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imported_bobc 04-11-2005 04:55 AM

Replacing a self priming hot water tank
I have a self priming hot water tank which I want to replace with a indirect hot water tank. The water is heated by a back boiler.
I know that as well as the new HW tank I will have to install a header tank with a return expansion overflow pipe to it but my questions are these.
Are there conventions as to where in the central heating circuit the header feeder pipe must be connected and like wise with where the CH expansion overflow pipe must be connected.
Any advice would be very welcomed.

dirkgently 04-11-2005 10:19 AM

No particular conventions but the cold feed is often taken to the boiler and the open vent off the top of the primary circuit at the cylinder. (This is assuming you are using a gravity circuit and not pumped).

imported_bobc 04-11-2005 10:52 AM

The CH system is pumped so what then?
for info, The hot water tank is down stairs (so is the bathroom and wc) and so the pump gets the CH hot water upstairs.
the new tank will stay downstairs.
The present cold feed and expansion pipe will be connected to the new tank and so I'm assuming that the CH system will need it's own cold top up feed and expansion pipe.
Just not sure what the best/safest point in the CH system for the connection points is.

dirkgently 04-12-2005 02:30 PM

Open Vent and Cold Feed Position

The vent pipe is a safety device fitted to the central heating system. It enables any steam to be removed quickly from the heat exchanger should the boiler controls fail. It also removes air from the system but this is not its primary purpose. The vent pipe should be a minimum of 22 mm diameter and should rise continuously from the discharge above the feed and expansion cistern. There should be no valves, either manual or automatic fitted in the route of the vent pipe.

The position of the open vent and cold feed is a critical part of an open central heating installation.With the introduction of more powerful pumps, and the increase in resistance in central heating circuits, due to high resistance boilers and radiators, the correct positioning of these pipes is even more critical.

It is important to avoid pressures below atmospheric which may draw in air through the open vent pipe, or small leaks in the system, such as through valve packing which may be watertight but not airtight.

It is also important to avoid a rise in water level in the vent pipe such that water is forced over into the feed and expansion cistern thus aerating the water in the system.The ingress of air will eventually cause the corrosion process to accelerate, leading to sludge being formed and subsequent blockages within the system or rusting of radiators. The pump bearings may also 'run' dry which will ultimately lead to early pump failure.

The boiler manufacturer's installation instructions will often give advice on the correct pipework configuration, and this advice should be followed.
Some manufacturers recommend a configuration where both the cold feed and open
vent are taken from the flow pipe between the boiler and pump position. This is
normally referred to as a close coupled system (Fig. 9).

Fig. 9 Close coupled system

With this type of layout the connection points of the feed and vent pipes should not be more than 150 mm (6 in) apart and the design of the system should be such that there is always a cold water feed path to the boiler's return connection when all automatic valves (if fitted) are closed. Some manufacturers also recommend a combined feed and vent pipe for their boilers.

imported_bobc 04-15-2005 09:11 AM

thanks for your last post and the diagram, both are very useful fo me.
last question, does it make any difference that the boiler is only a 45 to 50k btu back boiler?
many thanks

dirkgently 04-19-2005 12:45 PM

If the back boiler was able to cope with the heating load before then there should be no problem.

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