MWT (Mean water temperature), which is the average temperature at the radiator versus room temperature, has a significant impact on the design due to the delivery output of radiators. Where new build radiators are sized to the MWT, existing retrofits are reliant upon the capability of the existing infrastructure, unless forming part of the replacement works.  

It is essential to understand that flow temperatures alone do not determine heat outputs, as there is a significant difference in delivery outputs between systems such as Traditional and modern-day gas boilers, to High-temperature CO2 heat pumps, each with 80 o.C flow temperatures.

MWT of typical heat sources

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As demonstrated above, albeit High-Temperature CO2 ASHP provides 80 o.C flow temperatures, due to the low return temperatures, MWT is low. Where such a system is installed to replace existing gas-fired boilers, pre-existing infrastructure must be oversized by a minimum of 67%, where radiators have been sized to the building's heat loss.

Where sources with lowered MWT can operate satisfactorily, upon careful calculation, is in buildings that have undergone fabric improvements to reduce the building's heat loss, thus existing infrastructure then becomes oversized to the capacity required.

Carbon trust publication CTV014 – Building fabric advises on percentages of heat loss through different building fabric elements, as shown in Figure-1 Below. Most of the heat is lost through ventilation and air infiltration loss, windows, and the roof. Different fabric elements have different thermal (heat transfer) properties. Glazing, for instance, is usually the part of the fabric least able to retain heat.

Figure 1 Carbon Trust CTV14 - Building Fabric - heat loss from a commercial building
Figure 1 Carbon Trust CTV14 - Building Fabric - heat loss from a commercial building

Typical efficiencies see all heat pump options demonstrating reduced energy usage to gas boilers, with 100% fossil fuel carbon savings. However, with current energy prices, savings in running costs are not realised until comparing traditional (atmospheric) gas boilers to low-temperature heat pumps.

Syntegral can assist you in selecting the system most suited to your needs, whether for carbon reduction, budget, or cost savings, please get in touch with us on 0121 0474 233 or