Which wall your hot water unit is on matters more than how old it is

Everyone near the coast knows salt air is hard on things. What people are rarely told is how uneven it is. Two identical hot water units, installed the same week on the same street, can be years apart in condition purely because of which wall they were fixed to.

That is worth knowing because it changes what you should do about yours. Age is the number everyone asks about. Position is the one that actually predicts what you will find when somebody looks.

The short answer

Metal in a coastal environment corrodes several times faster than it does inland, because chloride breaks down the protective oxide layer that would otherwise slow it down. A hot water unit is usually the largest piece of exposed metal on the property and is usually on an external wall, so exposure decides its life. An exposed, seaward or west-facing wall with no shelter is a completely different situation from a sheltered southern one.

1. Find out which wall your unit is actually on, and how sheltered it is
2. Read the compliance plate for its real age
3. Have the sacrificial anode checked earlier here than the usual advice suggests
4. Look for staining and seeps rather than waiting for a burst
5. Specify materials for the position when it is replaced, not just the model

On this page

1. Why salt is different
2. Why the wall decides it
3. Anodes matter more here
4. What to watch for
5. Specify for the position
6. What to ask

1. Why salt is different from ordinary weathering

Most metals protect themselves. Copper, steel and galvanised iron all form a thin oxide layer on the surface, and that layer slows further corrosion down. It is why plumbing inland lasts as long as it does: the process is largely self-limiting.

Chloride ions break that layer down. Once the passive film is compromised the metal has no defence, and corrosion continues rather than slowing. That is the mechanism, and it is why coastal figures are not a slightly worse version of inland ones. Metal in a coastal environment corrodes something like five to ten times faster than it does inland.

It also explains the failure signature. Copper here tends to fail as pinholes: tiny perforations that seep slowly, rather than the dramatic burst people expect. That changes what you should be watching for entirely.

2. Why the wall decides the outcome

A hot water unit is typically the largest single piece of exposed metal on a residential property, and in most houses it is fixed to an external wall. That makes it the item most directly exposed to the thing that does the damage.

• Seaward-facing, unsheltered. The worst case. Direct salt-laden air, wind-driven, with nothing intercepting it.
• West-facing. Salt plus afternoon heat and UV on the casing, which is hard on everything from the jacket to the fittings.
• Under an eave or in an alcove. Meaningfully better, because shelter reduces both the deposition and the wetting cycle.
• Southern or courtyard side. Generally the kindest position available, and worth considering when a unit is being replaced.
• Inside a cupboard or laundry. Different problem entirely, and a different set of questions about ventilation and overflow.

None of this is a reason to move a unit for its own sake. It is a reason to know which situation yours is in, because it changes how often it is worth looking at and what to expect when somebody does.

3. Sacrificial anodes matter more here, and sooner

A storage tank is protected by a sacrificial anode: a rod that corrodes deliberately so the steel of the tank does not. When the anode is consumed, the tank starts corroding instead, and the countdown on the unit begins.

The general advice about checking one is written for average conditions. Here the consumption is faster, so the check is worth doing earlier and it matters more when you do. Replacing an anode is a small job. Replacing a tank because nobody looked at the anode is not.

It is the single most cost-effective thing available on a unit you intend to keep, and it is almost never done, because nothing goes wrong until everything does.

4. What to watch for, given it seeps rather than bursts

Because the characteristic failure here is a pinhole rather than a rupture, the signs are quiet ones. People wait for water on the floor and get a slowly rising bill instead.

1. Staining or a green or white deposit on or below pipework and fittings.
2. A damp patch on a wall or ceiling that never quite dries and never gets worse quickly.
3. Water pressure that has drifted down over months rather than dropping suddenly.
4. A water bill creeping up without any change in the household.
5. Rust bleeding from the base of the hot water unit or from its brackets.
6. A meter that moves with everything in the house turned off.

That last one is the definitive test and it takes ten minutes. Everything off, read the meter, wait an hour, read it again. Movement means water is leaving the system, which is the whole question a pinhole leak makes difficult to answer any other way.

5. When it is replaced, specify for the position

This is the practical advice that gets left out, and it is where the money is.

Replacement is usually discussed in terms of type and capacity: storage or continuous flow, gas or electric, how many litres. Those matter. What also matters here, and rarely gets raised, is what the fittings and brackets around the unit are made of, because they are exposed to the same environment and they are not all equally resistant.

Stainless and brass are not one thing. Grades differ in how they behave in a chloride environment, and specifying appropriately at installation costs a little once instead of a lot twice. It is a conversation worth having before the unit is ordered rather than after the brackets have rusted.

The same applies to position. If a replacement can reasonably go somewhere more sheltered, that is worth raising while it is still a plan.

6. What to ask when somebody quotes you

• What position is the unit in, and would a more sheltered one be practical?
• What grade are the fittings and brackets you are proposing?
• When should the anode be checked, given where this is installed?
• Has the existing pipework around the unit been looked at, or just the unit?
• Is there any staining or seeping on the connections?
• What is the actual age off the compliance plate?

A quote that answers all six is a quote from somebody who has worked on this coast. One that answers none of them is a quote written for a house anywhere.

The short version

• Metal corrodes several times faster in a coastal environment because chloride breaks the oxide layer.
• That means corrosion does not self-limit here the way it does inland.
• The hot water unit is usually the largest exposed metal on the property, so position decides its life.
• Sacrificial anodes are consumed faster here. Check earlier than the general advice suggests.
• The failure signature is pinholes and seeps: staining, drifting bills, falling pressure.
• At replacement, specify fittings and brackets for the position, not just the model and size.