Main navigation

Social sharing options

You are here:

  1. Home
  2. Blog


Installing a heat meter in the domestic RHI – say “Ahhh”

It’s not long to go now until the Domestic RHI launches and lots of installers have already started swatting up on the scheme requirements and changes to MCS that come into effect on the 16th March.  These include guidance on metering for the domestic RHI, which is well worth being familiar with.

DECC have also recently published some encouraging results from a large-scale heat pump metering programme.  Overall, the results are good news; heat pumps are performing significantly better than they were when the EST measured them in their field trial several years ago.

However, we have noticed a lot of variability in how MCS installers fit heat meters in the process of installing heat meters on more than 700 heat pumps (see pages 26, 27 and 46 – 48 of the report).  Incorrectly installed heat meters don’t give reliable results.  That’s bad news if you’re a customer whose domestic RHI payments depend on what your meter says.

So look out customers, you could lose out!  But by how much?  Well let’s assume the domestic RHI funds 60,000 heat pump and biomass boiler installations in the first two years of the scheme.  If 20% of those are metered and their metered payments are 10% too small because the heat meters measuring them aren’t installed right, then by my calculations, customers collectively stand to lose out by about £10m over the lifetime of the scheme.  Or, in other words, a bit over £800 each.

There are some drawings of how heat meter temperature sensors should and shouldn’t be installed below but first it’s worth knowing how a heat meter works.  Heat is measured by monitoring flow rate and two temperatures and combining them with some fluid properties.  The equation below shows which component does what (the pictures shown are the components used in the RHPP metering programme, others are available).

RHI model showing equation components

As a heat meter installer, you need to plumb in the flow meter and both temperature sensors and connect them up.  If this isn’t done properly, one or both could read a value that is too small and therefore the heat reading reported will be too small.  In RHPP, the biggest problem was the plumbing for the temperature sensors. These should be hanging in the centre of the heating pipe.

Here’s how a temperature sensor should be installed…

Picture showing how a temperature sensor should be installed

The sensor is inserted into a protective pocket that extends to or past the centre line of the pipe.  This way, the sensor takes an accurate measurement of the fluid travelling through the pipe.

Here’s how not to do it…

How not to install a temperature sensor



Wrong installation of heat sensor

Temperature sensors installed like this won’t be measuring the temperature of the fluid travelling through the pipe because the fluid around the sensor will be stagnant.

These heat meters will generally under-report heat and therefore RHI payments will be too small.

Another trap to avoid is not plumbing in the temperature sensors at all.  No amount of cable ties, tape, conductive paste and insulation is going to make a temperature sensor read the temperature of the fluid inside the pipe if it’s strapped onto the outside of it. Installing temperature sensors like this will mean that the heat reading is too small and therefore the RHI payment is also too small.

Most people in the domestic RHI won’t need to be metered because their payments will be Mouth showing how the temp sensors should be installed: "The temperature probes should dangle in the heating pipes like that thing at the back of your throat!"based on deemed heat use but if you do need to install a heat meter, remember how those temperature sensors need to be installed.

And if you can’t remember… look into a mirror and say “Aaahhh”.  The temperature probes should dangle in the heating pipes like that thing at the back of your throat! Good luck.


12 Responses to “Installing a heat meter in the domestic RHI – say “Ahhh””

  1. John Cantor says:

    I started the week trying to source a relatively low cost RHI compliant class 2 heat meter for a small inverter air source heat pump. I remain a bit baffled, and wonder if anyone can enlighten me.
    Many variable-output heat pumps have a fixed-speed circulator pump. At full load the flow-return dt could be 7 degrees (k). At minimum speed (possibly 40%) the dt could be somewhere around 3k.
    I was told by 2 heat meter suppliers that the class 2 meters on offer will return a reading of zero if the dt is below 3 degrees. The only thing I can glean at present is that there is some sort of MID regulation.
    I have a heat meter on my own Ecodan ASHP that spits out water flow-rate and temperature readings every 10 seconds or so, and I can see that at present, on minimum speed, my flow-return dt is on average a healthy 2.8 degrees (kelvin).
    So it seems that if a commonly available heat meter were fitted to my Ecodan, it would read zero heat when the compressor is running at minimum speed. I expect this would be true for many inverter heat pumps.
    I can see how the accuracy will drop off considerably as the dt gets closer, so I could understand the wish to state accuracy for 3k upwards, but by making the heat zero, the error is much worse … 100% error!
    I could see the sense in ‘capping’ the minim dt reading, but surely 0.5k would be adequate. (i.e. for situations with underfloor heating and a constantly running circulator- anything less than ½ must be zero?).
    The next thorny topic is defrost of air source. It seems that many meters read heat-only. My monitor shows snap-shots of the reverse-cycle defrosts. These can typically occur hourly, and start when the air is 7C or less. Over a 3-5 minute duration (depending on conditions), the heat reading goes negative as water is fed to the house at a lower temperature. By comparing areas under the graph, it seems that typically we can look at a 6 minute duration where there is no net-heat (as it swings negative and back to positive). The net loss is therefore in the region of 10% (if 6 mins lost in 60 mins), and this happens commonly in winter.
    What happens with a normal heat meter? How often are temperature readings taken? Will it capture or ignore defrosts? If the minimum value of dt is 3k, then it may be possible that entire defrost period returns a zero heat. Has somebody worked out that the 3k ‘cap’ actually captures defrost energy reasonably well??
    I would have thought it to be extremely easy for all heat meters to capture defrosts properly. Do they? They should.
    Final issue is glycol. Most ASHPs need glycol, and I think the error due to the different specific heat would typically be 8%. When I enquired to a heat meter supplier, there was a general vagueness about this issue. Are some people ignoring it?
    I look forward to getting the bottom of these issues.

