Boiler retrofit to Heat Pump

We all know that buildings which use Gas or Oil boilers will need to change to Heat Pump technology as the world demands the elimination of fossil fuels. The Irish government has committed to an ambitious 51% reduction in 2018-level carbon emissions by 2030. This will require a dramatic reduction in energy consumption, which is being supported by the SEAI through education and grants, as HVAC in buildings is the biggest energy consumer in our cities.  

It is the job of the Building Engineers to design the solution for these retrofit upgrades to clean energy with Heat Pump technology.

However it is not as simple as just swapping out a boiler for a heat pump.

At EICL.ie, we are constantly getting inquiries about these retrofits so I’ve decided to give you a brief overview of some the concerns that we’ve encountered and the solutions that we’ve proposed for these concerns.

I’ve broken these down into five mains points:

1)           The main concern that comes up in almost every conversation is about the operating temperatures.

Most boilers were selected to operate with a supply temperature of 80C and a return temperature of 60C and they satisfy the demands of both the Domestic Hot Water (DHW) and the space heating via radiators, high level radiant panels, Fan Coil Units (FCU), Air Handling Units (AHU) or underfloor heating (UFH).

Most Heat Pumps are not designed to not operate at such high temperatures. For example the maximum temperature for the Aermec NRK range (which uses a special refrigeration compressor and a common refrigerant) is 65C outlet. There are some alternative Heat Pumps with alternative refrigerants (such as CO2), but there are risks associated with such refrigerants that most companies would rather avoid, especially as this high temp may not be needed.

2) The second concern is about the efficiency of heat pumps v’s boilers.

3)   Another other issue that is often asked is about the flow rates and delta t (differential temperature between the flow and the return).

4)   The other issue that must be addressed is the location of the new equipment. 

5)   What size boiler is needed?

So let’s tackle these one by one…

1)           Do we actually need 80C?

Sometimes you may, but I believe it’s fair to say that you don’t need this high a temperature for the vast majority of the time you are heating. Remember the boilers are generally sized for the max possible demand and often oversized. Even if they aren’t oversized, the actual demand for heating is rarely anywhere near its peak load.

Many such traditional boilers would have a ‘weather compensation / dynamic setpoint’ which reduces the setpoint as the ambient temperature drops automatically.

Consequently we know that the max temp of 80C is typically not required for a vast period of the year.

Top tip: if you have the opportunity to reduce the hot water setpoint to 60C during the winter and the building temperature is maintained, you know you don’t need 80C and a heat pump will work for you.  

If you find you do need 80C, then the next question is how to satisfy the demand when the ambient temperature drops in the winter and the heat pump can’t deliver a water temperature over 65C?

Well there are a few options…

The actual capacity (heat load demand in kW) of the high temperature will determine what is the next step. If the load is quite low and only required let’s say once per week (i.e. to eliminate legionnaires disease), you could probably satisfy this demand with an electric emersion element in the calorifier/s or DHW storage tanks (i.e. if used in conjunction with a PHE hot water generation system). The electric element would be programmed on a timer to come on once per week for 1hr and thermostatically controlled.

For much higher loads, you could use a ‘Water to Water Booster Heat Pump’ such as the WWB from Aermec. The WWB takes its source from the lower temperature heating system (such as from an Aermec NRB-H) and using a refrigeration circuit it delivers a much higher temperature (up to 80C) to the supply side.

It’s very important that the source side of the WWB must be added to the primary (heating circuit) load so it is accounted for in sizing on the NRB-H.

If you are using a Multifunctional Heat Pump (for the space heating and cooling) such as the Aermec NRP the potential for energy saving is far greater as the source is potentially free when the NRP is in cooling mode. If you are not familiar with Multifunctional Heat Pump technology, just see my technical vlog where I describe the various modes of operation here ….

Another option is to use the old boiler as a means to satisfy the high temp once per week, but this is not an ideal long term option considering the desire to eliminate the fossil fuel usage.

