Central Heating maintenance without draining down

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The generally accepted wisdom is that if you want to make changes to the pipework of your central heating system or make other significant changes like swapping a radiator, then you need to drain the system down first. However there are times that this is not always desirable. For example you would like to:

  • Keep a system partially in operation while other parts are altered
  • Save time when a drain down and refill is too time consuming
  • Save the cost of replacing lost corrosion inhibitor or anti-freeze
  • Avoid problems refilling a system that tends to airlock

This article will describe some of the ways in which you can carry out work on a system without a full drain down.

Understand the type of system you have

Most domestic CH systems come in one of two forms, traditional open vented systems, and sealed (sometimes called "pressurised") systems. What needs to be done to work on a system successfully without draining down will change depending on its type.

Vented systems

Vented system are characterised by having a small "feed and expansion" tank/cistern at the highest point in the system (traditionally in the loft). The tank will usually be refilled from mains cold water via a ballcock or float valve. There are normally three connections to the cistern. One that connects directly to the lower part or base of the tank; this is the feed and expansion (F&E) pipe), and another that will loop over the top such that the open end of the pipe is directed into the tank. This is the open vent pipe. The final pipe is the overflow pipe, this is normally connected near the top of the tank and will allow excess water to flow out of the house should the F&E tank get too full.

As the system heats up and the water held in it expands, some of it will push back up the feed and expansion pipe, raising the level of water in the tank. Hence the tank should not usually be "full" - it needs to have room to accommodate this expansion.

When the system requires water, either because it is cooling and its water is contracting in volume, or because water needs to flow into the system because a radiator has been bled, then water will feed into the system from the tank. If the water requirement is substantial, then the level in the tank will fall enough that the float valve will refill it from the mains supply. This means that the system can refill itself automatically (and indefinitely) if required.

The vent pipe is a safety system that ensures should the primary water ever boil (say when heated from an uncontrollable source like a back boiler) their is a safe way to release the over pressure and temperature water from the system.

Sealed systems

Sealed systems are as their name suggests - sealed. There is no tank that is open to the atmosphere, The pipework is arranged as a closed loop. In order to allow for expansion and contraction, there is an expansion vessel which contains a rubber bladder full of air with some pressure in it. As the water heats and expands, it pushes into the expansion vessel and compresses the air. As the pressure in the system falls, the air expands and pushes the stored water back into the system.

In addition there will be a "filling loop" - a link pipe with a tap at either end that allows mains fed water to feed into the system (usually with a non return valve to stop stagnant CH water feeding back into the mains!). Lastly there is a pressure gauge that shows the current water pressure in the system.

The keys to success

To work on a system that is still full or partly full of water you need to employ one of a number of tricks to stop it all coming out when you don't want it to. These can be one or more of:

  • Drop the pressure
  • Block the flow of new water (or air) into the system
  • Create a hydraulic lock
  • Isolate part of the system
  • Work above the water line
  • Be ready to work fast
  • Protect and mop up

If a system is under pressure and you cut a pipe or undo a pipe union, then water is going to escape quickly (and probably in your face!). So lowering the pressure will help greatly. With a sealed systems there are two types of pressure in the system - static pressure from the total "head" of water in the pipes, and elastic pressure provided by the expansion vessel.

Dealing with the elastic pressure is simple - if you bleed some water from the system (via the system drain, or by opening a union nut on a radiator for example, you will draw off the stored pressure in the expansion vessel. Once the gas bladder in this has reached maximum size it can push no more. You can see from the system pressure gauge that you only need draw off a few litres of water to dramatically drop the pressure. (note you will not drop it to zero unless the gauge itself is near the top of the system since it will still read the static pressure from the head of water stored above it).

With a vented system there is no elastic pressure, only static.

Block entry of new water or air
A typical kit of rubber bungs for blocking off vented systems. This one is marketed as a "Radvalve change kit" but in reality is good for any work on vented systems without draining down.

