Communal Heating and Central Heating

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This article is aimed at describing the methods of feeding central heating in properties connected to district heating, and in particular the decision to go direct or indirect.

Direct Heating

Directly fed central heating is standard outside the UK, where it is the same water that circulates around the district network that feeds into radiators in properties. However, in the UK we almost exclusively use indirect fed systems, making use of a heat exchanger to provide a break between the district system, and the central heating system in each property.

The reasoning why, is more than a simple technical argument however, and one must also look towards safety and maintenance as factors to steer one's decision.

Direct heating is most certainly more efficient, with numerous advantages:

  • There are no additional temperature drops introduced by a heat exchanger.
  • return temperatures are lower to plant.
  • flow temperatures do not need to be as high to drive a given heat load.
  • No heat exchanger, central heating pump, safety valve and expansion vessel.
  • No discharge pipes.
  • No requirement to test individual systems for water quality, or to add corrosion inhibitor as its done centrally.
  • Lower heat losses from a smaller HIU.
  • Cheaper
  • No need to maintain pressures in domestic heating systems
  • No filling loops to be left open

Indirect Heating

Given these advantages then, why are indirect units so popular ?

Safety

We have often been in discussions with local authorities regarding the use of direct fed systems, only to be met with a brick wall of fear about feeding district heating water into a property. The district network can run over 90C, runs at up to 10 bar pressure, and has thousands of gallons of water ready to discharge. If you have ever bled a system that runs at these pressures and temperatures, you can understand the fear. What happens if a householder tries to bleed a radiator (or a radiator splits) and is faced with scalding water spraying all over the place ? Is there a duty of care to prevent this ever happening ?

And then there is theft. Some even say we have a superior class of vandal in the UK. We have been on sites where directly fed radiators have been ripped out only to leave district heating water flooding 4 floors of a building, requiring the fire brigade to attend. These experiences do build up a resistance to direct connection.

An indirect system overcomes these fears, although there is still some risk from primary pipework and HIUs. Temperatures are limited to what is required for function, pressures are in the region of 1.5 bar, and there is only a small volume of water to spill. The district network is kept protected.

Why then is there such a disparity between UK and European views on this subject ? Unvented cylinders were common in Europe for years before they were even made legal in the UK in 1986, and when they were legalised, the safety regulations were more strict than those elsewhere. One could argue that this is paranoia, although the fact that we haven't yet had a death in the UK related to unvented cylinders may be a testament to the view that in the UK we do things to higher standards.

One answer is to use leak detection in partnership with shut-off valves for emergency protection.

Loss of District Function

Issues within properties on a direct system that lead to the discharge of primary water into properties, may well result in the district system going down. Protecting the plant with the use of indirect systems should therefore reduce the incidences of loss of services, and any issues within properties will have no impact on the function of the plant.

Cross Contamination

This is certainly more of an issue on retrofit systems, where one may be connecting a brand new plant to an old system. In such circumstances one may need to look at the possibility of debris from older radiators working its way throughout the network.

One also needs to understand if the connection of a variety of radiator materials, from cast iron to aluminium, will have a negative impact on plant, that over time results in a build up of sludge, or pinholing in radiators.

Specification of Equipment

If you are connecting to a district heating system that runs at high pressures and temperatures, then radiators, valves, and pipework in properties needs to be up to specification. Motorised valves that only close against small differential pressures may be no good, and it may completely rule out the option to use plastic pipework and fittings within properties.

Ownership

There seems to be a preference in the UK for individual people to own products and properties and this has a knock-on effect with heating systems. Even in some bigger developments many properties are purchased and it is now rare that an entire development is owned by just one organisation.

Property owners want a clear demarcation line between the district network and their own equipment, so that any issues with the system can be correctly attributed to the responsible party. This is also important where a property is being let, because the owner of the property is in the hands of the district heating network manager, and their own tenant.

For instance, when there is an issue with the heating in the property caused by the tenant removing a radiator to decorate, this is clearly apartment side of the demarcation line and can be charged back to the tenant. If the problem is down to incorrect dosing of the primary network, this is clearly district side of the demarcation line and is therefore a building management problem and not the tenants.

If the district heating system is a direct system, a fault anywhere on the system can have a knock-on effect with any other part of the system, therefore diagnosing building-wide cause and effect is made very difficult and an argument regarding responsibility could be unavoidable.

Most other utilities (i.e. Water, Electricity, Gas) have a clear demarcation line generally where their service branches off the mains to the property or at the meter, and it is not in their interest to warranty their equipment in areas outside of their control or ownership, although they often offer additional warranties for the system beyond the demarcation line at an additional cost. This brings in additional revenue and could be attractive to some landlords, however the demarcation line still needs to be clearly stated when the property is sold, as the additional cover may not be taken out every year, or the property may change hands.

An indirect system cuts through this, allowing houses to be plumbed up using the desired plumbing methods and materials.