If you are going to leave the boiler connected, the system design is very important for a number of reasons such as to avoid high temperatures going to the Heat Pump (which will cause high pressure alarms), but also for efficiency. If you are designing a retrofit system such as many of the schools and colleges shall be, just call the EICL.ie HVAC Solutions Team on 01 8255155 or email [email protected] and we shall send you a sample schematic of how you can achieve this efficiently.  

2)           Heat Pump efficiency

Air to Water Heat Pumps (the most common type) take energy from the air so when the ambient air temperature drops the capacity and efficiency also drops.

I’ve made a few selections using the Aermec Magellano (Eurovent certified) software to show you the drop in capacity and efficiency as the ambient drops:

NRK0330HEJ00                                                                            

Supply @ 65C, return @ 60C                                                                  

Ambient temperature    capacity              input power      COP                     SCOP    SCOP @ 35C

7C         83.2kW 34.1kW 2.44                     3.03      3.68

6C         80.2      33.4      2.4                       3.03      3.68

5C         76.2      33.3      2.29                     3.03      3.68

4C         71.8      33.1      2.17                     3.03      3.68

3C         67.7      32.9      2.06                     3.03      3.68

As the capacity and the efficiency drop somewhat in proportion to the ambient, there is an argument that the old boiler may actually be more efficient when we encounter low ambient conditions. Consequently you may decide that the Heat Pump does not need to be sized for the total load and you may only use it when the ambient is above say 2C. Consequently you may assume a capacity of let’s say 90%, but I’ll discuss the capacity sizing in more detail below. 

You will notice even with a drop in efficiency (COP) as the temperature drops, the efficiency is still far greater than that of an oil or gas boiler, so this is what you’d call a ‘no brainer’.

3)   Heat Pumps cannot operate with a 20C delta t.

Most Building Engineers should have the formula Q=M.C.DT tattooed in their brain as it is the most used formula that they will need for calculating either loads, flow rates or the delta-t.

With the knowledge that a heat pump can’t operate with a 20C delta-t (max 10C allowed) and an understanding of this formula you will immediately realise that with a lower delta t, you will have a much higher flow rate.

If you are doing a retrofit (as opposed to a new installation) and the pumps and pipework have been sized for a high dt and low flow rate you may believe that the system is not compatible with a heat pump…but there is a simple solution…

All you need to do is separate that heat pump from the heating circuit using a hydraulic separator (a decoupling tank) so we can satisfy both the lower flow rate and higher dt on the heating circuit from the higher flow rate and lower dt on the heat pump circuit.

Please note when I say separate, I am not talking about using a PHE. It is the same water that circulates on both sides, just at different flow rates and dt’s.

The type of decoupling tank is very important to ensure that you eliminate ‘short circuiting’ on each loop (primary and secondary), such as the Fiorini VKG (indoor) or VKGE (outdoor) range.   

4)   Location of the Heat Pump

Boilers are located in plant rooms, but heat pumps are typically located externally. There is the possibility to locate the Heat Pump internally if the air inlet or/and outlet are ducted, but you will need adequate space for this ducting and you must ensure the inlet and outlet are adequately separated to avoid recirculation of the air.

The other concern with regard to locating the heat pump internally is the type of refrigerant being used. Many new low GWP refrigerants are deemed flammable so there are risks associated with indoor location.  

Consequently most Heat Pumps will be located externally, so you must have the space to house them outdoors and with adequate space for servicing and for the air flow rates. This is called the ‘minimum technical spaces’.

You should also be considerate of the noise that will be created from locating the Heat Pump externally, although they are a lot quieter these days. A data sheet will be provided with all Heat Pumps, so you can explore if this is an issue for the locality or not.  

Obviously you shall need to be able to route the heating pipework to the heat pump. This is typically ran directly form the boiler house to the external heat pump.

