Microbore Heating: Open or Sealed Systems?

Among the great strides taken in heating in recent times, the most spectacular development has been that of microbore central heating. In many situations, this is not only quicker to install than conventional, small-bore heating, but more efficient, more economical to run-and less likely to give trouble. The choice is between open and sealed systems – at a range of operating temperatures.


Microbore central heating is an extension of the small-bore system, which is sometimes called minibore. It also utilizes two-pipe, flow-and-return arrangements, using small-gauge pipework.

Technically, the advantages of using microbore pipes are: low pipework heat loss, good response and running economy of low thermal-capacity boilers, a reduced number of fittings, and simpler to install.

There is also less disturbance to the fabric of the home during installation, easier balancing of the circuit and less chance of ‘hydraulic’ noises within the pipes, once in operation.


The heart of the system is a manifold, usually located somewhere in the centre of the house. Pipes to radiators – a flow and return in each case – usually run radially from it.

Where a home has a solid floor, a central manifold may serve radiators or convectors on a drop-pipe principle. The radial arrangement of the manifold system ensures a high degree of circuit self-balancing. With suspended floors, both an upstairs and a downstairs manifold may be used.

The manifolds are fed by main flow-and-return ‘header’ pipes from the boiler. On a larger heating installation, several manifolds may be used.

Microbore pipe is made in gauges of 6mm, 8mm or 10mm and supplied on reels of 20m or more. The pipe is malleable and, therefore, easy to manipulate. This greatly lessens the number of components and reduces the need to cut and manipulate sections of tube.

The distance from a manifold to a radiator or convector should, ideally, be no more than 7.5m, with a total combined pipework run of 15m.

Microbore can be further divided into open and sealed systems, though sealed systems can also be used on standard small-bore circuits.

Low-water content

Microbore systems work on a principle of circuit low-water content and may use modern, low water-content (low thermal-capacity) boilers. The heat exchange units of these boilers are usually made of copper or stainless steel. This means that as there is little residual heat retained by the boiler, system controls can react swiftly and accurately to precise temperature conditions.

High thermal-capacity boilers are those with cast-iron heat-exchange units. These hold a much larger quantity of water and the jackets ensure the maximum local heat retention.

Residual thermal retention makes such boilers less accurate in response to actual circuit conditions, and these are less satisfactory in performance when used on microbore systems.


Heat dissipation below floorboards on microbore pipes is very low. Unless the system is likely to be shut down in cold weather for a protracted period, it is unnecessary to lag these tubes; this would be necessary with small-bore or larger pipework.

Since microbore pipework is almost invisible when painted, and because of its relatively low heat emission, it can be concealed, if necessary, in plasterwork without likely problems of cracking.

It can be bent by hand or over the knee, but with proprietary hand-held pipe-benders, neat, even bends can be produced.

This type of system has, in most cases, definite advantages over small-bore circuitry and is usually cheaper and easier to install, assuming, of course, that the layout design is good.

Pumped primary

Microbore circuits also use the pumped-primary method of heating hot water. For this reason, a high-recovery cylinder or a microbore heating element, which fits into the immersion heater boss, on a direct cylinder, is necessary to maintain the fast response and high heat recovery associated with the system.

Sealed systems

The feed-and-expansion, or ‘header’ cistern, as with conventional small-bore systems, is incorporated at the highest point in the installation. It serves various roles: it absorbs expansion of heated water, maintains a constant pressure within the system, and provides a source of replenishment for evaporative loss.

Sealed or closed systems differ from the common ‘open’ system, which employs a feed-and-expansion cistern for venting. Sealed systems should not be permitted access to the atmosphere, so expansion has to be catered for in another way.

Generally, open systems are those working at a flow temperature of 82°C, while sealed systems work at temperatures of 93°C and above.

When a system is sealed many advantages are gained. The feed, or ‘header’ system is eliminated, together with the associated components and circuitry. This makes the system suitable for bungalows where solid floors dictate a drop-pipe arrangement. The problem of low static head, causing water to be induced over the vent pipe or air to be drawn in, are eliminated. However, certain additional control circuitry is needed.

Sealed systems rely on the fact that much higher temperatures can be utilized. With open systems, the upper temperature limit is dictated by the boiling point of water. Allowing a safety margin, this temperature limit is placed at 82°C; and no such system should work at higher temperatures.

Because sealed systems are shielded from the atmosphere and pressurised by the molecular expansion of water during heating, much higher temperatures can be attained without boiling.

Temperatures of up to 110°C (medium temperature) can be used, provided system components are capable of working at this heat level. In practice, 99°C (elevated temperature) is rarely exceeded in the domestic field, largely because of psychological fears than to any other factor!

Air in open systems causes noise and may produce air locks. Once initial venting of a sealed system is complete, by allowing it to run at design temperature for several hours, these problems should not arise.

Absence of air in a sealed system reduces the likelihood of corrosion. Air is a retarding factor as far as heat transference is concerned, and its absence increases the efficiency of transmittance between water and the heat-emitting appliances.

The freedom from air locks allows certain types of layout which would be avoided when designing small-bore systems. For instance, in the two-storey house, two distribution manifolds could be placed at each end of the building on the first floor only, with inverted loops of pipe serving the ground-floor radiators.

Good microbore design is, therefore, a compromise of the ‘radial’ ideal and small-bore design practice, without the same concern of possible air locks.

10. November 2011 by admin
Categories: Featured, Handyman Tips | Tags: , , , , , , , , , | Comments Off on Microbore Heating: Open or Sealed Systems?


Get every new post delivered to your Inbox

Join other followers: