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).
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…
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…
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 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.