Indirect Heating and Unheated Zones

Not every zone in a non-domestic building has its own heater, radiator, fan-coil or air-conditioning unit.

What is an indirectly conditioned zone?

• it has no fixed conditioning equipment of its own; and
• at least 50% of its surrounding envelope area is adjacent to directly conditioned space.

The surrounding area includes:

• party walls;
• party floors;
• party ceilings;
• and other enclosing surfaces.

Convention 6.13 states that where at least 50% of the surrounding envelope is adjacent to directly conditioned space, the zone is assumed to be indirectly conditioned.

Example

• the west envelope is adjoined to two zones, both conditioned via a gas boiler
• the south and east envelopes are adjoined to three zones, which are conditioned via panel heaters
• the north envelope is an external wall
• We can assume the floor is external due to being ground floor zones
• The space above is conditioned

The convention states surrounding envelope area is the determining factor for the 50% rule. This means the assessor will first need to work out the area of all the envelopes. This is the done by creating the nomenclature.

Once the total envelope area has been calculated, followed by the total conditioned envelope area, the percentage can then be worked out as:

This would mean the corridor is Indirectly Conditioned. 

Which HVAC system should be assigned?

Convention 6.13 says the assessor should normally use:

the HVAC system serving the largest proportion of the adjacent conditioned spaces.

Following the example above, we can do this by adding together the envelope areas based on their HVAC type.

Example 

• two zones conditioned via a gas boiler
• two zones conditioned via panel heaters
• one zone conditioned via a heat pump

In the example below, there is a door between the corridor and reception which is permanently kept open and is the only way in and out of the office block. This would likely mean the corridor zone should be assigned the same HVAC system as the reception. 

What if the zone does not qualify for indirect conditioning?

Convention 6.11 applies to existing buildings where at least one zone has no fixed conditioning equipment.

Certain activities can legitimately be entered as having no HVAC, including:

• circulation areas;
• plant rooms;
• store rooms;
• warehouse storage;
• industrial process areas;
• car parks;
• B1 workshops;
• and, in limited circumstances, toilets.

However, an office, classroom or retail area would normally be expected to be conditioned.

If such a zone:

• has no fixed heating;
• does not qualify as indirectly conditioned;
• and should reasonably be conditioned for its activity;

The published flowchart uses:

Appendix 10.07:

Can one indirectly conditioned zone heat another?

You should not create a chain such as:

The second zone should not simply inherit the status of the first indirectly conditioned zone.

The assessment needs to establish whether each space genuinely meets the Convention 6.13 criteria based on the directly conditioned spaces surrounding it.

Example:

Zone A is directly conditioned via a gas boiler.

Zone B is considered indirectly conditioned due to the 50% rule.

Zone C is considered as Zones without HVAC following Convention 6.11.

Zone D is unconditioned since Zone B is indirectly conditioned, Zone C is recorded without HVAC, and there is one external wall. 

Zone D will be recorded as having assumed heating following the flowchart in Appendix 10.07.

What about adjoining buildings?

Unless evidence shows otherwise, adjoining buildings are assumed to be conditioned.

The exception is buildings falling within Planning Classes B2 to B8, which are assumed to be unconditioned.

This can affect the 50% calculation.

For example, an internal zone may have a party wall adjoining another commercial property.

If that neighbouring building is assumed to be conditioned, the party wall can contribute towards the area considered to be adjoining conditioned space.

The assessor should retain evidence supporting the adjoining-building classification.

Evidence to retain

• that there is no fixed conditioning equipment in the zone; and
• that at least 50% of its surrounding envelope is adjoining conditioned space.

Useful evidence includes:

• floor plans;
• zone drawings;
• site photographs;
• calculations showing surrounding envelope areas;
• and notes identifying the HVAC serving adjoining spaces.

A simple annotated floor plan is often one of the clearest ways to demonstrate the decision.

Common mistakes

• A zone may be indirectly conditioned.
  

Using floor area for the 50% test

• The test uses surrounding envelope area.
  

Assigning the nearest HVAC automatically

• Use the system serving the largest proportion of adjoining conditioned space unless significant air transfer justifies another choice.
  

Daisy-chaining indirect heating

• Do not assume one indirectly conditioned zone can automatically condition the next.
  

Ignoring the zone activity

• Some activities can legitimately have no HVAC. Others would normally be expected to be conditioned.
  

Checking unheated spaces too early

• Condition spaces that require heating first. Then assess the remaining zones.
  

Summary

• 50% or more of the surrounding envelope must adjoin conditioned space;
• use the most appropriate adjacent HVAC system;
• apply Convention 6.11 where a normally conditioned activity has no suitable heating;
• only treat genuinely appropriate activities as unconditioned;
• and always work through the building in the correct order.

If the logic is clear on your floor plan, the SBEM modelling becomes much easier to justify.