Understanding Your HIU Heat Meter Reading

What a Heat Meter Actually Measures

A heat meter does not measure water, and it does not measure gas. It measures energy, and it works it out from two things.

First, a flow sensor measures how much water passes through, in cubic metres. Second, a pair of temperature sensors measures the water going into your flat and the water coming back out. The difference between those two temperatures tells the meter how much heat you removed from the water on its way through.

Multiply the volume by the temperature difference and you have the energy you used, expressed in kilowatt hours. That kWh figure is what you pay for. The water itself is not consumed — it circulates back into the building system to be reheated.

Heat meters used for billing in the UK are normally MID-approved instruments, meaning they are type-approved to a legal accuracy standard for trade use.

What Each Reading Means

Press the button on the front of the meter and it will cycle through several displays. Most meters show some or all of the following.

Delta T: The Number Worth Watching

If you only look at one figure beyond the kWh total, make it ΔT — the gap between the flow and return temperatures.

A healthy HIU extracts a lot of heat from the primary water, sending it back much cooler than it arrived. A high ΔT means your unit is doing its job. A low ΔT means hot water is passing through your flat and going back to the plant room still hot — heat you were charged for but did not use.

Persistently low ΔT usually points to one of three things: a limescaled plate heat exchanger that can no longer transfer heat properly, a valve passing and letting primary water bypass the exchanger, or an incorrectly commissioned unit whose return temperature limiter was never set up.

All three cost you money every month, and all three are things an annual service is designed to catch. In London's hard water areas, scaling is by far the most common.

How to Take a Reading

1. Find the meter. It is normally in the same cupboard as your HIU, fitted on the pipework entering the unit. It is a compact box with a small LCD display.
2. Press the button. One button cycles the display. Give it a moment between presses.
3. Find the cumulative kWh or MWh figure. This is the headline total. If yours shows MWh, multiply by 1,000 to get kWh.
4. Record the serial number too. It is printed on the meter body and proves which unit the reading came from.
5. Photograph the display. A dated photo is far more useful than a number written down if you ever need to query a bill.

Many newer blocks have meters that are read remotely over M-Bus or wireless M-Bus, so you may never be asked for a reading. It is still worth taking one occasionally — it is the only way to check that what you are billed matches what your meter says.

How Heat Network Billing Works

A heat network bill almost always has two components:

• A standing charge. This covers the building's fixed costs — plant maintenance, metering and billing administration, and the heat lost from the distribution pipework. You pay it whether or not you use any heat.
• A consumption charge. Your kWh usage, taken from the meter, multiplied by a unit rate set for the building.

This is why a flat that barely uses heating can still receive a meaningful bill, and it is the single most common source of confusion for residents. The standing charge is not an error.

Since 27 January 2026, heat networks have been regulated by Ofgem, and suppliers are expected to provide clear, transparent information about what customers are paying for. If your bill does not break these elements out, you are entitled to ask for that detail. Our guide to the Ofgem heat network rules covers what operators now have to do.

If You Think Your Bill Is Wrong

Work through it in this order — it is much more persuasive than simply disputing the total.

1. Take two readings a week or two apart and photograph both. That gives you a real consumption figure for a known period.
2. Compare against the bill. Does the consumption charged match the difference between the readings on your meter?
3. Check the standing charge separately. Confirm how much of the bill is fixed and how much is usage.
4. Look at your ΔT. If it is low, you may genuinely be consuming more than you should because of an HIU fault — the meter is right but the unit is wasteful.
5. Raise it in writing with your heat supplier or managing agent, including your photographs and readings.
6. Escalate if unresolved. Residents are expected to go to their supplier first; if it is not sorted out, the Energy Ombudsman can take heat network complaints.

When a High Bill Is Actually an HIU Fault

Not every high bill is a billing error. Some are a working meter accurately recording a wasteful unit. The patterns we see most often:

• Consumption rising year on year with no change in habits — usually progressive scaling of the heat exchanger
• Significant consumption during periods you were away — often a valve passing and letting heat through continuously
• Consistently low ΔT — heat being returned to the building unused, but charged to you
• Much higher consumption than comparable flats in the block — worth asking your managing agent for an anonymised comparison

Each of these is diagnosable on site with flow, temperature and pressure readings, and each is normally fixable.

This guide is general information for residents on heat networks. Your own charges depend on the tariff set for your building and the terms of your lease or tenancy — check with your heat supplier or managing agent for figures specific to you.