Engineering practice · Heat pumps
Article 02 / 10
A heat pump and existing radiators: what to check before replacing the boiler
‘A heat pump must have underfloor heating’ is too categorical a conclusion. The opposite claim, that any heat pump can be connected to any radiators, offers no guarantees either.
We would start by checking each room. Radiators can remain where they provide the required heat output at the selected flow and return temperatures. Heat losses and emitter capacity are the determining factors, not simply the building’s age. (SEAI — Heat pumps: system assessment and input data)
A 2 kW radiator — under what conditions?
Consider an illustrative radiator rated at 2 kW at 75/65/20 °C: flow, return and room temperature.
The mean temperature difference for a simplified selection is:
At 55/45/20 °C, it is just 30 K. At 45/35/20 °C, it is 20 K. Note that ΔT here means the difference between the mean radiator temperature and the air temperature, not the difference between flow and return.
For illustration, we will use Stelrad’s correction table: at ΔT30 the factor is 0,513, and at ΔT20 it is 0,302. This gives: (Stelrad — temperature correction factors for radiator output)
| Flow/return/room temperatures | Calculated radiator heat output |
|---|---|
| 75/65/20 °C | 2000 W |
| 55/45/20 °C | 2000 × 0,513 = 1026 W |
| 45/35/20 °C | 2000 × 0,302 = 604 W |
For a specific emitter, use its own tables or selection software. These factors should not automatically be applied to all types of heat emitter.
An undersized radiator does not mean rebuilding the entire heating system
Suppose a room needs 1,2 kW at the design outdoor temperature. Our illustrative radiator provides approximately 1,03 kW at 55/45 °C — slightly less than required.
The equivalent rated output required at ΔT50 is:
For this example, selecting an emitter with an output of around 2,34 kW at ΔT50 is therefore sufficient. In another room, however, the existing radiator may already have enough spare capacity.
That is why I would not recommend replacing all radiators before carrying out a room-by-room calculation. A sensible upgrade may involve only selected emitters, balancing and changes to the controls.
55 °C in freezing weather does not mean 55 °C all season
In milder weather, the building’s demand is lower. A weather compensation curve allows the flow temperature to be reduced, provided the heat emitters continue to maintain comfort. A lower required water temperature creates better conditions for heat pump efficiency. (iDM — the effect of flow temperature on efficiency)
However, the curve must suit the most demanding rooms. If one office is persistently underheated because its radiator is too small, raising the temperature for the entire property may be a poor way to compensate.
On-site testing complements the calculation
During the heating season, a trial at a lower water temperature can be agreed and the response of the rooms monitored. It must be arranged within the operating limits of the existing boiler and controls. Success in mild weather does not prove that the system is adequate at the design outdoor temperature.
Ultimately, the customer should receive more than a general statement that ‘the radiators are suitable’. They need a list showing what stays, what changes, which flow and return temperatures are required and when backup heating comes on.
For a heat pump, the label ‘radiators’ or ‘underfloor heating’ is not what matters. What matters is the temperature at which the system can actually deliver the heat the building needs.