What is the difference between the cold feed & open vent

Ever wondered why there is a cold feed & open vent also know as the feed & expansion? In the picture to the right of you is an example of an open vent (expansion pipe). To the right of the picture is an example of no open vent also known as the expansion pipe. Just below, Martin Smith an experience emergency plumber explains in more detail.

9 Mysterious questions about the feed & expansion pipe you may never of known

• Can I have a cold feed as my expansion pipe?
• Why do we need an expansion pipe?
• Why is the expansion pipe in 22mm pipe?
• What happens if the feed is turned off and there is no expansion pipe?
• Why is there a service valve on the cold feed pipe?
• When the central heating water is getting too hot, what cools it down?
• Do we have to have a service valve on the cold feed to fill up the central heating system?
• Is it possible there can be a combined feed and expansion pipe?
• Why do we get cold feed and expansion problems?

Can I have a cold feed as my expansion pipe

If you had a cold feed used also as the expansion pipe, it doesn’t mean that the central heating won’t work, it just means that you are more open to problems, with a combined cold feed & open vent, where the central heating system isn’t pacifically designed for this. The problems can be small, but feed & expansion pipe combined can also be very dangerous, which is the reason, when you working with the cold feed & expansion it is done in such a way, it is made safe to use at present and in the future.

Why do we need an expansion pipe

The reason for the expansion pipe is, when the water expands, it need somewhere to expand to, so the job of the expansion pipe is to take the expanding water.

Why is the expansion pipe in 22mm pipe

There are a couple of reasons why the expansion pipe is at 22mm rather than 15mm. The first reason why the expansion pipe is at 22mm is because, it is more likely not to block up if the rest of the central heating is at 22mm circuit. The second reason that the 22mm pipe is bigger than the cold feed 15mm pipe is because, the flow is not as restricted and easier to let the water expanding to find the expansion vent pipe. The is only a typical example of domestic plumbing in a 1 to 5 bedroom home or maybe more if the home is to a standard central heating system.

What happens if the feed is turned off and there is no expansion pipe

If the feed was turned off and there is no expansion pipe, when the water heats up to it’s maximum, it is in serious danger of exploding, as pressure will get higher and higher.

Why is there a service valve on the cold feed pipe

There are some emergency plumbers that don’t believe in fitting a service valve due to extra dangers, but is handy for any maintenance required. If a service valve is fitted, also known as a gate valve, it must be in such away, that it can’t be turned off by the occupants and only a central heating engineer. This is why you will find on some service valves, no handle, but another unusual fitting over the valve, so that a handle can not be fitted.

When the central heating water is getting too hot, what cools it down

When the central heating water is getting too hot, then the cold water feed in the central heating tank will cool it down, how this works is, the expanding water will pass through the open vent expansion pipe and the feed then draws the cold water from the central heating header tank. This may only happen for such a period as the water in the central heating tank will get warmer. This is one of the reason why an emergency plumber doesn’t like a service valve fitted as a safety precaution to the water over heating and drying out.

Do we have to have a service valve on the cold feed to fill up the central heating system

There is no reason that we have to have a service valve on the central heating cold feed pipe, however, if there was a service valve that only the heating engineer can operate on the cold feed, then this would make it easier to work on the central heating maintenance.

Is it possible there can be a combined feed and expansion pipe

Yes it is possible to have a combines feed & expansion, however, the central heating has to be designed specially, so that it can take a combined cold feed & open vent. Due to the extra cost involved, for this reason it makes more sense, to have a separate feed and expansion.

Why does the hot water expand through the expansion pipe and not the feed

This is due to the weight in the water of the central heating tank versus the weight of the water in the expansion pipe. Because the weight is much lighter in the expansion pipe, the water then expands through the expansion rather than the cold feed. As the water is much heavier in the central heating tank, when getting the supply of water into the central heating system, it then draws the heavier sauce off water supply.

Why do we get cold feed and expansion problems

The reason why we get cold feed and expansion problems, is because the central heating system is fitted by a basic plumber, that doesn’t have an understanding for how and why a central heating system is fitted in such away. Due to the lack of understanding, there then becomes central heating problems, costing the owner of the home more money.

If you would like an emergency plumber to deal with your cold feed & open vent problems or known as feed and expansion, then feel free to give us a call and we will recommend an engineer to visit your home. If your an emergency plumber, that has experience with the F&E, feed and expansion, then please have a look at the plumbing register. If you feel you can add to our cold feed and open vent or disagree with what we say, then please send us an email to emergencyplumber@me.com