    • John Cantor says:

      I realise I might have exaggeretae energy loss du to defrost; about 10% on worst days- nothing on good days – maybe 3% on average?

      • Derek Pye says:

        Hello John

        You need to look at meters uk Ltd
        There product is Class 1 ( better than Class 2 or 3 ) It is a modern digital product. It will read a Delta T as low as 1 degree c difference and it takes reading every second giving superb accuracy. There is a Glycol Version available also

  2. debra storr says:

    I have just installed an ASHP but need to install a meter as it’s a second home. Do I apply for the RHI first and then metering is installed or do I need to install the meter up front before I apply – and if so, who are the meter installers (for Highlands).

  3. Sam says:

    Chris, great to see this kind of info/advice coming out of government.

    Great to see support for more ‘intelligence’ and use use of cheap technology to improve energy efficiencies.

    Quick question, are you able to highlight where one can source suitable sensor pockets for use in typical T connections. All my googling leads me to swimming pool sites, where the prices are ridiculous, or the length of the pocket too long such that they aren’t suitable for use in a typical brass T as depicted.


  4. […] Department of Energy and Climate Change Blog» Blog Archive » Installing a heat meter in the domestic RHI – say “Ahhh”… […]

  5. Scott Carpenter says:

    Hope you can help me – I am trying to find a domestic heat meter that could be installed into a HMO, either bedsits or S/C flats so that one boiler could supply all the units of accomodation with hot water and central heating, and the landlord could apportion the charge dependent on the tenants use of the central heating/hot water. The reason for this is that in old properties, central heating will be the best solution to the problem of Cat 1 Excess Cold under the HHSRS – particular in old HMOs where there is NO fixed heating. But multiple boilers and meters are problematic. all the best,

    Scott Carpenter
    Senior Private Sector Housing Officer

    • Chris Wickins says:

      Thanks for your comment, Scott.

      I’m not really in the business of recommending one heat meter. The one in the pictures is quite high-spec. It’s made by Sontex and supplied by DMS, their UK distributor.

      But there are lots available. A google search for “domestic heat meter” just turned up lots of companies that you can buy one off: Itron, Bellflow, DMS, Kamstrup etc.

      I’d give a few a call. They’ll want to know the volume flow rate through your system (usually in m3/hour). You can also talk to them about whether you need a class 2 meter or a class 3 meter. The non-domestic RHI requires class 2 meters; the domestic RHI requires class 3.

      Good luck

  6. Nigel Dent says:

    This is a great blog & extremely important. We see this issue all the time and like you stress to our installers the importance of location of heat meter (always on the return pipework if possible ) & the correct installation of the temperature probes.

    We are working with CTC heat pumps to provide training for installers on how to correctly install heat meters and temperature sensors. For more info email me on


    • Chris Wickins says:

      Thanks. I think providing training is an excellent idea and i know Ofgem do too. It’s great that a meter manufacturer is taking responsibility. It might be worth talking to Kelly Butler at BEAMA, who i know is also doing some work on metering training.

      The trick will be to encourage installers to go on training before it’s too late. No one will benefit if the application system for metered systems in the domestic RHI grinds to a halt.

      Meter manufacturers could simplify their installation instructions too to focus on the most important things! How about 1 page rather than 20?!?

      And what about shipping meters with all of the components needed for installation. Like the ‘tees’ the sensor pockets screw into?

      Thanks for getting involved.


Leave a Reply

DECC @YouTube

DECC's YouTube feed

Share DECC's YouTube Channel

view all

Partners & Help