Once any elastic pressure is removed, water can only flow out of a system if something can flow into it elsewhere replace the volume that comes out. Without that you have a hydraulic lock that will keep the water captive. Sealed systems will tend to naturally do this - if there are no open bleed valves or other holes in a the pipework, not much water can escape other than by air flowing back up the pipe you are working on.

With vented systems, you will need to first block the F&E pipe and the vent pipe at the header tank (no need to worry about the overflow). Once this is done replacement water or air can't get back into the system. You can buy a kit of conical rubber bungs from a plumbers merchant designed to block these pipes. One you stuff up the vent pipe, the other to reach into the tank of water and stuff the other into the outlet to the F&E pipe (fun if it happens to have a nice layer of sludge and mould growing on the top of it!)

(An alternative is a couple of decent sized fresh carrots!)

Hydraulic lock

So long as you maintain control over new water and air getting into the system, the water already it is is mostly "stuck" there. Just be aware of breaking that lock by opening a bleed valve or cutting into a pipe elsewhere.


If there are valves in the system, you may be able to isolate the part you need to mess with. A typical example would be a radiator - these normally have valves on both inlet and outlet. Shutting them both off will stop any water from the system (other than that which is already in the radiator itself) getting out. So you could change a radiator by shutting off both valves, placing a tray to collect water underneath one of the connections to the bottom of the radiator, and undoing the nut that secures the valve to the radiator. Water will then start to "glug" out of the rad slowly (hydraulic lock again!) You can ease or restrict the exit of water using the bleed valve on the rad - opening it will allow air in and water out. That allows you chance to turn off the water for a bit to empty your collection tray into a larger bucket.

If you can't isolate, then perhaps installing a way of doing so is the first job on the agenda.

Note on rad valves - one end will normally be a "lockshield valve". This is not usually used for manual control, it just sets the maximum flow rate through the radiator. So make a note of the number of turns required to turn it off. Opening it to that number later will keep the rads "balanced" as they were previously.

With Thermostatic valves, not all have a proper "off" setting. It might just go down to "frost protection". That might mean you get an unwelcome surprise if the room you are working on gets cold (say because it is mid winter and someone has turned off the radiator!). So use the "decorating cap" in place of the thermostatic head if you still have it (normally supplied with the rad valve), or remove the valve head, insert a 5p coin under it, and refit it. That will keep the plunger in the valve stem firmly pushed down into the "off" position.

Work above the water line

When working on pipework upstairs, you can do a very partial drain down by isolating all but one of the upstairs rads, and draining down just that one rad and the pipework from the main drain cock. Working in a loft for example will require even less draining down to work on a pump or CH valve installed up there.

Working fast

Planning what you are going to do, and having the parts ready and to hand, will also control the amount of water spilt. Say for example you need to extend of move a radiator tail. With a hydraulic lock on the system, some plastic sheeting and old towels handy, you should be able to cut mostly though a pipe with a pipe slice without loosing much if any water. Have a full bore service valve to hand (compression or push fit), assembled and ready (e.g. compression nut tightened so that it is snugged up to the olive) to be slid onto the pipe.

A quick wiggle on the pipe will snap it off at your cut. Do that and put your thumb over the open end. Grab your fitting and do a quick swap. With push fit - just push it on. With a compression fitting push it down onto the pipe, and hold it down while tightening the nut a bit. It should only take a half turn to stop most of the water. You should be able to do this with loss of less than a cup full of water.

Note on rad tails - in places with a suspended floor the pipe may tend to drop if not fixed to the rad - if there is a danger of the cut end dropping into the floor void, then gently clamp some mole grips onto the pipe near the floor to stop this happening

Protect and Mop

When working on a full system, some water is likely to escape. So you need to plan to deal with this. You might have an unfortunate incident and lose more - so plan for that as well. Beware that CH system water can be contaminated with black iron oxides, that might permanently stain furnishings, carpets and decorations. So you need to protect the area where you are working with plastic sheeting and / or old towels or dust sheets. You need a bucket to empty water into. Have more than you expect to need. A wet'n'dry vacuum can also be great foll collecting spills, or even catching escaping water before it gets far from the pipe.