Boiler & Central Heating FAQs

[FAQ]

(16) HOW TO TOP UP A WORCESTER BOSCH COMBI

Thanks to Bengasman

NOTE: There are two slightly different Keys which WB's use.

To top up a W-B combi when you can’t find a filling loop, chances are you need one of these.




Look up from under the boiler and you will see this. That is how the key is positioned to get it in.
A bit of silicone grease on the o-rings makes inserting them a lot easier. A tiny bit of hand cream will also help.





Now turn the key about 45 degrees clock-wise and it looks like this.



Just to make sure it does not fall out halfway through, tug it with twice the force it took to push in.
Now you can start filling by turning the white knob open, the same direction as you open a normal tap. 3 turns of 360 degrees is fully open.

When the pressure is correct, close the white knob GENTLY and pull the key out.
It is normal when a little bit of water spills out. If it keeps dripping, turn the white knob just a little further.

Remember that plastic is called plastic because it IS plastic. It will deform when closed with too much force and left that way at 60 degrees for a thousand hours.

 

[FAQ]

17) MID POSITION VALVE EXPLANATION AND DIAGNOSTICS
Thanks to D Hailsham

The Y plan uses the mid position valve, so first an explanation how it works. The following diagrams show the various states the valve can be in: HW only (top left); HW and CH (bottom left); CH only (top right); CH satisfied (bottom right)

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How the mid-position valve works

When the valve has no power applied, it is in the "rest" position with Port B (hot water) open. In this situation power is applied via programmer HW ON, HW thermostat Common and Call terminals to the boiler. This is shown in the top left diagram (HW Only).

If CH is required at the same time as HW, voltage is applied to the white wire and the motor (M) runs via S1 (top left). When the valve reaches mid position, switch S1 moves over (bottom left). The motor is now fed via S2, resistor R1 and Diode D. The diode rectifies the AC current so it becomes half wave. The effect of this is to "stall" the motor in mid-position. Resistor R1 limits the current through the motor and consequently the residual magnetism. This ensures that, when CH goes off and 240v is no longer applied to the white wire, the spring in the valve is able to return the valve to its original (port B open) position. The boiler is still supplied via the HW thermostat as in the HW only position. The motor is also supplied via the orange wire and resistor R2 with a lower ac voltage, this also prevents the motor becoming permanently magnetized and ensures the spring return will work.

If HW is now satisfied or turned off, 240v is applied to the grey wire, via either the programmer HW OFF or HW stat SAT terminal (top right), This applies 240v to the motor via switch S1. This is sufficient to move the motor from mid position to fully across (Port A - CH only) open. The boiler is no longer supplied via HW ON and CALL terminals but, as the valve reaches the end of its travel, switch S2 moves over. 240v is now applied via the white wire, S2 and the orange wire to the boiler.

If CH is now satisfied, or turned off, the white wire no longer carries 240v, so the boiler goes off. The grey wire, however still carries 240v, so the motor is held in the Port A position. Switches S1 and S2 both remain in the "switched over position (bottom right). In this situation resistor R2 now performs a second task, it reduces the voltage applied to the boiler via the grey wire to between 50-150V. This is lower than the gas valve or pump requires to operate, so the boiler does not light.

The valve will stay in the CH position until either HW is called or the power is turned off.

Valve problems

Mechanical: valve sticking or valve passing.

The symptoms of this are pipes getting hot when they should not. This means that either the valve is not moving fully over (sticking) or the valve is leaking when fully over.

The way to check this is to remove the actuator (box on top). How this is done varies depending on valve. On some old valves the actuator cannot be removed - the whole valve had to be taken out. In the case of old Honeywell valves (identified by the lack of a small bump on the actuator) it is possible to get a conversion kit (part no 40003918-007); but you have to drain down to install it.

When the actuator has been removed the actuator can be removed, the valve spindle will be visible. This should turn easily - how far depends on type of valve, in the case of the Honeywell is 10-15°. If the valve is sticking, a small amount of silicon lubricant - not WD40 - can be applied to the shaft. If the valve is very stiff a new one will be required. Some manufacturers sell the body separately from the actuator, so this can be installed, provided the actuator is OK. In the case of Honeywell valves, it is possible to get a repair kit consisting of the ball and 'O' rings (part no 272752A/U).

