Advice on Heating - Feed and Vent Locations (bit long)

[TheScullster]

Hi all

I am currently trying to "optimise" the heating system in 1970s house prior to install of new boiler and general upgrade. For regulars, my MagnaClean reports may be familiar. The system has been prone to pump over which may have been caused by: Separation distance between feed and vent pipes (see later) Only 15mm feed to upstairs rads - combined with...... Fitting of thermostatic valves to all downstairs rads Plenty of crud in system

So my question concerns the location of make up water feed and vent pipes. The separation distance between these two connections depends on whether the dhw zone valve is open, as the vent is on the feed to the cylinder with the make up water on the return from it. The shematic

shows the existing pipe layout, with main items in roughly the right place vertically IYSWIM.

To resolve the separation between feed and vent, can I simply tee the make up feed into the vent above the tee off to the hot water cylinder?

AIUI it used to be common practice to have a combined feed/vent. My version would be similar, but would have the feed connection about 1.5m lower (distance between F/E tank outlet and intended connection point on vent).

It has been suggested on this group before, that, if you have combined feed/vent, the boiler must be capable of operating under sealed system pressure. Don't understand this! The resistance to venting appears the same, as the boiler pressure has to overcome the same column of water in the vent pipe wherever the water make up connects in the system.

Anyone know enough to clarify all this please?

TIA

Phil

[Andy]

If you have combined cold feed and vent on your system (if im reading your plan correctly). The expansion from the bioler would have to pass through the pump, and if the zone valves are closed then the rads. The vent needs to follow a unobstucted route to the f and e tank. If the pump fails or there is a blockage the boiler sometimes vents very voilently.

It used to be common practice in the past to have combined cold feed and vent, The usual set up now is flow from the boiler to an air seperater or a "H" then to the pump, auto bypass and then the zone valves. What this does is reduce the pressure at the vent and cold feed, stopping pumping over at the vent.

Have a look at this

paits a thousand words and all that.

Andy

[TheScullster]

"Andy" wrote

Thanks Andy, comments included

The current scheme does not have combined feed/vent. As the system is pumped return, the flow side is unobstructed and vents beyond the tee for the hot water cylinder.

The make up feed currently connects to the hw cylinder coil return. I was considering moving this to connect roughly where the air separator is shown on the diagram you refer to. But I'm not sure of the implications.

Phil

[Dave Plowman (News)]

My system has 28mm from the boiler to the pump and three way valve, and later reduces to 22mm where it splits for upstairs and downstairs.

Both boiler flow and return are close together at the pump, and if you imagine an 'H layout of the pipes above the pump, with the bottom two legs of the H being the 28mm boiler flow and return, the 15mm feed going to the top leg of the H above the pump and the 22mm expansion pipe to the other top leg of the H. The horizontal part of the H is a 28mm bypass.

I've never really gone into the theory of water flow when it's pumped, but this arrangement - which I was told was the best (at that time) by a very experienced central heating installer - has never caused any problems with pumping over - despite it being a three story house with TRVs on all rads except in the living room where the stat is sited.

So I wonder if the lack of a bypass circuit is at least one of your problems?

[TheScullster]

"Dave Plowman wrote

Thanks Dave

Where-abouts in your house (elevationally) are the boiler, pump and zone valves located?

Phil

[Dave Plowman (News)]

They're all in the 'mezzanine' level bathroom (rear addition). The boiler at one side of the room and the tank, pump and valve at the other. Both feed and expansion have a fairly tortuous route to the header tank in the roof void at the top of the main building

[Ed Sirett]

I suggest that you make the new system unvented then you won't have to start moving the layout around. The current layout probably arose because the a valve was added to make the system fully pumped with control of the HW. See the SealedCH FAQ. and the Boiler Choice FAQ.

[TheScullster]

"Ed Sirett" wrote

Thanks Ed

Yes I have seen a number of posts recommending this.

My reservations are:

The existing system is old and we have already had to repair an underfloor leak (granted this was on the domestic hot water side rather than heating, but it still makes for loads of hassle in solid floors). In a recent(ish) extension, plastic non-barrier pipe has been used to supply

2 rads. Being a bit old-school, I am rather nervous about using this stuff at heating temperatures and elevated pressure. Getting to the pipe work for these to replace in copper would be a nightmare - tiled floors etc.

Phil

[Ed Sirett]

Well if you really want to stay with a conventional system then you need to make sure the pipe work is properly laid out. [1] You need to find a boiler which is compatible with open vented (still plenty to choose from ATM but the trend is away form open vented primaries).

The plastic pipe is certified for 3bar @ 80C (realistic operation at 1.5 bar @75C). The barrier/non-barrier issue is not that great and can be dealt with by changing the inhibitor on a regular basis. The copper pipe is more vulnerable to corrosion from the outside than the inside. If the outside is dry then there is no problem with the pressure.

The customer is however always right in these matters.

[1] That's Boiler flow, then either 22+mm vent+flow or feed just before vent or air seperator then pump then zone valves. then coil/rads then boiler return. .