Does Size Matter?

Yes, size matters a lot! But perhaps not in the way most homeowners think.

The biggest mistake is assuming bigger = better. With heat pumps, the goal is to install a unit that closely matches the building’s heat loss and heating demand. A correctly sized heat pump will generally outperform one that is too large or too small.

If the heat pump is too small

It may struggle to maintain indoor temperatures on the coldest days and run continuously at maximum output. This will result in higher electricity consumption and increased wear on components.

If the heat pump is too large

It will short cycle which will reduce efficiency. Starting and stopping will cause the compressor to wear prematurely and reduce the lifecycle of the heat pump. A compressor draws more electricity at start up than it would running at a lower rate for longer periods.

Modern inverter heat pumps change the picture

Most current air source heat pumps, such as Vaillant aroTHERM Plus, Viessmann Vitocal, Panasonic Aquarea and Grant Aerona R290 can modulate their output over a wide range. This means a modest amount of oversizing is usually less problematic than it was for older fixed-output systems. However, even inverter driven units have a minimum modulation limit. If the heat pump’s minimum output exceeds the home’s heat demand during mild weather, efficiency can suffer due to cycling. That’s why it’s important to hit the sweet spot when designing a system.

What actually determines the right size?

A proper heat loss calculation should consider:

·       ODT (outside design temperature) which can vary quite dramatically in Aberdeenshire

·       Floor, walls, ceiling, roof areas including building fabric/ insulation levels

·       Performance of windows and doors 

·       Air leakage or air changes per hour (ACH)

·       Desired indoor temperatures

·       Domestic hot water requirements

Industry guidance for heat pumps must facilitate a full room-by-room heat loss calculation determined by the Domestic Heating Design Guide from the Chartered Institute of Building Service Engineers (CIBSE).

Example...

If a house has a design heat loss of 7kW at -4.5C

·       A 5kW heat pump will be undersized

·       A 7-8kW heat pump would be ideal

·       A 12kW heat pump will fulfil heat demands but will short cycle more in spring and autumn if it can’t modulate down to a low enough level. 

There are occasions when a smaller heat pump maybe just on the cusp of meeting heat demand. On such occasions it is pertinent to weigh up the pros and cons of efficiency over comfort, especially if the customer has a secondary heat source like a wood burning stove. 

The ODT is set to the 99th percentile, which means for 1% of the year, the heat pump is required to meet the household demand at this temperature. In the case of secondary heating with a wood burner, most customers would light the fire in these conditions anyway. This would then make up the shortfall for the three and half days it’s required and reduce running costs with a smaller heat pump for the other 362 days.

Importance of correctly designing the system

There is no one size fits all when it comes to heating systems. This is why it is critically important for the design to be done by a qualified heating engineer and NOT a salesman! I have encountered occasions where my competitors use a copy and paste quoting process and have seen properties with a heat loss of 4kW being quoted for a 12kW heat pump. If the layperson doesn’t know any better, they will go with what they think is the best option and its human nature to think if you’re getting a bigger unit for a decent price, it’s a bargain. This is why it’s important to do your research, read the reviews and choose carefully before committing to your chosen installer. 

Size matters. Make sure it’s the correct fit and don’t be left with regrets and a high heating bill.

Robbie