If the valve is not sticking or passing, there may be a problem in the actuator

Checking Actuator

These instructions assume that the Y Plan has been wired as shown in the following "standard" diagram. If your system is wired differently, you will have to make the appropriate changes to terminal numbers. Before assuming the valve is faulty, check that the wiring is correct and all terminals done up.

View media item 31288
Most of the tests can be carried out without a meter, but voltages have been given as well.

Check valve for sticking/passing before doing electrical tests.
The actuator must be attached to the valve.
Carry out tests in sequence given.

CH Only Test (Port A open)

Turn Power OFF at main connection for the system (turning CH and HW OFF at programmer is not sufficient) This resets the valve to rest position
Disconnect white valve wire from terminal T5 and grey wire from terminal T7.
Connect white and grey wires to permanent Live (T1)
Turn Power ON
Valve should move over so port A is open and boiler light (check by feeling that only the pipe from port A gets hot)

If the valve does not move, Switch S1 is not moving over.
If the valve moves but the boiler does not light, switch S2 is not moving over. (check 240V on orange)

HW Only Test (Port B open)

Power OFF (valve returns to rest position)
Turn HW stat to lowest
Disconnect grey and white from permanent Live (T1) and temporarily make safe (e.g connect to spare terminals in the wiring centre)
Disconnect cylinder stat Common from T6 and connect to permanent live (T1)
Power ON
Turn cylinder stat to max.
Boiler should light. (check by feeling that only the pipe from port B gets hot)
If it does not light, the cylinder thermostat is suspect (check for 240V at Common and Call terminals of cylinder stat and at terminal 8 ).

CH and HW Test (Ports A and B open)

Power OFF
Turn cylinder stat to lowest
Disconnect cylinder stat wire from permanent live (T1) and reconnect to original terminal (T6)
Disconnect white wire from temporary location and connect to permanent Live (T1)
Power ON
Valve should move to mid position
Turn Cylinder stat to max
Boiler should light (check that pipes from ports A and B both get hot).
If boiler does not light cylinder stat is faulty
Power OFF
Reconnect white and grey to original terminals (T5 and T7)
Turn HW OFF and CH ON
Set CH stat high
Turn Power ON
Valve should move to Port A open, boiler light and Port A get hot.
Turn CH OFF, so boiler goes out.
Boiler should go out but valve stay in CH only position
If boiler does not go out, check voltage on orange wire.
If orange has 240v, the actuator is faulty
If orange has 50-150v, there is a problem with the boiler

If these tests are passed, the problem is elsewhere.

Testing other components

Cylinder thermostat: disconnect wires from CALL (1) and SAT (2) terminals when checking, this prevents false readings due to back feed.

There should be 240V on Call (1) when stat is turned up and 240v on Sat (2) when stat is turned down

Room thermostat: disconnect wire from Switched Live when checking to prevent false readings due to back feed.

There should be 240v on Sw L (3) when stat is turned up and 0V when it is turned down.

Note: you may find that terminals 1 (live) and 3 (Sw Live) of the thermostat have been swapped over. This is not too important for modern digital thermostats but should be corrected if you have the older mechanical thermostat.

Programmer: Check voltages at CH ON, HW ON and HW OFF terminals when programmer set to appropriate positions. Also check the voltages when the timer is used to switch on/off.

 

[FAQ]

18 ) WATER IS COMING OUT OF BOILER'S "PIPE" Thanks To ChrisR

Combis (and other boilers) have a metal Pressure Relief pipe going out through the wall, sometimes wrongly referred to as the boiler's "Vent" or "Overflow". For safety it is usually turned back to the wall or taken to a low level. This is from the Pressure Relief Valve and is an important safety feature. PRVs in boilers nearly always have red tops. Old Vaillants are grey.

It can be seen leaking water because:

1) The PRV has been opened (perhaps to let water out of the system) and it has failed to shut properly. This is because dirt gets stuck on the valve seat. You can try to twist the valve and it may "clunk" shut but that can also make the leak much worse.

