New Combi boiler

I want to fit a new combi boiler for my office, and I want to do as much work as possible myself, just getting the final commissioning done by a Corgi.
There is a (condemned) floor standing boiler at present, and I want to replace this with a wall-hung combi immediately above, to minimise the pipework changes. Hot water is currently from an electric water heater which will be discarded.
The current boiler is about 20 KW and heating is adequate. New combi needs only to supply a sink for washing up, but I did think of adding a shower cubicle some time in the future. Office is ground floor with a handful of rooms. Cellar underneath.
I originally thought of a Potterton 24, but I am now more inclined to a Vaillant TurboMax Plus 28, even though this is 25% more expensive. Obviously, this is overkill for the heating, but I want to leave a bit in reserve for the shower if it ever gets fitted. Anybody any suggestions/comments, please?
One reason for the Vaillant is that it takes a 15mm gas supply whereas the Potterton takes 22mm. The boiler is quite close to the meter but the gas pipe is only 15mm. Does it matter what the inlet pipe to the boiler is? I was told by a fitter that I needed a new 22mm pipe for a combi, but he never bothered to look where the meter was and how close it is. The max gas flow needed is about 3.5cubic metres/hr for the Potterton and I assume it must be similar for the Vaillant, so I am a bit surprised that the Vaillant takes a 15 mm gas pipe. Looking at the cda calculator, the 15 mm pipe would need a pressure differential of 2mbar to deliver this flow over a length of 6m, whereas only 1 mbar is acceptable. Why is the acceptable pressure drop so low? The burners operate at between 12 mbar and 2 mbar. If the pressure at the meter is 20 mbar, at peak flow, the pressure at the boiler reduces to 18 mbar. Why is that not okay? Do I definitely need to rerun the pipework in 22mm?
The present system is gravity fed with a pressure of about 0.25 bar. I would be right in thinking I need to pressure test the existing pipe-work? What is the best way of doing this?
I want to flush out the old pipework with Fernox sludge remover/cleaner. Am I right in thinking that I might as well do that with the old boiler in place? That avoids washing all the old crud through the new boiler. Do I need to flush out again with the new boiler fitted, as well? The Fernox is about 15 quid a bottle, so I don't want to (literally) throw money down the drain.
What is the best way of getting inhibitor into the new sealed system? Do I just use the filling loop? Presumably BEFORE pressuring the system with water? And can you just pour it in with a funnel?
Fitting the new flue sounds interesting. The boiler is mounted on an exterior wall, so a core cutter bit should do the trick, mounted on a big drill. How do you fill in around the flue once it is fitted? Just sand and cement?
Sorry to ask so many questions, but I want to get this completely clear in my mind before starting.
Thanks
Geoff
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The Vaillant is a good boiler. Get the smallest combi you can and run a 22mm gas pipe right up to it.

yes.
There are special kits to do this. But you could connect up the combi, pressurise and look around the system for leaks. Any leaks fix them.

yes.
No need to use fernox again, just a flush out with fresh water.

Install a filling point of 22mm pipe. Have a compression cap on the end and use a funnel.

Yes. Using the right sized drill will mean the flue will fit snugly.
The blow-off pipe should be taken to outside at low level, with an elbow pointing to the wall. It should be copper.

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On Sun, 7 Sep 2003 12:52:18 +0100, "GB"

How about a condensing version such as the EcoMax?

The issue is the flow rate that the boiler requires and the pressure drop allowed. As you deduced from the CDA site, these can easily be worked out.
It is quite common, for convenience to run the last metre or so of pipe at the boiler in 15mm even if most of the run is 22mm. It's the overall behaviour that matters. Often boilers are fitted with an isolating cock with 1/2" BSP inlet and this goes more naturally with 15mm tube for a short run.

So he's flying by the seat of his pants and assuming characteristics of the installation. It depends on the required gas rate and the run length of pipe.

The CDA site information is based on the British Standard for doing this. From what you are saying, you probably do need to run in 22mm apart from possibly the last short piece to the boiler.

It wouldn't hurt, although remember that you are only going up to 3bar maximum which is the same or less than the cold water mains.
The most likely problems are going to be that the radiator valves will weep at the seals. I had one which did this and decided for the sake of completeness to change all of them. The easiest test is simply to do the conversion to sealed operation, capping off the boiler connections and check for pressure loss on the gauge. If you think that there might be problems, run the system after installing for a few weeks before adding the inhibitor in case you need to drain and fix.

That's what I did a couple of weeks before ripping out the old boiler.

It's a good idea to put a strainer on the return to the boiler anyway.