Finally it is very important that you install a buffer tank. A Heat Pump is a refrigeration system and like with all chiller or heat pump systems, you need volume for a number of reasons.

These include to prevent short cycling which would reduce equipment life span and reduce efficiency. Inertia volume will also stabilise the temperature of the water to/from the heat pump. The buffer tank and decoupling tank (mentioned above) can be the same unit.

5)   What’s the boiler heating capacity?

The nameplate is often either gone or doesn’t state the capacity of the boiler. Many boilers can do a range of capacities and a specific burner jet is fitted during the installation to suit the building.

If you don’t know the capacity, just record the gas usage and send it to us on [email protected] and using a calorific value we will calculate the capacity for you.

In addition, it would be prudent to cross reference the capacity by using a w/m2 calculation. The figure you choose can vary greatly depending upon the fabric (heat losses) of the building. Let’s say for example you select 55w/m2 and you have a 1,000m2 building, the capacity required would be 55,000 watts or 55kW. A building Engineer would need to investigate the value to be used in this calculation based upon the building fabric.

If you decide upon more than one heat pump (with min two compressors p/circuit), perhaps using the 2/3rds rule, I would strongly recommend using the Aermec MultiUnit Sequence Controller which will optimise the efficiency of the heat pumps. Request a link to my vlog if interested in learning more. 

Another consideration could be that you may wish to introduce an element of fresh air to improve the indoor air quality (i.e. for heavily occupied school classrooms).

If you do decide upon this route, then it would be sensible to introduce a Heat Recovery Unit (HRU) such as the Aermec RPF range, as this will reduce the amount of heating required for the supply of the fresh air.

You should assume approx. 10 l/s per person to calculate the air volume flow rate required.

As always a Building Engineer should be consulted for any of the above.     

Summary:  

In summary, as you can see it is absolutely possible to retrofit a boiler with a heat pump, so let’s all do our part to reduce the use of fossil fuels and consequently reducing the environmental impact from these buildings, by changing to clean green Heat Pump technology. 

If you are a Building Engineer with a boiler retrofit project and would like the support of the EICL.ie HVAC Solutions Team, just call or email us now on:

Tel - 01 8255155

Email - [email protected]

EICL.ie are the exclusive Irish distributor for the world renowned Italian manufacturers Aermec, Hitema and Fiorini.

At EICL.ie, ‘We don’t sell boxes, we sell solutions!!!’

I am happy to share this information freely to highlight our companies (EICL.ie) philosophy which is ‘to help everyone reduce their carbon footprint and the consequences of climate change’.

I drafted this today 10th May 2021 purely from my industry knowledge which I hope you will find of some use. It should not be considered absolutely conclusive, especially as technologies are consistently evolving, but it should give you a good insight into the possibilities for retrofitting of boilers. Consequently all information hereby provided is given in good faith but without responsibility. I must reiterate that a Building Engineer should be consulted for any of the above.     

Peter McMahon is the CEO and founder of an Air Conditioning and Heating systems company EICL.ie which have a strong team of Building Engineers who have delivered solutions for some of the most iconic buildings in Ireland such as Salesforce, Pfizer and the new Central Bank of Ireland.

Peter is a qualified Refrigeration Craftsperson (C&G207), Refrigeration Technician (C&G257), a Building Engineer (B Eng Tech), Council member of the Institute of Refrigeration Ireland (IRI) for many years, Guest lecturer for level 8 students at Technical University Dublin and an Engineers Ireland CPD approved Registered Training Provider with 33 years industry experience and who currently delivers CPD presentations to Consulting Engineers on the latest HVAC technological innovations and system design and control methods.

The latest of which is a 3-part series titled ‘Designer Training Program – the evolution of hydronic system design’ which builds a hydronic system step by step with the use of schematics to satisfy space heating, space cooling and DHW.

You can book Peter to deliver this CPD series completely FOC if you are a building engineering consultancy based in Ireland. All others must incur a fee. For bookings or further details just contact Amber at [email protected]