2) The System pressure has gone above 3 bar and the valve is doing its job releasing boiler pressure. This is often due to the boiler's Pressure Vessel needing attention.

3) The pump may have failed. This can lead to the boiler boiling, the steam from which escapes out of the Pressure Relief Valve.

ANY Pressure Relief Valve which has let water out is likely to need replacing!

Water coming out of a (normally white) PLASTIC pipe will be from a condensing boiler and is normal. It must go to a drain and not be allowed to dribble on the ground!

This is a typical PRV.

 

[FAQ]

19) BOILER SIZING

Thanks to Bengasman

The following is a fair rule of thumb estimate but not a substitute for the calculation of an experienced professional on site.

For a combi you can ignore the heating demand in 99% of cases and base the size on the hot water demand.

For 1 bathroom, any boiler over 20 kW will do the job, but a 25 - 30 Kw will give you a lovely shower.

For 2 bathrooms (used simultaneously) you will require 35 kW or more. The amount of bathrooms used at different times is irrelevant.

Other important issues are the size of the incoming water all the way back to the street valve and the size and length of the gas pipe.

For heating only: 1 single panel radiator of 600 mm height and 1 metre length with “fins” on it equals 1 kW.
Add up the length of all the radiators and count “double panel radiators twice, the total will give you the size of the boiler in kW.

Due to the influence of insulation, and the current building regs, there is no need to include the cylinder demand in the calculation.

 

[FAQ]

20 ) CHECKING A PUMP using " The Finger Test "

Thanks to Agile

This is a simple way to find out if a pump is working properly using just your finger!

First remove the pump head but leave it wired up to the supply. It is important that an earth wire is connected to the pump and ideally it should be tested for continuity from pump body to earth.

The pump head is placed with the impeller upwards on a flat and firm surface. Hold the pump body firmly with the left hand and switch on the power. The impeller should spin! Use the right index finger ( 2 ) to press down on the left hand side of the impeller which should be moving away from the end of the finger.
By pressing down on the outside of the impeller it should be quite difficult to stop it moving. You will probably be able to stop the pump starting with the finger but again only by applying considerable pressure.

If the pump is spinning but can easily be stopped then its quite possible that the capacitor has lost some of its value and this can be measured to see how the value compares with the marked value which is often about 2.6 µF assuming that you have a multimeter with a capacitance range.

 

[FAQ]

21 ) Checking Flow through Heating pipes - The "BUCKET TEST"

Thanks to Agile

This test checks the flow through pipework on either radiator valves, or pump isolation valves.

First, turn off both valves and undo the valve union connection (collect water contained between the valves) then remove the radiator or the pump ( or usually just the pump head! ).

You then place the bucket under the valve and open the valve and see if a good flow of water runs freely into the bucket. Its best to run at least half a bucket from each radiator valve in turn.

With a pump its only necessary to check that the flow rate is good and that the pump valve is indeed opening.

This test will often clear a blockage on a radiator feed pipe as there is far more pressure than the pump head provides.
Pump valves sometimes shear off and dont open or they can become blocked by dirt.

Sometimes after using DS40 it loosens calcium based dirt which then becomes mobile and blocks at a pump valve on the inlet to the pump volute.

 

[FAQ]

22 ) Vaillant Diverter Valve Failure symptoms Thanks to Agile


SYMPTOMS:-

Stuck on CH:- Gives CH as normal. On DHW heats radiators instead when a hot tap is turned on but the water stays cold!

Stuck on DHW:- Gives DHW as normal. On CH does not give any CH but boiler fires up every 10-15 min to keep itself warm.

Stuck halfway:- Gives DHW when a hot tap is used as well as CH. Seems to work pretty much as normal on CH but takes longer to heat the rads.

The early diverter valve had small holes about 3.8 mm diameter and a round ball which tends to stick.

The more recent Vaillants had a diverter valve driven by a stepper motor powered from the PCB.

Later diverters had a rugby shaped ball and larger holes about 5.8 mm diameter, ( a few large hole types still had the round ball though.) This later type rarely sticks, but it still can!

The above only applies to Turbomax Plus 824, 828 and 837 combi boilers.
The newest type diverter valves have a code on the side such as E17. Earlier ones were Axx or Cxx. Some of the newest ones fail too!

 

[FAQ]

23) AVOIDING BURST PIPES

THANKS TO STEM

When water freezes it expands and can burst pipes. There are only two solutions....... get rid of the water, or prevent it freezing.

In an empty property turn off the water supply and drain everything down, including water tanks, water cylinder and radiators. Even then, if any water remains in a dip in a pipe run you could still get problems.

It is slightly less likely if the system is drained because there will be air in the rest of the system instead of water and air can be compressed as ice expands taking some of the pressure off, unlike water which doesn't compress and splits pipes instead. But this would only apply to any small amounts of water remaining, not a pipefull.

Where the water is left on, the building must be kept warm enough to protect all of the pipework. Some boilers have a frost setting that will warm the boiler when the temperature is getting close to freezing, and if it operates the pump and no motorised valves are fitted, the radiators too. However it will not do much, if anything, for any other pipework. For example the cold water supply. Insulation will help, but it will only slow heat loss.

If there's no heat reaching some pipes because water is not run through them, eventually even lagged pipes could freeze. You could consider installing a frost thermostat in the coldest location in the property or where frozen pipes are most likely. This will keep the room air temperature above freezing and protect all pipework, rather than the boiler frost thermostat which only keeps the water in the heating system above freezing. Even then, low inside temperatures over a long spell could prevent sufficient heat reaching some pipes in the loft.

If no one is living in the property, and the internal temperature is low for a long time, then you could lift the loft hatch slightly, just to let some heat through.

 

[FAQ]

24 ) CENTRAL HEATING PROTECTION

In order to look after your CH system it is imperative that you protect it from corrosion.

Inhibitor is a liquid chemical that you add to the circulating water. It stops the steel radiators from "rusting" away from the inside and clogging the boiler and pump with black sediment.


Sentinel and Fernox are two good brands of inhibitor which are the market leaders and have been recommended by many users of this site.

Everyday your system is without inhibitor, the steel rads get more and more rusty inside. This creates gas and your radiators will eventually spring pin hole leaks.

MAGNACLEAN /Magnetic filters

A Magnaclean is basically a powerful magnet and will catch much of whatever is circulating in the system water so reducing new accumulations or blockages.

It can be fitted on any 22mm or 28mm boiler return pipe and costs around £100. Boiler Buddy is a similar device, amongst others and a mechanical strainer is also useful for removing non magnetic particles.

If you have a Primatic cylinder you should NOT add inhibitor

There is a small risk of losing the air bubble inside the cylinder which separates the CH water from the domestic hot water. This will result in your hot water being contaminated with chemicals!

 

[FAQ]

25) BOILERSTAT, ROOMSTAT, TRV AND YOUR GAS BILL

Thanks to Bengasman

C H controls will only work properly when the rads are large enough to heat the space where they are, and the draught is controlled to normal ventilation levels. If there is draught, or the radiator is too small for the space it is in, it will not work. With modern controls, radiators can not be too large as they will be automatically turned off when the room is warm enough.

A Thermostatic Radiator Valve is an auto opening and shutting valve that opens when the room gets below the set temperature, and closes itself when the room is warm enough. It does NOT make the radiator hotter or colder, and setting them higher does NOT warm the room up quicker.

A TRV is not a calibrated thermometer, hence the number/setting on it is a mere indication.

The optimum balance between comfort and saving gas/money is by finding the appropriate setting for a room, and leaving it at there.
Comfort levels are personal and subjective; the actual temperature is not particularly important.

After installing a system I tend to set them as follows:
Spare room and utility room: 1.
Bedroom, hallway and landing: 2
Kitchen and dining room: 3
Bathroom: 4
This is a only a DEFAULT setting, acceptable for most people as a starting point.
How and when we feel comfortable changes under influence of things like being hungry and tired. There is no point in adjusting trv settings constantly, and I recommend to all my clients to adjust trv’s no more than once a day by half a point.
After two or three days the temperature in each room will have been found, and you never need to look at it again.

