09 July 2024
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High temperature heat pumps can provide greater options for building owners to decarbonise heat in existing buildings, but they should not be considered a silver bullet, says Baxi’s Ryan Kirkwood.
Investigative engineering and optioneering are key success factors for optimal heat pump performance and value.
As building owners review boiler replacement projects and the achievable options to decarbonise heat, a hybrid heat pump solution can nearly always be a fast, efficient and affordable approach. This approach can enable a large portion of the heat to be decarbonised while overcoming some key challenges such as budget and risk.
But what if the client brief is to move straight to full electrification of heat? Are high temperature air source heat pumps (HT ASHP) the silver bullet to a straightforward swap out of boilers for heat pumps in existing buildings?
Unfortunately, the solution is not as simple as it is sometimes made out to be.
Operating temperatures
The main challenge with decarbonising heat in older buildings is that older systems typically operate at flow and return temperatures of 82/71°C or potentially 80/60°C, with the emitters sized accordingly.
We know that the most modern of R290 (propane) ASHPs, like our own Baxi Auriga+ range, can deliver up to 80°C flow temperature. However, 80°C, which is at the very top end of the performance envelope, falls slightly short of the 82°C flow required by 82/71°C circuits. What’s more, the coefficient of performance of HT ASHPs falls off at higher temperatures, affecting efficiency and subsequently operating costs. A further consideration is that most HPs prefer to operate in the 5-10°C ΔT range, making a straight swap on 80/60°C circuits not impossible, but challenging.
Understand the existing system
So how to ensure best performance and value from HT ASHPs? Understanding the system fully before designing a hybrid or standalone ASHP system is key.
To do this, we need to know the actual heating demand at a system circuit level. As design information for dated buildings is limited at best, we recommend carrying out a significant amount of investigative engineering at the outset.
This should include (but not be limited to) the original design temperature and loads, any hydronic inefficiencies, changes to the building and/or heating system and any bounding constraints. Understanding electrical capacities, whether the budget allows for standalone ASHPs, and any factors that could offset the potential higher running costs, such as PV, will also influence the design.
At this point, installing ultrasonic heat meters, undertaking a full heat loss calculation where possible and utilising data such as gas meter readings will provide better insight into the building profile.
Optioneering
With real measured data, heat experts can engineer different solution scenarios. This optioneering is a valuable process that allows clients and designers to make the best choice for a specific building within the project parameters.
For example:
- Reducing flow temperatures by fixing hydronic inefficiencies
- Bracketing of the heating system to avoid running all circuits at 80°C or 82°C all year, improving running costs
- Solving spatial challenges by sizing real requirements through measuring and calculation.
Bracketing
Let’s consider in more depth the bracketing option, which involves consolidating the heating system into frames of known and weighted data.
For example, if the survey data shows a sizable CT (constant temperature) circuit serving an air handling plant exclusively, one option might be to “bracket” this out of the overall heating system.
Serving it directly from its own heat pump plant would allow a change of the tempering or reheat coils to suit a 55°C f low temperature (or lower). This decision alone could increase the heat pump efficiency up to 150% from the current design temperature of circa 80°C.
The same principle can be applied to VT (variable temperature) circuits when the CT circuit is unable to deviate away from its current design flow temperature.
VT circuit bracketing can deliver considerable efficiency rewards as the weather compensation can now be done directly at the plant without the use of mixing valves.
With direct weather compensation on HT ASHPs, the flow temperature can now range from 35°C-80°C. If heat losses have been carried out, emitters may be changed when and if possible, to allow a more aggressive curve.
The portion of the year the HT ASHP(s) must remain at 80°C flow may be offset in terms of net efficiency by the portion of time that flow temperatures are not required at 80°C via direct weather compensation.
The weighted aspect of bracketing involves understanding the split in capacity required for each circuit. If VT equates to 80% of the overall load requirement, then addressing that purely in isolation with CT remaining on 80°C flow may impact the overall efficiency of the building sufficiently without the need to replace AHU coils.
Building up the solution
Figure 1 shows a typical 2 boiler reverse return header set up with a CT and VT circuit aligned to most 82/71°C legacy designs. Weather compensation is done directly to reduce plant load rather than overall primary generation temperatures. With HPs, standalone or hybrid solutions, the goal is always to directly weather compensate to reduce HP generation temperatures.
Even a modest reduction in flow temperature will ensure higher efficiencies.
Most HT ASHPs would achieve a coefficient of performance (COP) of approximately 2.2 at 65°C flow and -2°C ambient conditions.
High temperature requirements (DHW generation for example) should be isolated, when possible, to allow primary temperatures to track compensation curves directly.
Dropping the weather compensation to below 60°C will allow for a blend of Medium Temperature (MT) and HT ASHPs, potentially reducing any siting or budget complexities that a full HT solution may struggle to overcome.
In the blended MT/HT example solution shown in figure 2, the HPs are cascaded with a three port divert valve being used to deliver heat to the calorifier.
Typically, the MT ASHP(s) would act as lead for the directly weather compensated circuit supported by the HT ASHP(s) during peak demands. When higher temperatures are required for more challenging design conditions the HT ASHP can ramp the thermal store up to 80°C.
If figure 1. (reverse return) had been designed on a ΔT of 20°C, one solution is alternate cascading method shown in figure 3. This utilizes the thermal store lower and upper portions to cascade temperature rather than load, an identical philosophy used with most hybrid solutions.
Temperatures are still key, and with good weather compensation a blend of MT and HT ASHPs can still work. However, this solution is more suited to HT ASHPs as at higher design temperatures of 80/60°C MT ASHPs are unable to deliver any useful heat.
Perfect is the enemy of good
One area that has not been discussed until now is ‘fabric first’. As space constraints are difficult to avoid, precision engineering is essential to ensure the HPs are not over-specified in terms of kW capacity. Considering fabric options at the outset of every heat decarbonisation project is therefore vital to reduce heat losses and heat demand.
Where fabric upgrades are not an option, designing a perfect solution that delivers in terms of performance and value is almost impossible without compromise. However, building an investigated and measured thermal profile of real usage allows for a clearer understanding of the impact that each of the solution phases will have.
At Baxi, we work closely with designers and engineers to break down the complexities associated with decarbonisation and provide complete engineered solutions and technical support to overcome the challenges each project brings.
Together we can provide our customers with the support they need on their journey to decarbonise heat.
