The Reliance range of heating controls is specifically targeted at providing solutions for the sealed heating system, which is fast becoming the norm in the UK. All sealed systems have a number of basic requirements, which are the same irrespective of the application:
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The system must have an acceptable method of filling, whether via a temporary connection to the water supply, from storage via a pump, or in domestic applications by permanent connection of a CA type backflow preventer (BA device in industrial systems).
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Particles, which can arise from installation, from the water or even from degradation of components, have to be eliminated. This can be done by flushing the system using water and/or chemicals, depending on the amount and type of contamination, or by built-in filters.
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Water expands when heated and this volume of expansion has to be accommodated, generally by the use of expansion vessels.
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Air has to be eliminated from the system, usually by the use of air separators and air vents.
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Safety (pressure and temperature) has to be maintained.
With the increased use of controls to limit the amount of heat supplied to a given area (i.e. thermostatic radiator valves), protection for the pump from pumping against a closed circuit is becoming more important. It is common practice to fit a low flow bypass pipe to link the flow and return together and so provide a minimum flow rate through the pump, but this is not ideal. The correct solution is to fit a differential by-pass valve, set so that it is shut when the system is demanding flow and open when the system has shut down. As this type of valve is proportionate in operation it will keep a more even load on the pump irrespective of system load and it will help prevent rapid cycling of the boiler / heat source.
The most common case of combining heat sources is by adding solar heating to an existing system. This can result in two independent sealed systems and an interface system / heat store. The main reason for the two systems is the fluid requirements of each. A basic UK water filled solar system will have a high percentage of glycol in the water to prevent it freezing, whereas a conventional sealed heating system does not normally require protection against freezing. Having said this there is no reason why heat from both the solar and heating systems should not be used to provide domestic hot water. The simplest way to achieve this is to use a hot water cylinder with two primary coils. The solar system is connected to the lower coil and the boiler and heating primary circuit is connected to the upper coil. This allows the solar system to be used for providing all the hot water when weather and solar conditions are at their optimum and for pre-heating the water at other times. In the latter case the primary coil then heats the water to the required temperature, which should always be above 60°C in order to avoid risk of legionella occurring.
Another system growing in popularity is underfloor heating. Also a sealed system, it requires the same controls as the typical sealed radiator type installation. But because underfloor heating systems use a much lower circulating temperature and because the radiant panel is the surface that people are walking on, it is crucial that there is a good control system in place to maintain the circulating temperature at an even level. In this type of system the pipe runs, or loops, are fed to each zone from a central distribution point which has both a flow manifold supplying the higher temperature water to the pipe system and a return manifold which receives the water back after it has transferred its energy to the floor. At this point of distribution a mixing valve is used to blend the flow and return water together to maintain an even distribution temperature, a bypass is also fitted - or in some cases integrated into the valve - to return a portion of the water back to the boiler to be reheated.
Sealed Systems
The Sealed System is the Primary System or Heating System.
It is concerned with providing the heat for the building and, in some cases, the heat for the hot water.
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Filling the System
The main method used to fill a sealed system is a temporary connection to the main water supply. The temporary connection is made by a filling loop made up from four components -
- A means for controlling the incoming water
- A short length of pipe that can easily be disconnected
- A WRAS listed double check valve
- A means for isolating the heating system after filling
1. Controlling the incoming water can be done in two basic ways
a) Manually using a screwdriver operated ball valve or a screw down stop valve.
b) Automatically using a pressure reducing valve. The pressure reducing function can also be combined with shut off valves and either a RPZ or CA type backflow preventer, to form a control group.
2. Connection to the system is usually via a flexible hose with connections that can be made and broken by hand.
3. A WRAS listed double check valve.
4. An isolating ball valve.
Accommodation of Expansion (Expansion Vessel)
Easifit® Sealed System Kit |
Once the system is full of water, if heat is then applied to the system the water will expand and pressure will rise quickly so a method of accommodating this expansion must be used. The simplest method is the expansion vessel.
The size of expansion vessel used is dependent on the system volume and the pressure it is to work at. This can be related to heat input (normally 12lt per 4,000 BTU) but in systems using convertor radiators for instance this method will over size the expansion vessel. The following table can be used for conventional systems.
