One size does not fit all

Published:  15 September, 2014

Correct sizing of heat emitters is critical to ensure heat pump systems deliver the highest running cost savings and are eligible for domestic RHI payments, explains Stuart Gadsden

Correct sizing of heat emitters is critical to ensure heat pump systems deliver the highest running cost savings and are eligible for domestic RHI payments, explains Stuart Gadsden

Renewable heat pump systems operate most efficiently when used with correctly sized heat emitters with the lowest possible flow temperatures.

Underfloor heating systems can operate with flow temperatures as low as 35°c; heat pump convectors can also be selected to operate at similar low flow temperatures; and radiator systems can be designed to work at low flow temperatures of 45°c.

Not only will systems operating at lower flow temperatures deliver lower running costs, they will also have a higher seasonal performance factor (SPF).

The Heat Emitter Guide aims to help installers select the correct heat emitters to optimise the performance of heat pumps. The guide is also known as MCS 021 and is now incorporated into the Microgeneration Certification Scheme and forms the basis for estimating Renewable Heat Incentive (RHI) income.

The Heat Emitter Guide uses temperature star ratings from zero to six stars to indicate how efficient the proposed system is likely to be. A six star system is the most efficient and so has the highest SPF. As the flow temperature increases so the temperature star rating decreases.

To qualify for the domestic RHI, an air source heat pump system installed and commissioned on or after 9 April 2014 must have a minimum three star rating, equivalent to an SPF of 2.7 and a maximum design flow temperature of 50°c.

The Heat Emitter Guide is intended to be used to review a home’s heating requirement and the choice of emitter, to help optimise performance and lower running costs as well as determining if the proposed system will be eligible for RHI payments.

Using the Heat Emitter Guide with existing radiator systems:

1. Calculate the heat loss, room-by-room                     

Domestic heating systems must be designed to maintain homes at comfortable room temperatures, even when the outdoor temperature is at its coldest – in other words, the heating system must be able to compensate fully for 100% of the heat loss. Knowing how much heat is lost allows the installer to size the system and its emitters.     

To be eligible for RHI payments, a system must be installed by an MCS-accredited installer. MCS requires a room-by-room heat loss calculation that complies with BS EN 12831, essentially calculating the fabric and ventilation heat losses. This is covered by the latest version of the MCS Microgeneration Installation Standard for heat pumps, MIS 3005 v4.0.                         

It is important that calculations are carried out for each room because each will have different demands. This means emitters can be sized appropriately for each room.

2. Determine the rated output of the existing radiators               

It is unlikely there will be a detailed specification available for an existing radiator system. Therefore, the output of the existing radiators can be estimated based on their length, height and type (e.g. single panel, single convector). To help with this, the table of heat emitter outputs is available on the MCS website, or alternatively radiator manufacturers may provide data tables.                          

The rated output of the radiator must be determined at a mean water to air temperature difference of 50°c. This relates to a flow temperature of 75°c and a return temperature of 65°c which gives a mean water temperature of 70°c. Assuming a room temperature of 20°c gives a mean water-to-air temperature difference of 50°c. This is typically the design condition that radiator manufacturers use to present their data.

3. Calculate the oversize factor of the emitters for each room                              

The next step is to determine the oversize factor. This is calculated by dividing the room’s heat loss by the rated output of the emitters at a mean water to air temperature difference of 50°c.

4. Determine the temperature star rating of each room

Using a graph within the Heat Emitter Guide, the oversize factor is then used to determine the temperature star rating of each room

5. Determine the temperature star rating of the whole system                                

The temperature star rating of the heat pump system is equal to the lowest temperature star rating  of the worst performing room. For an air source heat pump system, if this is three stars or higher, then the system will be eligible for RHI payments as the SPF will be 2.7 and the flow temperature will be 50°c.                                      

However, in some homes the SPF will be too low, so further steps will need to be taken.

6. If the temperature star rating is below three stars                               

If the temperature star rating  is below three stars (an SPF of below 2.7) in any room, it can be improved by one or all of the following:

• Reducing the room’s fabric and ventilation heat losses (using insulation, for example)

• Changing the radiator to a larger one

• Adding an additional radiator

• Switching to a different emitter type that can operate at a lower flow temperature.


Once a change is proposed, the steps procedure needs to be followed again until all rooms have a temperature star rating of three or more.

The Heat Emitter Guide can be downloaded from the MCS website – www.microgenerationcertification.org – where installers can find a wealth of information to help them deliver efficient and appropriate renewable heating systems that meet RHI criteria for their customers.

Stuart Gadsden is a heat pump specialist at Daikin UK

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