Microbore, as the word implies, is the use of narrow pipes with tube bores ranging from 6mm to 10mm diameter. Microbore is now readily accepted within the heating trade because of its many advantages. It can be used with open or pressurised systems using a two-pipe flow and return arrangement.
The design and calculations for micro-bore must be made more carefully than for smallbore; however, the majority of components used in a smallbore can be installed into a microbore system such as boiler, radiators and pump.
The advantages of microbore outweigh traditional systems because:
1. Installations are quicker; savings of up to 10% are common.
2. Disturbance of the household is reduced to a minimum.
3. It can be a flexible installation, permitting extensions at a later date.
4. The water content of the system is less which gives a faster heat-up and better response to controls.
5. Most important, a neater installation is possible, as the very small tubes can be easily concealed.
6. Assembly techniques are similar to a smallbore system but simpler. No specialist know-how is required.
7. The absence of many pipe fittings reduces the risk of leaks.
8. You can enjoy the economy of a low thermal capacity boiler.
9. Microbore can be installed in existing buildings and existing floorboards may not have to be lifted because the pipework can be surface mounted. It can also be buried in the concrete floor when new buildings are constructed.
Designing a microbore system
The design considerations are the same as for smallbore systems:
1. U-values and heat losses.
2. Temperatures in each room.
3. Boiler sizing.
4. Radiators, their position and sizing.
5. Feed and expansion tank.
6. Circuit temperature drop, 81 to 70°C.
The exceptions to smallbore design and calculations are the pipework layout, pump sizing, and pipework sizing. The use of manifold or manifolds and the pipework layout will be covered in the installation section.
The pump will have a slightly higher head due to the smaller bore which creates a higher velocity of water flow. The velocity has to be kept above 0.3 metres per second.
Microbore components and materials Tube sizes have been reduced to 6, 8 and 10mm diameter, and are manufactured to BS2871 in soft tempered copper which allows easy manipulation by hand for all but very sharp bends. These can be formed with an external spring or hand bender. The following are the lengths available:
6mm 10 to 200m (33 to 660 ft)
8mm 10 to 150m (33 to 490 ft)
lOmm 10 to 100m (33 to 330k)
The longer the length of coil, the less fittings you have and the fewer leaks.
Care must be taken when handling the coils as they are liable to kink and both ends should be covered to exclude dirt.
Smallbore systems have a valve at each end of the radiator, but for microbore systems a valve has been designed which allows both inlet and outlet in one valve — known as a double entry valve.
When installing the valve the flow tapping is the one against the radiator.
In operation the valve controls the return from the radiator.
Manifold — the heart of the system
This is one of the major pieces of equipment that differs from the conventional smallbore system. It is simply a length of 22mm or 28mm copper tube with a central-
ly positioned separator disc, which provides both flow and return chambers in one unit. Into this length of tube a number of stub bosses are soldered to form compression fittings which allow pipework to be connected easily.
Any boss not required can be capped and used at a later date for additional radiators. Any number of manifolds can be used in one system.
There are basically three types:
A conversion unit is an immersion element used to convert an existing direct copper cylinder to an indirect cylinder. It can also be used in a new system but is not essential. Conversion units can be used on a pumped or gravity primary system. Various outputs are available:
output pumped size
11.1kW 760mm x 4 blade 8.8kW 680mm X 4 blade 5. hammer and cold chisel (for holes 3.5kW 400mm x 4 blade through walls)
no. of bosses for temp. drop I I °C I 7°C flow return
240mm 9 9 36kW 54kW
170mm 6 6 24kW 36kW
170mm 4 4 16kW 24kW
This is based on Wednesbury Micrafold range
Pumped primaries and heating
The layout for a pumped primary heating system is exactly as the pumped smallbore system and there is no variation in the position of pump, valves, controls and boiler.