The roomstat controls the temperature in the room where it is installed, NOT by the average temperature in the house. It turns the boiler on and off, it does NOT set the boiler higher or lower.

In the old days when there were no other controls, the roomstat was often installed in the hallway because that was the central point in the house. Although this works, a roomstat in the lounge gives higher comfort levels.

A separate old fashioned fire in the lounge will influence the temperature in the room. When the fire is off, the flue will still suck warm air out of the room and reduce the temperature in the room by doing that, 24 hours a day.
When the fire is on, it will add to the heat output and could leave the rest of the house cooler than planned.

Fires are extremely wasteful when combined with modern central heating systems; sometimes so much that the heat lost through the chimney is more than what they produce!
This may sound impossible, but remember that the chimney will still pull heat out that is produced by your central heating system when the fire is off.
For people who insist on keeping the old fashioned fire, it is advisable to locate the roomstat in another room. The decorative fires are mainly just that; they look nice but produce little extra heat and will be of little influence on the working of the roomstat.

Conventional roomstats, the round dial on the wall, are quite inaccurate. They usually have a couple of degrees between on and off, and also one or two degrees difference between the temperature set to come on, and when they actually do so. Therefore the temperature indicated on the dial can easily be 3 degrees off from what it actually is.

Digital roomstats are more accurate; the popular Honeywell 0.5 degrees and the Siemens 0.2 Celsius.

The boiler stat (boiler control) determines how hot your radiators get. It does NOT control the temperature in the house.
On older boilers, the non-condensing, lowering the boilerstat does not bring any noticeable savings in the gasuse, and is best left at a minimum of 70 degrees.

Condensing boilers are more efficient when operating at lower temperatures, and turning them down can bring savings of up to 10% compared to running at maximum. For optimum savings, set the boiler at a low rate, and only turn it up when the house is frequently too cold, or often takes too long too warm up.

Balancing a central heating system with a condensing boiler, trv’s and a digital roomstat, is a complex process requiring in depth knowledge and testequipment to get it fully right. When installed properly by a good RGI, should not need adjusting between services and is best left to the installer.

The above is the basis for small and medium sized houses using gas central heating. large systems, 20 radiators or more and dual fuel can more be complex and different.

 

[FAQ]

26) CHEMICALS IN CENTRAL HEATING Posted by Mogget

Corrosion Inhibitor: Common Brands - Sentinel X100, Fernox F1 & MB-1

Heating systems contain different metals - steel in the radiators, pipes made from copper etc. In the presence of water and oxygen an electrical current flows and this results in corrosion of the 'weaker' metal. Typically the steel radiators are eaten away leading to pinhole leaks and iron oxide 'sludge' which can cause all sorts of problems. For this reason it is important that any central heating system has inhibitor added to the water. This slows the corrosion process and prolongs the life of the system. It is NOT worth skimping on - even one week without inhibitor can be enough to cause problems.

Sludge Remover / Cleaner: Common Brands - Sentinel X400, Fernox F3

Sludge can collect in radiators, heat exchangers etc and cause inadequate water flow or heat transfer. When a system has fallen into disrepair, sludge remover can be used. This dissolves the sludge into the water which can then be drained out. These chemicals are quite gentle and take some time to work. X400 in particular requires up to a month to be effective.

Flushing new or newly modified systems: Common Brands - Sentinel X300, Fernox F3

When copper pipework is installed or altered, flux and other residues can enter the system. These must not be allowed to remain as they may cause corrosion problems later on. Flushing the system using the appropriate cleaner will dissolve these residues into the water. The system must then be fully drained before refilling with water and corrosion inhibitor.

Aggressive Cleaning: Common Brands - Fernox DS-40

If there are severe scale or sludge deposits, an aggressive cleaner may be used. It is important to follow the manufacturer's instructions. It should be borne in mind that aggressive cleaning can unmask the poor state of some systems. Radiators and heat exchangers leaking, boilers failing, and pumps seizing are not unheard of.

Mechanical power flushing is something altogether different and beyond the scope of this basic guide.