Water filtration for landlords
The landlord's calculation is different from the homeowner's. You are not the one drinking the water, but you are the one replacing the boiler, and in a hard water area those two facts point in a very clear direction.
Fit what protects the asset. Be cautious about anything that adds a maintenance obligation you will have to service between tenancies.
Worth fitting in a hard water area
A scale inhibitor on the boiler. This is the highest-return water treatment a landlord can fit. It is cheap, it goes in during a boiler installation or service at minimal extra cost, and in many cases it is a manufacturer warranty condition — meaning without it, a scale-related failure may not be covered.
A water softener, in the right property. In very hard water areas, on a property you intend to hold long term, a softener protects the boiler, the cylinder, the shower and every appliance. It also becomes a genuine letting feature. The catch is the salt: someone has to keep it topped up, and that someone will be your tenant, who has no incentive to.
Be cautious about
Under-sink filters. Fine to fit, but the cartridge is a consumable with a six-month interval, and a neglected cartridge is worse than no filter. If you fit one, either put it in the tenancy agreement whose job it is, or budget to change it at each check.
Reverse osmosis. More stages, more consumables, a tank and a drain connection. Rarely justified in a rental.
Anything with a service contract. You are signing up to an ongoing obligation across changes of tenant.
Your actual obligations
There is no legal requirement to filter water in a rented property. Your obligations concern the supply itself — you must not, for instance, have plumbing that contaminates the supply, and you have general repairing obligations for the installations for the supply of water.
Lead pipework is the exception worth taking seriously. If a property may still have a lead supply pipe or lead internal plumbing, that is a health matter rather than a taste one, and replacement rather than filtration is the proper answer. Soft-water areas raise this more sharply than hard-water ones, because soft water is more inclined to dissolve metals from pipework.
If a tenant raises a water quality concern, the water company will test the supply at the property free of charge. That is the sensible first step and it produces an authoritative answer.
For a portfolio
Check the hardness for each property rather than assuming it is uniform. Landlords with properties across a region routinely find that one is in very hard water and another twenty miles away is not — a softener that makes sense in Kent makes none in Cornwall.
Standardise on one scale inhibitor product across the portfolio so cartridges are interchangeable and one order covers everything.
Record what is fitted where, and when it was last serviced. At scale this is the difference between a maintenance programme and a series of surprises.