The short answer
Under about eight years old with a failed element, thermostat or valve is usually worth repairing. Over ten years old and leaking from the tank body is not, because the leak is the tank. Get both prices and the running-cost difference before deciding.
The first question is not which brand
It is whether the unit is worth saving. That comes down to two things: how old it is, and what has actually failed.
Usually worth repairing. A unit under about eight years old where the failure is a component rather than the vessel. Failed heating element, dead thermostat, faulty gas valve, blocked burner, a leaking relief valve. These are parts, and the tank itself still has years in it.
Usually not worth repairing. A unit over about ten years old, or any unit leaking from the tank body rather than from a fitting. When the tank goes, the leak is the tank. There is no repair, and a patch buys weeks rather than years.
The awkward middle. Eight to ten years with a component failure. This is a genuine judgement call, and it depends on what the repair costs against what a replacement costs, plus how much longer you plan to be in the house. Anyone who gives you a confident answer without asking that second question is guessing.
How to tell where a leak is coming from
Water on the floor near a hot water unit has a small number of possible sources, and they are not equally bad:
- Dripping from the relief valve is often normal, especially after the unit has heated. Continuous heavy discharge is not, and usually means the valve or the pressure feeding it.
- Wet at a fitting or connection is a repair.
- Water seeping from the base of the tank with no wet fittings above it is the tank. That one is a replacement.
Choosing a replacement
The honest determining factors, in order:
- What is already there. Swapping like for like is the cheapest path because the connections, the space and the supply all already suit it.
- How many outlets run at once. A household running two showers simultaneously has a different requirement to one that never does, and instantaneous units are sized on flow rate rather than volume.
- What the existing supply can carry. Going from a low-input appliance to a high-input one is where changeovers get expensive, because the gas line may need upsizing to carry the load. This is worth knowing before you fall in love with a model.
- Running cost over the life of the unit. The purchase price is the smaller number over ten years.
Storage electric, storage gas, instantaneous gas and heat pump all make sense in different houses. Rebates on heat pump systems change what the sums look like, and they change over time, so they are worth checking at the point you are actually buying rather than relying on what was true last year.
What a proper install includes
The unit going on the wall is the easy part. The parts that matter afterwards:
- The tempering valve set so the temperature at a bathroom outlet is safe to touch, while the tank stays hot enough to stop bacteria growing
- The relief line run to a legal discharge point rather than onto the paving
- The unit strapped and seated so it is not carrying its own weight on the connections
- Connections lagged where they should be, which is not decoration, it is standing heat loss
- Commissioned, meaning checked running, not just connected
If a quote is notably cheaper than the others, it is worth asking which of those five it includes.
Before you make contact
Have the age of the unit, whether it is gas or electric, and a photo of the data plate and the connections. That is usually enough to give you a real answer about repair versus replace before anyone attends.