Central Heating Review, Address Removed, Kings Clipstone, Nottinghamshire

September 2006


The property is a large detached dwelling built in the Spring of 2003. The Central Heating System had apparently been running satisfactorily until approx Christmas 2005 when it was noticed that the radiator in the Breakfast Room was not heating up. The occupant tried to remedy the problem himself by bleeding the radiator through the combined radiator/drain valve and by opening fully the Gate Valves in the Cylinder cupboard. It was also noticed that the radiators in the Bathroom and Landing were heating up intermittently.

In the Spring of 2006 a plumber was called who balanced the system and identified that the Circulating Pump required replacing with a higher capacity unit which he said he would call back and replace at a later date. Unfortunately that plumber did not return and after several months of waiting the occupant contacted Ball and Sons.

The Property

The property is a modern brick built dwelling which consists, on the ground floor, Lounge/Dining Room, Kitchen/Breakfast Room, Hallway, Utility Room and Cloak Room, and on the first floor, 5 bedrooms with 2 ensuites, one with shower and one with bath, a Family Bathroom and a Landing.

The property has a total of 16 Radiators with a total output of 10.4 Kilowatts, all pipework is copper with the terminal heating pipework being 10mm microbore, a hot water storage cylinder of 210 litres capacity all of which is served by a Potterton Suprima 50L wall mounted boiler situated in the integral garage.

Recent Developments

In recent weeks it was decided to replace the Circulating Pump, which was 5m head capacity, with a higher capacity 6m head unit. A Grundfos Alpha 15-60 variable speed pump was fitted as it was thought the extra capacity would overcome the circulation problems.

After fitting the pump it would appear that the increase in pump performance has revealed more circulation problems including reverse circulation which caused the radiator circuit to heat up when the system was only calling for hot water. After eliminating any possibility of defects in the zone valves a more detailed examination of the system layout was undertaken. This examination revealed several flaws in the pipework layout.

System Layout 

The system is piped in the “S Plan” configuration where separate zone valves control the hot water and central heating; these are independently controlled by the system programmer and thermostats. When the system calls for hot water the boiler fires and the hot water zone valve opens allowing heated primary water to circulate through the cylinder coil heating up the domestic hot water. The same occurs with a call for heating, the boiler fires and the zone valve for the radiators opens allowing heated primary water to circulate through the radiators. When hot water and heating are both satisfied the zone valves close, at this time there is excess heated water in the boiler and in order to safely dissipate this heat a by-pass circuit is used which allows the heated primary water to flow through a circuit of pipework until it cools.

The installation requirements for this system are supplied by Potterton, on Page 11 is the layout plan. The Potterton Suprima 30L – 100L Installation and Service Instructions Pages 9 –11 are produced as Document One

The installation requirements are that the order of flow connection immediately after the circulating pump are

  1. By-pass
  2. DHW Cylinder
  3. Radiators

This installation plan follows the Thee Tee Rule which governs the order of connection of various features of the system.

An explanation of this rule is given in Document Two which is an extract entitled 1.8 Central Heating System Layout from the book by John Reginald – Central Heating Fault Finding and Repair.

Images of this installation and a schematic are produced as:-

Image One – Zone Valves

Image Two – Return Pipework

Image Three – Final Radiator Return

In this installation the order of flow connection is (Image One)

  1. Radiators
  2. By-pass
  3. DHW Cylinder

This reveals two major faults, firstly the by-pass is in the wrong place and secondly the Radiators and DHW Cylinder connections are reversed thus the Three Tee Rule has been broken.


Image One – Zone Valves                                                            

The installation requirements for the return connections immediately before the boiler are

  1. By-pass
  2. DHW Cylinder
  3. Radiators

In this layout the order of return connection are (Image Two)

  1. By-pass
  2. Radiators
  3. DHW Cylinder


Image Two – Return Pipework

This reveals a further major fault in that the Radiators and DHW Cylinder connections are reversed again breaking the Three Tee Rule.

The consequence of breaking the Three Tee Rule is Reverse Circulation.

A full explanation of this condition is given in Document Three -  a further extract from Reginald’s book entitled 1.9 Central Heating – Reverse Circulation

I also refer to the Plumbing Engineering Services Design Guide issued by the Institute of Plumbing and Heating Engineering (IPHE) which states at Page 59 Piping Installation Item 10. “Always connect the DHW cylinder primary return as the last connection on the return pipe to the boiler, after any heating return connections to avoid reverse circulation problems” This extract is produced as Document Four.

The installation requirements for the by-pass is listed on Page 9 of the Potterton Installation Instructions (Document One) and requires the fitting of a Lock-shield valve adjusted to the required flow rate. A Lock-shield valve is a valve that is adjusted by the plumber and then locked to prevent interference with the setting by the householder.

In this installation a Gate Valve has been used which may well have been correctly adjusted on installation but because as it is a manually operated valve the householder was able to fully open it in his attempts to solve the circulation problems. As an absolute minimum the wheeled head of the Gate Valve should have been removed after setting as this would have prevented any later intereference. Better still would have been the fitting of an Automatic By-pass Valve which can be set to a specific flow rate and are not very expensive (about £30). The IPHE Plumbing Engineering Services Design Guide (Document Four) suggests that an Automatic By-pass Valve should be used to avoid boiler inefficiency.

The by-pass should also come off the pipework at right angles so that flow is only directed through it when the zone valves are closed, at present the position is more or less straight from pump to by-pass and consequently unrestricted flow is going straight through it.

A further flaw in the pipework was noted at the last connection from the Radiators to the Radiator Return Pipe which is situated in a void under the Cylinder Cupboard.

An image of this pipe junction is shown in Image Three.

 Image Three – Final Radiator Return

This connection consists of a 22mm Tee with the direction of the return coming in from the left, the return route is then upward through the top connection, the right connection is reduced from 22mm to 10mm and connected to a return pipe from a radiator or radiators. The direction from this return is coming in from the right. It is difficult to establish precisely which radiator this return serves although the adjacent room is Bedroom 4 and this is an obvious choice however no major problems have arisen with this radiator.


Due to the inadequacies of the system layout major circulation problems have arisen.

When the Radiator Circuit only is activated a significant amount of primary water is going through the by-pass directly into the return to the boiler. This is highly inefficient as none of this water is reaching any radiators. It is also possible that some of this water is finding it’s way down the radiator return in a reverse direction, the result being that the pump is then working against itself.

The final return from the Radiator into the tee directly opposite to the direction of flow of the other Radiators is a major cause for concern; the amount of water getting through this fitting into the return can only be minimal at best. This will cause significant impairment of the circulation to whichever of the radiators is served by that return.

When the Hot Water Circuit only is activated the combined effect of the open and straight by-pass and the return from the Cylinder being before the Radiators results in reverse circulation to the whole Radiator Circuit.

When both circuits are open all the above circulation problems are combined.


The manufacturers installation instructions have not been followed in this system layout and circulation problems have arisen as a result.

In addition extra fuel will have been used due to system inefficiency and the extra strain placed upon the boiler.

Additional expense has also been incurred for example the fitting of a higher capacity pump.

Enclosed is my estimate for the required remedial work to the cylinder cupboard pipework.

Yours sincerely

Martin Ball

Update March 2007

The builders of the property paid for the remedial work to the pipework in the cylinder cupboard and the problem was solved.

The by-pass and valve order was rectified and a swept tee used on the DHW connection into the return. 


Before                                                        After

The dodgy microbore return was capped off at the tee against the flow and re-connected higher up the return.


Before                                                                          After