Prioritising performance from hybrid heat pump systems


30 March 2022
Prioritising performance from hybrid heat pump systems Images

A solutions-based approach is essential to ensure high-performance and maximum efficiencies from hybrid heat pump systems, says Ryan Kirkwood, Baxi’s Heat Pump Business Development Manager.


Greater collaboration is instrumental in avoiding conflict between the technologies and achieving optimal outcomes

By now, everyone is hopefully aware that we must change the way we heat buildings if we are to achieve our ambitious 2050 net zero target and climate goals. As energy prices soar, the drive to heat buildings more sustainably will also assist businesses in mitigating the spike in costs. But designing the future of heat requires increased collaboration so that we can successfully prioritise performance – and practicality – every time, particularly in older commercial building stock.


Take heat pumps. In its Heat and Buildings Strategy, the government recognises both heat pumps and energy efficiency as key focus areas in the short term for decarbonising heat, while longer-term technologies like green hydrogen scale up.


Certainly, the credentials of heat pumps to deliver low carbon heat in newer, well insulated commercial building stock are well established. And as the UK’s electricity grid continues to decarbonise apace, heat pumps will increasingly deliver close to zero carbon heat in buildings like these.

We see Air Source Heat Pumps (ASHP) as the most popular and cost-effective choice of heat pump and are pleased to add our new Remeha E-HP AW ASHP range to our expanding portfolio of sustainable heating and hot water solutions.


Powered by the UK’s rapidly decarbonising electricity grid, air pumps can provide a highly efficient, sustainable method of supplying low carbon heating or indirect hot water requirements for a range of non-domestic buildings.

But here’s the problem: it’s unrealistic to think that we can engineer all projects and budgets fully with just ASHPs at present. In older, poorly insulated commercial buildings, air filtration, physical space, available electrical power and/or CAPEX budget are just some of the restrictions typically encountered.


Hybrid Systems

So rather than wait, how do we design around the limitations imposed currently? One option is to deploy a hybrid solution. Utilising existing or new high efficiency combustion fuelled appliances with heat pumps is time proven to decarbonise a large portion of the annual heat demand.

A hybrid system working in perfect balance should only use condensing boilers to overcome limitations imposed by operational factors and to ensure building performance at all time.

If a hybrid boiler and heat pump set up is deemed optimal for the building type, then we must be clear on a few key items.


Firstly, we calculate a sizing proposal that maximises contribution performance while taking all limitations into account.


Limitations may be (although not necessarily limited to) physical space, available electrical power or CAPEX budget. These should not inhibit the use of integration but guide the sizing principles required to deliver best performance.

Secondly, we require hydronic integration with peak or back up heat generation that does not penalise efficiency or performance of the system.


Flow and return temperatures, ∆T’s, controls and ultimately the detailed design must be taken in careful consideration when blending the technologies. All too often we have seen a primary and secondary heat generation methods fight against each other, ultimately at the cost of system efficiency.


Mixing Deltas

Heat pumps provide a unique challenge in a hybrid system when paired with condensing boilers due to the temperature differential (∆T). Typically, heat pumps work best at low flow temperatures, and a ∆T of 5-10°C. While condensing boilers also operate more efficiently at lower temperatures, the ∆T range for a typical commercial boiler is 10-40°C.


The obvious approach may simply be to modify or design the system fully to a 10°C differential – an almost full circle back to the 11°C ∆T of older commercial heating systems. But this would remove all the advantages that larger differentials offer, such as pipe sizes, circulated volume and pump duties.

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Traditionally a LZCT generator would run lead in a baseload configuration with gas boiler(s) to assist as heat demands increased. ASHP hybrid systems can also be configured in this way, however care must be taken when the system ∆T is above what the ASHP alone can produce.


Running a full hybrid system on a 10°C ∆T (pipe and pump sizes aside) would not be problematic for most pre-mix condensing boilers. However, as we know, in order to benefit from maximum boiler efficiency, the return temperature must be kept under 54°C to promote condensing – no matter the ∆T.


One example of how the ASHP and boiler can operate together in a system where the temperature differential is above what the ASHP can provide alone is shown below (see Fig 1).


If we assume a 60/40°C flow and return temperature in this example and with a boiler and ASHP plant feeding into the same thermal store, the principle of operation is a two-stage process. The ASHP provides a portion of the temperature lift allowing the boiler to top up to target set point.


During periods of low or no load, the ASHP is able to iteratively charge the store using low carbon heat, up to a set point. This can be particularly effective with cascading heat pumps and thermal stores.

Under dynamic load conditions, care should be taken to ensure the selected boiler plant is capable in this instance of a 10°C ∆T. In some cases, an overshoot in set point can occur and therefore an introduction of a mixing valve to blend flow temperatures down on the demand side of the thermal store can help resolve this.


During periods high demand and when the thermal store is generously sized, we can boost the discharge pump to utilise the stored energy. Care must be taken that the thermal store is never allowed to fully discharge as this shall disrupt the stratification layer.


The controls should ensure that the boiler contribution is held back until absolutely required. This is achieved through close monitoring of tank temperatures, at multiple points, allied with boiler and ASHP temperatures.

Speaking of controls, we cannot stress enough how absolutely key this is ensuring hybrid systems perform. Our advice is to ensure that the controls strategy is aligned as early as possible in the project. Care should also be taken in the selection of the thermal store, to one that avoids mixing by making use of baffles, sparge pipes etc. 


Greater collaboration

As system design grows more complex, greater collaboration and increased sharing of our collective knowledge become increasingly important to prioritise practicality as well as performance.


Experienced manufacturers not only know their products but how to maximise their efficiency and effectiveness, so designers should regard them as a useful resource to utilise from the outset. Early engagement and collaborative working will make it easier to identify and deliver a solution based around each individual building and bespoke to its unique requirements.


Both hybrid heat pump systems and standalone, purpose designed ASHP systems offer the opportunity for important efficiency gains and emission reduction to drive heat decarbonisation in commercial building stock. With hybrid heat pump systems, considering the optimal operational design conditions of both technologies, including the hydronic design, is essential to maximise heat pump utilisation while maintaining system performance and overall efficiency.


By working more collaboratively from the outset and taking a more collaborative approach, we can ensure that both performance and practicality are prioritised so that the optimal outcome is achieved.