Heat Pump Kit Builder FAQs


What heat pump output do I need?
For an MCS certified heat pump retrofit the installer is required to do a room-by-room heat loss calculation measuring all walls, windows, doors etc. and calculating the heat loss to BS EN 12831. This will give an overall heat loss to be met. They may also include some allowance for hot water heating as this can be significant. We have included a ‘fag packet’ calculator to get to a reasonable number for a budget estimate but this in now way replaces doing a full and proper calculation. Whichever method is used a total heat load in watts is generated and you have to find a heat pump which has sufficient output to meet that load.
How do heat pump outputs work?
Heat pumps generally don’t produce the amount of heat on their nameplate and there is no convention established for what nominal output a heat pump labelled with. To further complicate this output varies with outside air temperature and water flow temperature off the heat pump. A heat pump producing water at 60°C while the air is -3°C will produce a lot less heat than if it was producing water at 35°C while the air is at -10°C. Different “5kW” heat pump can produce 4 or 6kW at design condition or anything in between. The output you probably want to know is the output ‘at design condition’ i.e. the hottest water you’re realistically expecting and the coldest outside temperature you’re designing for.
The good news is that this kit builder consults the manufacturers’ capacity tables to give the real output to meet your actual heat load. By default our kit builder looks at the output of a heat pump at 45°C flow temperature at -3°C external air temperature as this is a very common design condition. You can adjust it by clicking on the button as shown below.
What size hot water cylinder do I need?
The right size of hot water cylinder is to some extent a matter of choice. MCS produce a document with recommendations that can be found here, which has some recommendations. If the heat pump capacity is less than 10kW then a minimum of 45 litres per occupant is recommended, if it is greater than 10kW then a minimum 35 litres per person is recommended because the heat pump will be able to reheat the cylinder faster.The number of people in the house might be more than the number of bedrooms or it might be less, the person choosing the hot water cylinder will need to take a view on that. Often the cylinder is selected somewhat cautiously against this. The table below represents an approach of 45 litres + 45/35 litres per person and is appropriate in many cases.
If the bathrooms are particularly large with high flow outlets you may want to make an allowance for that. If the house has low flow fittings or Waste Water heat Recovery then you may feel comfortable using a smaller cylinder.
How does the logic work to assemble heat pump kits?
In a heat pump system there will always actually be more or less the same bunch of stuff. There’s a heat pump outdoor unit, something for it to stand on, some way to connect it to pipework, a main controller to take care of inputs, outputs and behaviour of every component, a thermostat or room controller, a hot water cylinder and a whole bunch of other little bits and bobs to tie it all together and make it all work.
The challenge of course is that these components can arrive built into or bundled with a range of other components. Often they arrive built into a pre-plumbed hot water cylinder, the heat pump outdoor unit or sold along with the heat pump. Some manufacturers will bundle a set of components into an install pack.
Once you have chosen a heat pump there are then a limited range of cylinders you’re likely to want with that heat pump, so we offer those to you in the kit builder in logical families of cylinders to suit different types of install.
Once you have a heat pump and a cylinder you know which components you’re getting already bundled/included and the rest of the kit just has to fill in the blanks to make a whole system. This might end up being many, many extra things to order or it can be only a few.
Sometimes there are options where you can do things more than one way (e.g. some heat pumps can have anti-freeze valves or glycol as preferred by the installer) or you might have a brand preference (e.g. on filters there are range available). Where we sensibly can offer options the kit builder does so to make it easy for you to get exactly the kit you want for the job.
What do I do if the kit I want to build isn’t there?
If the heat pump you want isn’t in our kit builder or the style of kit you want to build isn’t possible within the kit builder, don’t worry, the clever people who set up the kit builder logic are there to put together the kits for you for things we haven't yet put in the kit builder. Simply email saying what you want and we’ll get a kit list to you as soon as possible. If we get asked for something a number of times we’ll add it to the kit builder. If you really think we should include an option please let us know.
Is this the best price I can get?
If you’re a trade customer you may be able to get discounts from the price visible on the web. Please contact your branch or account manager with the product codes from the kit builder to get trade pricing.
What are all these bits in the kit?
Our kit builder shows you a whole series of headings: Heat Pump, Cylinder, Heat Pump Controller, Primary Pump, Buffer/Header, Secondary Pump, Diverter Valve, Feet/Mounting, Flexible Hose, Filtration, Expansion Vessel, Pressure Relief Valve, Filling Loop, Anti-Freeze Arrangement. All these functions are always there in all heat pump systems.

Heat Pump

This is the big bit that goes outside the house and, as a minimum, contains the compressor and fan. It is sometimes referred to as the outdoor unit. Often the heat pump will contain many more parts of the overall system including the primary pump, main controller, strainer and/or pressure relief valve. Sometimes other parts of the system are also bundled with the heat pump and sold as the same line on the order. Most commonly this will be some control components or filtration equipment.


Heat pumps cannot work like a combi boiler, they are too low in capacity for instantaneous hot water and too slow to respond. This means that if a heat pump system requires hot water it will need to be stored. Usually the simplest way to store hot water is to use a hot water cylinder. Cylinders vary from ‘naked’ cylinders where you have to plumb everything up on site through to fully pre-plumbed and pre-wired cylinders with nearly all system components on the cylinder. Pre-plumbed cylinders are faster to install and harder to get wrong, but are more costly. We’ve arranged compatible cylinders into ‘families’ which define the type of install the kit works for. Once you have the heat pump and cylinder selected this then defines what else is needed to make up a full kit.

