Central Heating Layout

[Matt Beard]

I am planning to install CH in my house, however there is a huge concrete floor between the only logical place for the boiler (will probably be a Merlin Series 2000 oil boiler) and the hot water cylinder / radiators. Because about half the ground floor is concrete I am planning to run all the heating pipes upstairs and drop down to radiators.

The problem is that with the boiler a long way off downstairs (and the pump too hopefully to prevent too much noise) how should I manage the venting etc.

As far as I can tell I would need to have a vent pipe on the boiler side of the pump for safety, but to ensure that air is extracted I would also need either an automatic air vent (AAV) or a deaerator (such as a Myson Aerjec).

My first stab at a plan can be found at

I am concerned about the vent pipe - I guess that being as long as it is there is no chance of the pump sucking in air, but it's a very long run that I am not sure if I need.

Any comments?

[Mr Fizzion]

I can't imagine many people here recommending a gravity system, but let's see. :-)

If there is nothing installed already, surely a big combi isn't much more expensive and easier?

Mr F.

[John Rumm]

If I read you symbol index correctly you have the vent shown in 15mm pipe. 22mm is more normal for this to eliminate any possibility of it getting blocked with scale etc.

Having said that, this would seem like an ideal candidate for being installed as a sealed system if the boiler can support operation like this.

[Matt Beard]

With a mains pressure that struggles to reach 1 bar it's not a goer!

[Matt Beard]

I was wondering - I just didn't want to run 8m or so of 22mm if I could get away with 15mm!

I'd rather avoid going sealed - but maybe that's just me being old-fashioned!

Opinions welcome...

[<me9]

Sealed system? That will solve many problems. No vent pipe.

[Dave Plowman (News)]

Drivel seems to be rather lacking in advice here given he seems to think everyone has 4.5 bar.

I'm in the same position as you here in this part of SW London. My mains pressure is just over 1 bar, and the flow of cold I get to my kitchen mixer via a 15mm main is near identical to the flow to the same mixer from a hot water storage cylinder fed from a header tank.

So for adequate flow showers and bath filling, I've got to rely on large bore pipes from a header tank and storage cylinder. A high rated combi would be a waste of money.

[Doctor Drivel]

Dr Drivel hasn't given any advise on this thread. Boy you are thick!

[John Rumm]

What is it youi have against sealed systems? Given your need to have the main piping run at 1st floor level with drops to the radiators, a sealed system would be better from the point of view of eliminating air locks etc.

[Matt Beard]

I guess that I still regard sealed systems as "newfangled" and best avoided unless needs must. (OMG am I starting to get old already?)

More practically, there are three problems for me with this:

1) The slightly higher pressures makes leaks a little more likely (but then if my soldering is up to spec...)

2) Space is very limited where the boiler is going (only just enough room for the boiler).

3) There seems to be close to nothing useful available on how to design a sealed systems, whereas there is quite a lot of info kicking around for vented systems. (Although maybe not quite enough for my uses, otherwise I wouldn't have to ask questions here!). It seems that the general attitude seems to be "vented central heating? no problem, this is how you do it... sealed system? nah, you need to get a professional in for that"

[John Rumm]

;-) can't answer that!

Well if your mains cold water does not leak, then you could say there is no reason for your heating to either. There is a chance if you were upgrading an existing system that you would need to replace some old radiator valves if they weep after conversion. There is also the possibility that any radiators that are on their last legs would give up also - but in which case it is only a matter of time anyway.

Since you are talking about a new install (which no doubt will be protected by inhibitor from the start and be using new components and valves etc), it does not sound like it will be a problem. Look at the bright side - if it does leak at least there is a limit to how much water can come out!

A sealed system need not take more space round the boiler. ("system" boilers will include all the required components for a sealed system inside their case anyway).

One of the traps you may be falling into is that most sealed system installs they are actually far simpler than conventional vented ones. Hence there is less information available since there is less you need to know.

Have you read Ed's FAQ?

folks will start with a system boiler. This includes the pump, expansion vessel, overheat protection, and possibly even the filling loop. To turn this into a complete heating system you just add pipes, radiators, and a room thermostat.

If you want stored water heated as well, then you would need a cylinder, a stat (and/or flow switches if you want to get clever!), and suitable valve arrangements just as you would on a vented system.

[Matt Beard]

Maybe I'll go for a Worcester Danesmoor system boiler then... there isn't much choice for an oil-fired system boiler lwss than 400mm wide!

Bumps the price up a bit, but I guess it's worth it if I don't have to do all the venting.

I guess I will still need some sort of AAV at the top of the system though.

[Ed Sirett]

The boiler will almost certianly contain one it would be a good idea to add one at the top. In any case a bleed point of some description will be essential at the high points.

[Andy Hall]

I did this conversion a couple of years ago to a system that was 15 years old then but had been well looked after. The only thing that I did in terms of renewal was to change the lockshield valves which had been original and not of particularly good quality with some Pegler Terrier ones. Apart from that - no issue.

You do need to calculate the total water volume of the system from radiator data sheets and amount in pipes. There is a table of numbers for the required size of a pressure vessel to go with the water volume and initial pressure of the vessel.

Note that it is not a problem if you oversize the vessel.

Some boilers have an internal vessel which may be large enough for the system - typically they are. In my case, the capacity was not quite enough so I added a second vessel externally.

One option is to install the vessel in the loft in the space formerly or otherwise occupied by the feed/expansion tank. It needs to be insulated of course, and you need a bleed vent on the pipe near it to let out the air.

If you want a comprehensive heating design book that includes this and a lot of other useful stuff, I can recommend the Domestic Heating Design Guide published by the HVCA

at £19.50

Sealed heating is not an issue if you can plumb competently. Pressurised HW storage systems have a bunch of regulation around them.

[Dave Plowman (News)]

That's the one true statement you've made in years.

[Doctor Drivel]

Boy you are sharp.

[Dave Plowman (News)]

That's the second one.