Step 5.2: Downsizing

Owned by Dennis De Beuckeleer
Last updated: Oct 20, 2022
5 min read

What is downsizing?

Downsizing is lowering the temperature regime of the end units. Most heating systems are designed with oversized end units (e.g. radiators) because of safety margins. When supplying an end unit with a lower supply temperature, the maximum heat flow that the end unit can deliver also decreases. When the heat emissions system is oversized, it does not matter that the total heating capacity of the emissions system drops. You can for example change the temperature regime of an 80/60°C radiator to 60/40°C without having an impact on thermal comfort, depending on the original safety margin of the radiator

Why is downsizing useful?

When lowering the temperature regime of the end units, lower supply temperatures are required. This is beneficial for the addition of low-temperature heat production units (e.g. heat pumps) to an existing system. A lower temperature regime also lowers the return temperatures, which has a positive impact on boiler efficiencies.

When is downsizing applicable?

To check whether downsizing is possible in a system, two parameters need to be known:

  1. System peak load (measured on an hourly, half-hourly, or quarter-hourly basis)

  2. The total installed capacity of end-units

When the capacity of the heat emission units is much larger than the peak demand, downsizing is possible.

Illustration

The figure below shows a load duration curve of an installation in blue. The most left point of the curve shows that the maximum demand over a full year is approximately 1000kW, indicated with the green horizontal line. The total installed capacity of end-units equals 2000kW, shown by the orange horizontal line. This means that this system is oversized with a factor two, and hence, temperatures can be lowered while still ensuring thermal comfort.

Illustration of when downsizing is applicable

How to downsize?

Downsizing is done by changing the temperature regime of the end-units, however, before you can start lowering the temperature you need to know to what supply temperature you can lower down. The easiest way to do is is to lower down the supply temperature setpoint of the boiler through a sensitivity analysis.

At a certain point, the boilers will not be able to provide enough heat and the 'building load space heating” will drop down. At this temperature, the drop in building load means an inability to provide the necessary heat. In the example below the fourth simulation significantly drops down. The temperature for the third simulation is therefore chosen as supply temperature.

The temperature regime change needs to be executed by using the ‘Change regime and power’ function in Hysopt (shortcut: Ctrl+Shift+R), which can be found in the ‘Operations’ menu ().

IMPORTANT NOTE: the physics behind the change regime and power function are very specific to radiators only. Therefore, you can not use this calculation for air handling units and domestic hot water.

Before clicking this function, make sure all end-units that need to be downsized are selected. Then, the pop-up window below appears, where three parameters are required: the target supply temperature, the target return temperature and the target room temperature. After clicking the ‘Apply’ button, the downsized end-unit heat capacities are calculated by Hysopt and the downsized temperature regimes are applied. Finally, it is important to redo the ‘Compute design flows’ step to recalculate all powers, temperatures and volume flows. Once the temperature regime of the end-units is lowered, also make sure to lower the heating curves and/or temperature setpoints in the system.

Tip: If you want to avoid changing pumps or having to rebalance the system (e.g. for an existing system), it is recommended to maintain the original design volume flows when downsizing. This can be done by finetuning the delta T between the target supply and return temperature. How to do this, is shown in the illustration below.

Illustration

In this example, there is an equivalent radiator of 100kW, which is the top radiator in the figure below. The corresponding zone only has a design load of 60kW, which means there is a potential to downsize. The original end-unit has a temperature regime of 80/60°C and a volume flow of 1.23l/s.

In a first attempt, the radiator temperature regime is changed to 65/45°C, which results in a downsized end-unit capacity of 61.71kW. The new volume flow is calculated at 0.75l/s, which is a mismatch with the original volume flow. This downsizing attempt corresponds with the middle end-unit in the figure below.

A higher volume flow is desired, so the delta T between the supply and return temperature needs to be decreased. To have a heat capacity just above the zone design load of 60kW, the supply temperature is also lowered in the second attempt. This results in a new temperature regime of 62/49°C, witch an end-unit capacity of 64.25kW at a volume flow of 1.20l/s. In this case, the volume flow of the downsized end-unit is close to the original volume flow, which was desired. The radiator is now successfully downsized from a 80/60°C temperature regime to a 62/49°C regime, which results in a decrease in heating capacity from 100kW to 64.25kW, while maintaining the volume flows constant.

After changing regime and power, you always have to redo your loadmatch. Note that this is why you need to do the preliminary check for supply temperature using the sensitivity analysis. (this is only applicable using the percentage loadmatch method).

How to verify downsizing?

After executing a downsizing, it is recommended to verify that this were done correctly. It is important that the change in temperature regime has no impact on the thermal comfort in the building and that the same amount of heat is emitted into the building. After executing a one-year simulation, this can be easily checked in the ‘Pareto analysis’ or by exporting the ‘Energy report’. You want the 'building load space heating' to stay approximately the same before and after downsizing. If this is the case, downsizing was justified and executed correctly.

In order to make this process more streamlined, you could use the aforementioned absolute loadmatching method, where the relative value (in the example above, 60% for a 60kW design load) is filled into the absolute, meaning 60kW in this case. This will create a similar load on the radiator, even though the design conditions may have changed.