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Expansion Vessel Size
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Static Head (Meters)
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System Volume (Litres)
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Boiler Rating (BTU)
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| 5 |
5
10 |
87
69 |
29,000
23,000 |
| 8 |
5
10 |
139
111 |
46,333
37,000 |
| 12 |
5
10 |
208
167 |
69,333
55,666 |
| 18 |
5
10
15 |
313
250
188 |
104,000
83,333
62,666 |
| 25 |
5
10
15 |
435
348
261 |
145,000
116,000
87,047 |
| 40 |
5
10
15 |
696
557
417 |
232,000
185,667
139,000 |
The static head refers to the charge pressure of the vessel which must be at least two meters higher than the actual static height of the system.
A maximum system pressure of 3 bar has been used for the above table. This is not the design running pressure. The maximum running pressure should be a minimum of 0.5 bar below the setting of the system pressure relief.
For systems that fall outside the above table either in pressure or volume the expansion vessel size would have to be calculated by use of the standard formula.
Safety Valve
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101 series sealed heating system pressure relief valves
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As the expansion vessel requires servicing for it to maintain its correct function a Safety Valve has to be incorporated into the system. For conventional systems this is set at 3 bar. This valve is sized against the heat input to the system and will function as an expansion valve (unless no control is exercised when filling the system) or for a short time as a steam relief valve. |
Discharge Capacities of PREL 100 series in kW
Bar
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DN15 |
DN20 |
DN25 |
DN32 |
DN40 |
DN50 |
| 1 |
30 |
60 |
121 |
213 |
364 |
543 |
| 1.5 |
37 |
74 |
148 |
260 |
446 |
666 |
| 2 |
43 |
87 |
175 |
308 |
527 |
786 |
| 2.5 |
50 |
100 |
201 |
354 |
606 |
904 |
| 3 |
56 |
113 |
227 |
400 |
685 |
1021 |
| 3.5 |
63 |
126 |
253 |
445 |
762 |
1137 |
| 4 |
69 |
139 |
278 |
490 |
839 |
1251 |
| 4.5 |
75 |
152 |
304 |
535 |
915 |
1365 |
| 5 |
82 |
164 |
329 |
579 |
990 |
1477 |
| 5.5 |
88 |
177 |
354 |
622 |
1065 |
1589 |
| 6 |
94 |
189 |
379 |
666 |
1140 |
1700 |
| 6.5 |
100 |
201 |
403 |
709 |
1213 |
1810 |
| 7 |
106 |
213 |
427 |
752 |
1287 |
1919 |
| 7.5 |
112 |
225 |
452 |
795 |
1360 |
2028 |
| 8 |
118 |
237 |
476 |
837 |
1432 |
2136 |
Pressure Gauge
This is an essential part of the system as it shows what the working pressure of the system is. It is also used when manually filling the system to show the pressure at which the system is being filled.
Air Vent
There must be fitted into the system at the high(est) point(s) a method of releasing air from the system. Air will cause a variety of problems in the system from stopping it working completely to stopping individual radiators from working.
Additional Valves
There are other controls which can be used to make the system suit particular requirements. These valves are :-
Line Strainers
A line strainer fitted before the circulating pump can save a great deal of money. Most pumps removed, as being defective, from a new system were not defective when installed. Most instructions include the immortal phrase "flush pipe work before connecting". This very often can not be done on a system without installing most (if not all) of the fittings that can be damaged by dirt in the system. A line strainer fitted before the circulating pump (with service valves either integral within the strainer or fore and aft so that the strainer element can be cleaned) will take out the dirt as the system circulates.
Differential By Pass Valves
With the almost universal use of thermostatic radiator valves and the increasing use of zone valves pump pressure and system flow rate alter as each part of the circuit switches in or out. A Differential Pressure Regulator is used to stabilise the circulation flow rate and pump pressure in pumped central heating systems.
Air Separators
These are used on the flow of larger systems to help separate the air which is naturally trapped in the water. They perform most of their function in the first hours of operation in a new system. They are normally fitted with or have integral within their construction an automatic air vent.
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