AIR SOURCE & GROUND SOURCE HEAT PUMPS
A hybrid approach to heating and hot water
Combining powerful hot water performance with renewables is just one way to help move our industry – and the UK more generally – towards the ultimate goal of Net Zero emissions, writes Silviu Catana, specification manager at ELCO Heating Solutions
A
lthough the phrase was first coined in a philosophical discussion about the problem of evil, “the best of all possible worlds”, would actually be a pretty apt description for how
engineers and specifiers are combining air source heat pumps (ASHPs) with commercial condensing boilers in non- residential situations. It’s true that combining different sources to generate heating and hot water isn’t new, but the winning combination of renewables with high efficiency condensing boilers to fully meet demand, is reducing energy use and emissions in a wide variety of applications across the UK. The reasons behind these hybrid approaches
are simple. Often commercial buildings – whether offices, public buildings like libraries, schools or healthcare establishments – have large heating and hot water peak heat loads that can sometimes be challenging to meet. Given this high level of demand, simply utilising heat pumps may not be enough, or indeed cost effective from an installation capital cost point of view.
Using a bivalent approach gets over the issues of performance, by combining ASHPs, backed up by gas condensing boiler(s) to ensure the entire heating load is met – whatever the outside temperature. The ASHP is sized to meet the majority of a building’s energy requirements, while the condensing boiler “kicks in” when a low threshold is met. From a practical perspective, the bivalent switching point is set at the design stage and the boilers are switched on when an outside air sensor, connected to a Building Management System (BMS), detects the predetermined temperature. These hybrid systems are already showing outstanding results in projects across the UK. For example, a Derbyshire academy school recently refurbished its entire heating system by combining two 48kW AEROTOP M commercial heat pumps with two 120kW THISION L ECO wall-mounted condensing gas boilers from ELCO. BSN group’s senior contracts manager, James Robinson, who was responsible for the project was very happy the results, saying it: “…will make a dramatic impact to the day-to- day running costs of the academy. The new heat pumps and boilers provide the best of both worlds, delivering optimum efficiency and
sustainability throughout the year, while the installation was incredibly simple. The project is contributing to a combined carbon saving of 134 tonnes per year.”
Of course, it’s important to plan and design such systems carefully. When heat pumps are added into existing heating systems to create hybrid or bivalent versions, they will only be successful when they are weather- compensated, as well as hydraulically prepared, by having the bypasses removed and the mixing valves and modulating circulator pumps installed. The reason for this is to make sure the flow temperatures required in the system drop when the outdoor temperature increases. It is fundamental to make sure that the system’s two distinct heat sources avoid high return water temperatures in the weather compensated circuits. If not, there is a risk of
the heat pump being bypassed and the entire heating load taken on by the condensing boiler. This is a much more holistic approach in transforming an existing or traditional heating system into a modern future ready one, that includes renewables.
If we look a bivalent system in a non-residential situation from a design point of view, how would it work? It should be designed in a way that the heat pump(s) would carry 100% of the heating load for mild conditions above +4°C. This would ensure top line efficiencies for well over half the heating season, resulting in an attractive SPF (seasonal performance factor), even at flow temperatures of 50°C. Then, when temperatures fall to between -2°C and +4°C, a bivalent operation can be used, so the system calls on both the heat pump and a high efficiency modulating gas condensing boiler to work together. This allows a heat pump to deliver the same COP, provided its pump’s flow temperature is also reduced. And finally, should temperatures dip below -2°C, at which point the ASHP’s COP drops significantly, a condensing gas boiler can deliver the entire heating energy – maximising system efficiencies. So, what’s important to remember when specifying or designing a hybrid system that combines heat pumps and condensing boilers? Well, a system like this can be more complicated than one with just a single heat source. Therefore, to ensure success, it would be wise to use heat pump and boiler products from the same manufacturer. The benefit of this is that it will allow both types of appliances to communicate efficiently by ensuring they “speak the same language”, while enabling consultants to seek advice from a single supplier on how the products can be best matched to the design requirements of the project. Additionally, by utilising lightweight, low water content boilers, a hybrid system can be installed on rooftop plantrooms for complete flexibility. The UK journey towards Net Zero and a decarbonised economy may not go in straight lines or be without challenges; however, having an innovative approach that partners tried and tested technology in ever more efficient and effective ways, and utilises a partnership and hybrid approach, will certainly help us to get to our desired goal.
18 BUILDING SERVICES & ENVIRONMENTAL ENGINEER SEPTEMBER 2022
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