I did an initial flush round with remover and the old boiler, then switched boilers putting lever ball valves on either side of it (there is a pressure relief valve in the boiler for safety). I added in the sealed system components and then using the filling loop flushed at each radiator. THis drives the crud out at each radiator point and I flushed through both valves and took the radiator outside for a good pressure washing. I filled and used sludge remover again before finally flushing and filling with added inhibitor. Possibly the second go with remover isn't necessary but 15 in the context of the system is nothing.

You can get sludge remover and inhibitor chemicals from Fernox in gel form which can be injected through a radiator vent - I use an empty one, not a full one as the instructions suggest. Alternatively you can put in a chemical introduction point by using a valve and short piece of vertical pipe and then a funnel to introduce chemicals.

Generally unless the manufacturer says otherwise.

.andy
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don't mean to hijack the thread - just a quick question:
is BES part no 7533 ok (22 mm x 22 mm compression Y in-line strainer) and will it be ok fitted vertically below the boiler?
i know that was 2 qns - sorry!
thx paul
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On Sun, 7 Sep 2003 18:09:12 +0100, "Paul"

You're very welcome.

Yes it is.

You could, but I don't think that it's the best option.
If you look at the diagram of this fitting on the web site, the flow of water through the fitting is from right to left. I mounted mine horizontally as shown and then used very gentle bends made with a pipe bender to change direction vertically into the boiler.
I have a lever ball valve on the input side of the strainer, then I go through the boiler (which has a pressure relief valve) and there is another lever ball valve on the flow. The filling loop is elsewhere on the system. This arrangement means that I can turn off the boiler, close both valves and depressurise the boiler only by undoing the cap on the strainer. Any bits of crud in the system, which tends to be the odd piece of copper swarf gets trapped in the strainer which is a cylindrical stainless steel mesh behind the cap. Because of the angle of the cap facing downwards, once crud reaches the strainer it falls into the mesh. So all I have to do is undo and rinse it.
You could put the fitting vertically and it will work in the sense that it will stop bits going into the boiler (this is the main point) but the cap will be facing upwards. This means that when you undo it, some of the bits may well fall back down the return pipe away from the boiler. I don't think that this is a huge deal but it does make it that bit harder to get rid of all the bits.
Having said that, if putting the strainer vertically is the only realistic option then I think it's worth doing rather than not. If you put in isolating valves as I did, then having a pressure relief valve on the boiler side is a good idea, in case the boiler is inadvertently fired up with the valves closed.

No extra charge :-)

.andy
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Thanks andy. It's pretty easy for me to mount this horizontally so that's what i'll do.

you've kindly described your system for me before and I've adopted some bits.
i think it's also now wise to stick those valves on the boiler F & R - it gives some good flexibility when i'm considering adding some old cast iron radiators.

cheers
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Thanks IMM and Andy for those very helpful thoughts.
I will have a look at the condensing boiler, but I was put off a bit by the price and the extra complication. Do the govt. still give a 200 contribution?
Geoff
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On Sun, 7 Sep 2003 18:26:38 +0100, "GB"

I'm not sure that the Big Green Boiler Scheme is running any more, but if you compare even the SEDBUK efficiency of the two Vaillants they are 79% vs 91%. There are arguments for and against the measurement method used for this which is supposed to reflect conditions in a UK house. Another approach looks at the appliance efficiency directly under different conditions - this will also give a 10-15% difference between modern conventional and condensing technologies. Don't be fooled by more extravagant claims regarding condensing boilers. The figures are often stated in comparison with much older products that had 65% efficiency or even less.
I replaced one of these old 65% efficient products about a year ago and replaced it with a condensing model and the savings have been pretty much as the figures would suggest.
The conventional boilers have also improved anyway, but especially because 78% is now the minimum allowed.
A Turbomax Plus 828 at Discounted Heating is 569, an EcoMax 828 is 928 - a difference of 359. If you amortise this across the boiler lifetime (let's be conservative and use 15 years) then you need to save 24 per annum in gas to make it worth it.
If you take your existing gas bill and assume that the old boiler was probably about 60% efficient, you can work out whether this pays off. I suspect it will.
In terms of complexity, nowadays there is not a huge difference between non condensing and condensing system models. The main additions are the condensate drainage arrangement which consists of a plastic overflow pipe in most cases going to a suitable drain. Some of the early UK manufactured condensing products did have reliability problems, although this was largely due to poor design and choice of materials - the condensate is acidic and this has implications on the choices of materials. German and Dutch manufacturers have been making them for over 15 years and they are quite common in those countries. I don't consider that this is a factor if you are buying a good quality product like Vaillant. You can pretty much focus on the energy cost difference.
.andy
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