Heat Pump Controller

The heat pump controller is a fairly big printed circuit board with a bunch of terminals that take input signals from system components like the room controller, cylinder probe and the heat pump outdoor unit itself and sends output signals to components such as 3-port valves and tells the heat pump what to do. These can be built into the heat pump, built into the cylinder, supplied with the cylinder or possibly a whole load of other variations. The kit builder will tell you how it arrives in any given kit.


We’ve called it a thermostat in the kit builder, but more accurately it should be called a ‘room controller’ normally you can schedule heating and hot water and adjust temperature setpoints for the thermostat at this controller. It’s usually best to use the manufacturer's controls if you can for smooth operation of the heat pump and optimum efficiency. Often bundled with something else.


A buffer serves 2 functions in a heat pump system. Firstly it provides hydraulic separation to allow for different flow-rate of water through the heat emitters and heat pump, this is especially important if the heat emitters are likely to be shut off for some reason by heating controls. Secondly it provides additional system volume to reduce cycling when the heat demand is less than the heat pump’s minimum output. In a lot of cases the heating system won’t require either of these functions and the buffer can be dispensed with. If only the additional volume is required then the buffer can be plumed up as a volumiser, essentially a section of fat pipe to add volume to the system.
A system without hydraulic separation is generally referred to as a ‘direct system’, the advantages of such systems is that the temperature of the water leaving the heat pump is definitely the temperature of water entering the heat emitters meaning the heat pump runs less hot for the same radiator output so efficiency is usually better with a direct system. Direct systems generally have to be run as substantially ‘open loop’ systems with as little intervention from zone controls or TRVs as possible. This lets the heat pump maintain the high flow that it needs and have a large area of heat emitter available at all times to let it output the heat it generates with a low water flow temperature.
This is all a design choice for the installer. Some of our kits are designed around buffers, especially those with a buffer integrated into the cylinder or where it is the manufacturer’s preference that a buffer be used. Some of our kits are designed around use as a direct system run as an open loop.

Secondary Pump

When you introduce hydraulic separation this means the primary pump doesn’t put any flow through the heat emitters so you need a secondary pump after the buffer to provide that flow. These pumps are only needed if there is a buffer used. If you want to use the buffer as a volumiser include the buffer, but not the secondary pump.

Diverter Valve

This is the valve that selects whether flow from the heat pump goes to the heating system or the hot water cylinder coil. Heat pumps generally run at lower temperatures when they’re doing heating than hot water and they modulate their output to suit the heat load. This means a heat pump cannot heat hot water and the house at the same time so these valves are always a diverter, not a mid position valve. These valves are often included with the cylinder, but not always.


Most heat pumps need to be mounted on rubber feet or a wall bracket to keep them out of standing water on the ground, to provide vibration damping and maybe to provide some height for condensate drain off. The kit builder provides a range of compatible options for each heat pump to suit your needs.

Flexible Hoses

Heat pumps generate a little bit of vibration while running; to prevent that vibration transferring to the pipework they are usually connected to the rigid paperwork with a flexible hose. Use of a flexible hose can also enable the heat pump to move a little without disconnecting it which can be useful for cleaning and annual servicing.


All heat pump systems need filtration to avoid any risk of particles clogging the narrow waterways in the heat pump’s refrigerant to water heat exchanger. Typically there will be some combination of vortex action, magnet and mesh in the filtration set up. This might be done with a strainer or filter ball valve and a separate system filter or use an all-in-one filter. Heat pumps have high flow-rates and filters by their nature can have large pressure drops so we carefully select only filtration options with low enough pressure drops that they shouldn’t harm system performance.
The following 3 components make up all the required parts of a sealed system kit.

Expansion Vessel

The expansion vessel is there to take up the thermal expansion of the heating system water when it is heated, it has to be large enough to take up all the expansion with an acceptable pressure increase in the system. The more system volume you have the bigger the vessel you require. 12 litres is usually fine for most houses, 18 litres is needed sometimes on larger homes and even bigger volumes can be required in extreme cases. Some heat pumps have an expansion vessel built in.

Pressure Relief Valve

The pressure relief valve is there to release water if the pressure in the heating system reaches an excessive level that threatens to damage equipment. It is not uncommon for this to be built into the heat pump - that way the manufacturer has control of this component that is critical to protecting the heat pump from damage from high pressures.

Filling Loop

The filling loop is to enable safe filling and top-up of the system using mains water. These are often included as part of pre-plumbed cylinders.

Anti-freeze Arrangement

Part of a heat pump system is outdoors so in case of a power cut or a tripped breaker coinciding with freezing cold weather could theoretically freeze causing damage to the heat pump. To avoid this nearly all manufacturers require some kind of protection from freezing in the system. Anti-freeze valves are commonly accepted by many manufacturers. These valves work by dripping water to prevent freezing when the water temperature gets below 4°C. Some manufacturers want the installer to use glycol, a chemical that reduces the freezing point of the system water. Whichever options are allowed by the manufacturer are included in the kit builder.

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