Saunier Duval 3-Way Valve Isofast / Master 3 / Themafast Ref. S1025500
When you turn on the tap to enjoy hot water or switch on the heating, the Saunier Duval Isofast / Master 3 / Themafast 3-way valve is responsible for directing the water between the heating circuit and the domestic hot water circuit. If this valve fails, you will notice that there is hot water but the heating does not work, or vice versa, or even that both circuits are mixing. This part is compatible with several boiler models, such as Isofast, Master 3, and Themafast, among others. It is recommended to replace it when you observe these symptoms to restore the proper functioning of your system.
VAT included
Consult via WhatsAppHow to check if it is the part for your boiler
What determines it is not the boiler model, but the reference printed on the part you already have installed.
- Find the reference printed on the installed part and check whether it matches the catalogue reference —. If it is not visible, do not identify the part by its shape alone.
- Check the full boiler model on its data plate and look for it in the model associations on this page.
- Compare the connections and physical body with the original part before installation.
- If anything is unclear, send us a photo of the marking via WhatsApp before placing your order.
Symptoms and faults that the 3-way valve resolves
The 3-way valve diverts water between heating and domestic hot water. These are the typical symptoms and what to do:
Common symptoms and faults
- 01
Sometimes it heats well and sometimes it doesn't, without changing anything on the thermostat.
Cause: If the mechanism of this valve gets stuck halfway between the two positions, the distribution of water between circuits is irregular.
Check if the fault coincides with the simultaneous use of heating and domestic hot water before replacing the part.
- 02
When a hot water tap is opened, the heating completely shuts off and takes time to return.
Cause: The valve prioritizes domestic hot water by closing the heating, and if the mechanism is stuck, it remains closed longer than necessary.
Time how long it takes for the radiators to heat up again after closing the tap; a long delay points more towards the valve than to another cause.
- 03
A thump or vibration is heard in the valve when the boiler switches modes.
Cause: The plastic body and the internal mechanism can generate noise when forcing the position change if they are worn out.
Locate the noise by touching the body of the gas valve during the change, to rule out that it comes from the pump or another pipe.
- 04
There is a water leak right in the area where this L-shaped part fits.
Cause: The sealing gaskets of this type of gas valve degrade over time and allow water to pass through the joints.
Dry the area well and check if the dripping reappears after a while of operation, before considering the gasket or the valve as faulty.
These symptoms can have more than one cause. If your boiler displays a fault code, send it to us via WhatsApp along with a photo of the labeling, and we'll tell you if this is the part.
Data sheet for the 3-way valve
- Type of part
- 3-way valves
- Units in stock
- 1 unit
- Status
- Refurbished
- Warranty
- 1 year
- Shipping
- 1-2 business days
- Return
- 14 days
- Please check that the reference of your original gas valve matches S1025500 before placing the order.
Manuals and technical documentation
Official documentation of the boiler, useful for locating the part and cross-referencing the fault codes.
- 3-way manual Saunier Duval Isofast / Master 3 / Themafast (without (PDF)Document ©, provided as a technical reference.
Frequently asked questions about the 3-way valve
Is this 3-way valve compatible with my Saunier Duval boiler?
What type of valve is this?
Are there any fault codes associated with this gas valve?
Where can I find more information about the VATF25MOD valve?
What should I do if my boiler is not compatible with this gas valve?
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