Why size matters in air source heat pump design and installation

Why oversizing or undersizing a heat pump happens

Oversizing or undersizing a heat pump is often the result of errors being made in the initial heat loss calculation or by using so-called ‘rule of thumb’ data to make inaccurate assumptions about the property.

 
The unique factors for each property, such as the fabric makeup and their associated U-values, as well as heat lost through ventilation, can significantly change calculated heat loss. Historically, many heating engineers became accustomed to oversizing boilers, particularly with combi boilers now being so popular in the UK, with the thought process being that it would be better to have too much capacity than not enough. However, this approach doesn’t translate to heat pumps and can cause significant inefficiencies.

Each manufacturer uses a slightly different naming convention for their heat pumps, resulting in difficulties in heat pump selection. It is important to ensure that the selected heat pump’s capacity is achievable at design conditions. 

Any specific preferences by the homeowner should also be considered. For instance, sizing the heat pump based on an internal temperature that is lower than what they prefer it to be at may result in an incorrect size for those requirements, causing confusion and frustration that the heat pump is not performing as expected. 

What happens when a heat pump is oversized

What happens when a heat pump is undersized

An undersized heat pump will struggle to meet the demands expected of it, leading to homeowners using more energy and potentially having higher bills. It also affects the comfort level in the home when it comes to internal temperature.

 
There may also be an added (and unnecessary) reliance on backup systems if the heat pump is unable to meet demand. 

Carrying out heat loss calculations correctly

As we mentioned at the start of this article, the issues from both oversizing and undersizing an air source heat pump stem from errors when carrying out a heat loss calculation or using generic values rather than data from a precise site survey. Similar issues arise from misunderstanding the homeowner’s exact requirements and not taking the heating environment as a whole into account.

An accurate heat loss calculation works out the exact amount of heat that is lost from the property through a room-by-room calculation. This works out two types of heat loss: fabric loss (through doors, windows and floors) and ventilation loss through draughts. Certain materials are more efficient at retaining heat than others, particularly when insulation measures such as cavity wall insulation are in place. Area and volume measurements of each room will also need to be conducted, as these are key variables that will affect the final calculation.

There are many heat loss software tools available that can be hugely beneficial in getting an accurate heat loss calculation for the property and ensure the unit is the right size for the exact needs.

As part of our Baxi MCS Umbrella Scheme, we can support you by carrying out the site survey including the heat loss calculations taking the project from initial design to customer handover. This helps you ensure a fully compliant installation and enables access to grants such as the Boiler Upgrade Scheme (BUS). To get started, click here.

Correct design and UK design temperatures

In the UK, design temperatures for an accurate heat loss calculation are calculated using local weather data from weather stations. Design temperatures vary based upon location. Factors such as altitude, exposed property areas and coastal locations all need to be considered. Internal design temperatures are based upon latest industry guidance along with any unique customer requirements.

 
The MCS website has a table of design condition temperatures with data obtained from the CIBSE to provide accurate design outdoor temperatures. This resource can be used alongside the data provided for room temperatures on the same page.