Heat pump flow temperature and emitters in commercial buildings
The water temperature your emitters need is the single design input that most affects how efficient a commercial heat pump will be.
Why flow temperature drives SCOP
A heat pump moves heat from a lower-temperature source (outside air, ground or water) to a higher-temperature sink (your heating water). The larger the temperature lift, the more compressor work it needs for each unit of heat delivered. The seasonal coefficient of performance (SCOP, rated to BS EN 14825) is the seasonal average of that ratio, so a lower flow temperature raises SCOP and a higher one lowers it.
This matters financially because it feeds straight into running cost. Cost per kWh of heat is the electricity price divided by SCOP, and the break-even guide shows how sensitive the comparison with gas is to that figure. Two quotes for the same building can have very different SCOPs purely because one assumes a lower design flow temperature than the other, and that assumption has to be physically achievable by your emitters.
Two further points. First, flow temperature need not be constant: with weather compensation the controls lower it on milder days, so most operating hours run below the design value. Second, hot water for taps is a separate, higher-temperature duty; see the hot water heat pump guide.
How to find your existing flow and return temperatures
Do not assume the temperatures on the boiler nameplate are what the building really runs at. Boilers are often set to a high flow temperature that the emitters do not need on most days. To establish the truth:
- Read the controls. Many boilers and building management systems log flow and return temperatures; export a heating season of data if you can.
- Check on a cold day. Record flow and return at the main headers when the building is at its coldest and the heating is running flat out. The difference between flow and return (delta T) tells you how hard the emitters are working.
- Test the margin. A competent contractor can lower the flow set point in steps on a cold day and see whether rooms still reach temperature. If they do, your emitters are bigger than they need to be for the heat loss, which helps a heat pump.
- Note each circuit. A building often has several circuits (radiators, fan coils, air handling units, hot water calorifier) at different temperatures. The hottest circuit, not the average, drives the design unless it is separated out.
Give these readings to every quote. They sit alongside the fabric, schedule and energy data listed in the sizing guide.
Which emitters cope with lower temperatures, and why
An emitter’s output depends on its surface area and the difference between its water temperature and the room temperature. Lower water temperature means less output from the same emitter, unless the emitter has more area or moves more air.
| Emitter | How it copes with lower flow temperature |
|---|---|
| Conventional panel radiators | Output falls as water cools. Radiators sized for a high-temperature boiler may be too small, though many were oversized originally and have margin. |
| Fan coil units | A fan forces air over the coil, so output per unit of water temperature is higher. A common, lower-temperature-friendly option, particularly where cooling is also wanted. |
| Air handling unit coils | Coils can be specified with more rows or larger area. Existing coils sized for hot water may or may not have spare capacity; check against the design. |
| Underfloor heating | Large radiant surface means it works at the lowest water temperatures of the common options. Floor construction and response time matter. |
| Fan-assisted radiators | Add a small fan to a radiator to raise output at lower water temperature. |
No temperatures are quoted here because the right design values depend on the building, its heat loss and the product. If you want design conventions, look to CIBSE guidance, and ask each designer to state theirs.
Upgrade options when emitters need hotter water than a heat pump likes
If the survey shows the emitters cannot meet the load at a heat-pump-friendly flow temperature, there are several routes, often combined:
- Reduce the demand. Better fabric, draught-proofing, ventilation heat recovery and improved controls reduce peak load, so the existing emitters deliver enough at a lower water temperature. This is often the cheapest way to lower the flow temperature the design needs.
- Enlarge or replace the worst emitters. Upgrading only the under-performing rooms or zones can be cheaper than replacing everything. A heat-loss room-by-room check shows which ones.
- Add fans. Fan-assisted radiators or fan coils increase output without enlarging the footprint.
- Zone the hot circuits. Separate the circuits that need high temperatures so the rest run low. A dedicated hot water duty is the classic example.
- Hybrid. A heat pump covers most of the load and a retained boiler covers the coldest hours or the hot circuit. Note that GOV.UK states that you cannot get a Boiler Upgrade Scheme grant for a hybrid system, so the grant and the hybrid choice are mutually exclusive. The hybrid retrofit guide covers this.
- High-temperature heat pump. Some units are built to deliver hotter water, typically at lower efficiency and higher cost than standard machines. They suit buildings or processes where the emitters cannot practically change.
Choose on whole-life cost, not on the cheapest way to avoid touching the emitters.
Checklist to take to quotes
- Provide measured flow and return temperatures for each circuit, on a cold day if possible.
- Ask for the design flow temperature, and the heat-loss basis for the emitter capacity at that temperature.
- Ask which emitters need replacing or enlarging, listed room by room, and what that costs separately.
- Ask for the rated SCOP and the test condition, flow temperature and standard it refers to (BS EN 14825).
- Ask whether weather compensation is included and how the controls set flow temperature across the season.
- Ask how hot water is served and at what temperature, and whether it is a separate duty.
- If a hybrid or high-temperature unit is proposed, ask why a lower-temperature design was not chosen, and what it does to running cost.
Run the efficiency you are offered through the running-cost calculator using your own tariffs. Check any installer on the public MCS register at mcscertified.com.