Solenoid Valves for Hot Water and Heating Systems
Whether you are controlling a hydronic heating loop, a hot water service, a buffer store or a glycol-filled underfloor circuit, the solenoid valve you choose has to cope with continuous heat, thermal cycling and, in many cases, a low pressure differential. A valve specified for cold mains water will often fail prematurely on hot water duty because the seal hardens, the diaphragm cracks or the valve simply will not open against the available pressure. This guide walks through the key decisions so you can specify the right valve first time.
What counts as a "hot water" application?
The term covers several quite different jobs, and each has its own constraints:
- Hot water service (DHW): potable hot water for taps, showers and washing. Typically 55–65 °C, and normally requires a WRAS-approved valve because the water is destined for human consumption.
- Hydronic heating: closed-loop circuits feeding radiators, fan coils or underfloor heating, usually running at 40–80 °C. Water quality is controlled and the circuit is sealed, so brass bodies are common.
- Glycol and antifreeze circuits: heat pumps, solar thermal and frost-prone systems often use a water/glycol mix. Glycol changes both the seal compatibility and the viscosity, especially when cold.
- Thermal stores and buffer vessels: higher temperatures and larger flow rates, where pressure drop and valve sizing matter more.
Temperature is the first constraint
Every solenoid valve carries a maximum media temperature that is set largely by its seal and diaphragm material rather than its body. Standard brass valves are supplied with NBR (nitrile) seals, which are excellent for cold water and compressed air but harden and lose resilience as temperatures climb. For sustained hot water and heating duty, the usual choice is EPDM, which handles hot water and steam far better and is compatible with glycol. FKM (Viton) tolerates high temperatures too, but is a poor match for hot water and glycol over the long term, so it is generally reserved for hot oils and aggressive chemicals rather than heating water.
Because the safe temperature limit is dictated by the seal, always confirm the elastomer before ordering. Our seal materials guide explains the trade-offs in detail, and the temperature reference tables are handy when you are converting between figures on an overseas datasheet.
| Seal material | Hot water | Glycol/antifreeze | Notes |
|---|---|---|---|
| NBR (Nitrile) | Limited | Suitable | Fine for warm water; hardens with sustained high temperature. |
| EPDM | Good | Good | Preferred for hot water, heating circuits and glycol. Not for mineral oils. |
| FKM (Viton) | Fair | Check | Excellent for hot oils and chemicals; not ideal for hot water long-term. |
| PTFE | Good | Good | Used where chemical and temperature resistance are both critical. |
Body material: brass, bronze or stainless?
For sealed heating circuits with treated water, a good-quality brass or dezincification-resistant (DZR) brass body is usually more than adequate and cost-effective. Bronze is a robust alternative for hot water. Where the water is aggressive, softened, demineralised, or where hygiene is important, a stainless steel body is the safer specification. On potable hot water, the body and every wetted part must be approved for contact with drinking water, which is where WRAS approval matters.
Direct-acting or servo-assisted?
Many heating systems run at modest pressure, and gravity or low-head pumped circuits may offer very little pressure differential across the valve. This matters because a standard servo-assisted (pilot-operated) diaphragm valve relies on a minimum pressure difference between inlet and outlet to open and stay open. If your system cannot guarantee that differential, choose a direct-acting valve or an assisted-lift type that will open from zero bar. Getting this wrong is one of the most common causes of a valve that "clicks but does not flow" on a low-pressure heating loop. Our guide on how to choose a solenoid valve explains the difference and how to read the minimum and maximum operating pressures.
Practical points that extend valve life
A few installation habits make a real difference on heating duty. Mount the valve with the coil uppermost wherever possible, so rising heat from hot pipework does not soak straight into the coil; solenoid coils already run warm, and combining that with a hot ambient shortens their life. Allow room for the coil to breathe rather than lagging it inside pipe insulation. Fit a strainer upstream, because scale, sludge and magnetite from an older heating system are a leading cause of valves passing or sticking. Observe the flow-direction arrow on the body, and consider whether you need the valve to be normally closed (de-energised = shut, the safe default for most fill and top-up duties) or normally open (de-energised = flowing, sometimes chosen so heat can still circulate on power failure).
Glycol and viscosity
If the circuit contains glycol, remember that the mixture is more viscous than water, particularly at low temperatures on a cold start. Higher viscosity increases pressure drop and can affect the opening behaviour of smaller valves, so it is worth sizing generously and confirming both the seal compatibility and the temperature range with the supplier before ordering.
Getting the specification right
In short, for hot water and heating you want the correct seal (usually EPDM), a body material suited to the water quality, the right operating principle for your available pressure, and WRAS approval if the water is potable. If you are unsure which combination fits your system, use the Valve Search Wizard to filter by media, temperature and connection, or contact our technical team with your flow rate, temperature and pressure and we will help you confirm the right valve before you order.