh x .02512 = £4.109). If you are actually using 500 m3 your bill would still only be £142 a year.

Considering the average gas consumption for a UK house is 12,000 kWh per annun (using the official conversion of 11.2 kWh/kWh that is 1071 m3)I would say that my figure of 11,804 for last year looks spot on. You need to re-look at your figures which are obviously way out and re post the correct ones.

I think your plumber may have been a bit disingenuous in his comment about heat going upstairs. Of course it will do that, but the essential thing is that it doesn't leak out through the walls or the roof. Many years ago I doubled the insulation in most of our loft. I have no proof of how much difference that made, but the loft can be pretty cold in winter and hot in summer, while the house remains at a comfortable temperature.

When we bought the property it had about 2" of ancient loft insulation thus there was plenty of lost heat. The cavity walls had been insulated in the early 1970's but what use that is now is debatable. The pipework for the existing heating system ran beneath the timber floor with virtually no insulation. The new loft conversion meets the current building regs for insulation and its noticeably warmer upstairs than it is downstairs, need to get the cavities checked one day. The loft insulation is now 6" thick. The new heating pipework is mostly located within the internal walls with little pipework under the floors and what there is has modern insulation. In reality the upstairs radiators only really work first thing in the morning, the thermostatic valves soon close down as the heat is not lost and some is gained from pipework between the floors/ceiling.

Does the main pipe get smaller as it gets to its last branch on the existing layout? If so, you may find the pressure drop when reversing the flow direction to be rather high, possibly starving outlets at the new end of the system if the outlet(s) nearer to the boiler are in use.

If not, then I don't see why not, unless my brain has gone to sleep (I am in need of food, so it could've).

Good point Engineer Andy. Will check. Thank you.

The plumber should not have connected the 15mm hot water outlet from the Combi into a larger 22mm pipe. This is so inefficient and very bad practice. He should run the outlet to a part of the system where the connection is all 15mm and it provides a good supply to all hot water taps and outlets. That should not be too difficult to determine. I could do it in 10 minutes. Sounds like you need to recall this 'plumber'. He may be a plumber on his mothers side!!!

As for gas consumption, a Combi is generally rated above 90% efficiency for fuel burning. This alongside no hot water storage should be very efficient. Like everything else in this world the human factor counts. The Combi boiler is fairly fool proof, until you get a clever fool. They are factory set and need little or no burner maintenance. They are electronically controlled and operated once powered up. Only the odd clean out needed. Modern flue fans do not need regular lubrication any longer either. Pretty fool proof. It all depends on the installer for the rest of it. Give a good car to a bad mechanic etc etc.!!!!!

Cheers Concrete

It depends on what the local mains pressure is and if you have a large DWH load. Don't forget that the legal minimum for mains pressure is only 1bar at the meter (some areas have very low pressure as its easier and cheaper for the water company to reduce mains pressures to the legal minimum than to repair leaks in pipes), and an older home may well have a long pipework system to go to and from a HW cylinder with lots of bends and whose pipes have been there for decades, reducing in effective bore due to scaling, etc.

Additionally, if the home has a bath instead of a shower and/or (like mine) a hot fill washing machine, the DHW load is quite a bit higher than a very modern set-up. As such, all the above can have a large impact under the 'right' circumstances on the pressure drop. Not helped if a shower is a modern 'drench' unit requiring significant flow and pressure (over a standard unit) to work properly.

I'm not saying definitively this is the case here, but without all the details, we can't be certain either way.

Just spotted Concretes new post above where he says

The plumber should not have connected the 15mm hot water outlet from the Combi into a larger 22mm pipe.

I have read all the OP's posts again and simply cannot find where he has said that the plumber has done this. However I have spotted that the OP says in the original post

But the hot water is pumped round the rest of the system before arriving at the sink

and this is obviously not correct. The domestic hot water from the Combi Boiler is not pumped, it is simply a gravity flow.

Andrew T said above

But although our place was first built in 1970 and extended about 1980, the meter itself dates from 1989. No idea why it would have been changed then, as the combi boiler went in ten years after that, the only serious alteration I can trace.

At our old house we had gas installed in 1997 when we swapped from solid fuel heating to gas. The meter was swapped once between that date and 2017 when we sold the house simply because it was the policy of the transporter to swap them, never complained, nothing unusual with the readings.