PIV vs Dehumidifier: Which Is Better for Condensation?
A PIV unit is fitted once and ventilates the whole house. A dehumidifier is a portable appliance you plug in where the problem is. Both tackle condensation and mould, in different ways and at very different upfront costs.
The short answer
A dehumidifier is the quicker, cheaper and reversible fix, and the only one of the two a tenant can simply buy and use. A PIV unit (positive input ventilation) is a permanent, whole-house answer for owners whose home has poor ventilation throughout. Many damp homes benefit from both.
What a PIV unit does
A PIV unit usually sits in the loft. A small fan draws air through a filter and pushes it gently into the house through a ceiling vent on the landing. The slight positive pressure moves stale, humid air out through gaps and vents. It runs all the time and uses very little electricity for the fan itself. It does not remove water from the air. It replaces humid indoor air with outdoor air, which in winter is cold and holds less moisture. Some units include a heater to temper the incoming air, and using it raises the running cost.
PIV vs dehumidifier at a glance
The main differences are in installation, reach and what happens to your heat.
| PIV unit | Dehumidifier | |
|---|---|---|
| How it works | Ventilates: pushes fresh air in | Extracts water into a tank |
| Covers | Whole house | One room or one floor, movable |
| Installation | Fitted, usually in the loft, with wiring | Plug in |
| Upfront cost | Typically several hundred pounds fitted | From under £100 to about £300 |
| Electricity | Very low for the fan; more with the heater on | Roughly 4-9p an hour while the compressor runs |
| Effect on warmth | Brings in cooler air | Returns slightly warmer air |
| Dries laundry | No | Yes |
| Suitable for tenants | Needs the landlord's agreement | Yes |
When a dehumidifier is the better choice
You rent. The problem is one or two rooms. You dry washing indoors. You need results this week. You live in a flat with no loft. In all of these a dehumidifier does the job without altering the building: start with our dehumidifiers for mould.
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When PIV makes more sense
You own the house. Condensation appears in every room each winter. The home has been made airtight with new windows and insulation but has little background ventilation. You do not want to empty a tank. In that case a ventilation fix addresses the cause, and it works without anyone having to remember it.
What each one costs over five years
A dehumidifier is cheap to buy and costs electricity whenever the compressor runs. A PIV unit costs much more to buy and fit, then very little to run unless its heater is used often. For a single damp bedroom the dehumidifier stays cheaper for many years. For a whole house running two or three dehumidifiers every winter, a PIV unit can pay its way. Ask an installer for a written quote and check which heater setting the running-cost claim assumes.
Limits of PIV
PIV relies on air being able to leave. In a very airtight home, or one with closed internal doors and no trickle vents, it works less well. It needs a loft, or a wall-mounted version for flats. It brings in whatever is outside, filtered for particles but not dried. And on mild, wet days the incoming air carries plenty of moisture, so the benefit is smaller than on a cold, dry day.
Limits of a dehumidifier
It treats the room it is in. It needs emptying, or a hose to a drain. It makes some noise. A compressor model loses efficiency in a cold room, where a desiccant dehumidifier is the better type. And like PIV, it manages moisture produced by living in the house; it does not fix leaks, bridged damp-proof courses or failed gutters.
Using both
A PIV unit lowers background humidity across the house. A dehumidifier still earns its place for laundry days, for a cold north-facing bedroom, and for drying out after a leak. Neither repairs rising damp or water coming through a wall: that needs the building fixed. If you rent, see what your landlord must do under Awaab's Law.
Ready to compare models? See our best dehumidifier for mould and damp comparison →
Frequently asked questions
Is a PIV unit better than a dehumidifier?
For a house you own with condensation in every room, a PIV unit treats the cause across the whole home. For one or two rooms, laundry drying, flats and rented homes, a dehumidifier is the better and cheaper choice.
Do PIV units make the house cold?
They bring in outdoor air, so the landing near the vent can feel cooler in winter. Units with a heater reduce that, at a higher running cost.
Is a PIV unit cheaper to run than a dehumidifier?
The fan alone uses very little electricity, less than a dehumidifier. With the built-in heater switched on, a PIV unit can cost more.
Can I use a dehumidifier with a PIV system?
Yes. The PIV handles background ventilation and the dehumidifier handles laundry and problem rooms.