How are people finding the accuracy of Sontex heat meters, especially over time, as compared to ASHP flow meters?
In theory, the Sontex meter comes pre-calibrated and should be trusted much more than any built-in flow meters in ASHP, but in my experience, the flow rate reported by the Sontex meter isn’t always as accurate as you’d assume.
I had a “Sontex Superstatic 449” heat meter installed along with Valiant AroTHERM just over a year ago as part of a PasivLiving (government-subsidised) monitoring system. But when I looked into the low COP’s reported in the dashboard I found that the Sontex meter was reporting 20% less flow rate than the ASHP provided information. This in turn impacted the energy output, COP and also my RHI repayments.
As part of my first annual ASHP service, I asked my installer to replace the Sontex flow meter body and immediately the flow rates matched the Vaillant flow rate exactly. That said, by the end of the same day the Sontex meter was reporting -5% less than the ASHP and a week later is showing -7%.
The flow meter body is installed according to the installation instructions vertically on the return inside the house.
In theory, ultrasonic flow meters should not be impacted by dirt, but I’m struggling to get consistent figures that can be relied on from the Sontext meter I have installed and wondered if anyone has experienced anything similar or has any advice?
Hello @dfeist that’s interesting. I only have the one Sontex 531 on mine, no parallel meter to compare with. @glyn.hudson has a Sontex 789 and then also the Sika VFS that comes with his Samsung heat pump. The flow rates from both of those appear to be within 1% and there’s no visible change in that since it was installed last September.
I am comparing it with the ASHP flow-rate reports. Initially, I assumed the Vaillant flow rate must not be accurate. Others, from what I’ve seen on Twitter also trust heat-meter over their ASHP figures.
But given a large difference, I looked in more detail and saw that if I added 20% to the heat-meter reporting flow rate then power output and COP all started matching the spec sheets perfectly.
The flow rate of the new flowmeter body was within 1% here too initially, but within a day it was reporting 5% less! I can’t easily clean/replace it without draining the heating circuit and I don’t want to pay our installer every few months to look at it.
Over the past month the Sontex recorded an average of 8.9L/min which the Sika reported 8L/min.
If there was any discrepancy I would be more included to believe the Sontex since the Sika is a vortex flow meter which is prone to being more effected by air bubbles than the fluid oscillation principal that the Sontex uses.
Temperature sensing is a different story, I’ve found significant inaccuracies in the onboard temperature sensing in my Samsung heat pump compared to the Sontex which uses wetted pocket temperature sensors. This results in the heatpumps inbuilt monitoring reporting 30% less heat delivered than the Sontex. The Sontex has Class 2 accuracy guarantee while the manufacture’s inbuilt monitoring has no accuracy guarantee, therefore I’m more inclined to believe the Sontex.
@marko You are right, I wrongly assumed they were ultrasonic!
Air in the system is unlikely because the flow meter is installed vertically, the system supposedly has the necessary valves to allow air out, and I have run the purge cycle for 1hr.
Dirt is a possibility I guess. The only filter I know of (which is incorporated in the return shut-off valve) is after the flow meter, rather than before.
There is a 150-200mm straight pipe before the flow meter, and the flow meter is DN20. I believe this meets the installation requirements. There are no vibrations. The flow rates we are talking about are in the range of 800-1200 l/hr.
Any thoughts on how I could/should adjust the installation to avoid these issues?
Dirt in the system is not good news for any components, have you got a strainer and magnetic filter fitted? Assuming the installer cleaned these out during the service? If so your system should now be dirt free, maybe it wasn’t properly flushed after the initial installation? The Sontex should be able to handle dirt better than other ultrasonic meters, but I’m sure there are limits to this! Does the old meter body the installer removed look dirty inside?
Yes, the installation has both a strainer and magnetic filter, although the strainer is after the flow meter (between the flowmeter and the monoblock), rather than before it.
The strainer was cleaned as part of the service, unsure about the magnetic filter. Initially, the new flow meter body reported exactly the same as the ASHP.
The old meter body did have some dirt, but I didn’t see it personally so not sure how much. It’s possible that the system wasn’t fully flushed after the initial installation, although I would have assumed the installer would have done this.
I do actually have another Sontex heat meter in the system (on the UFH loop) and by comparison, this has had no issues at all from what I can tell. The only difference with this meter is:
Our set up has a Sontex heat meter (as part of Passivsystems MMSP) and we also have a circulation pump speed % from the NIBEuplink data feeds. Obviously the NIBE is just a % of the max speed at which it thinks the circulation pump can run. Nevertheless the 2 parameters are clearly in reasonable agreement in relative terms.
“Temperature sensing is a different story, I’ve found significant inaccuracies in the onboard temperature sensing in my Samsung heat pump compared to the Sontex which uses wetted pocket temperature sensors. This results in the heatpumps inbuilt monitoring reporting 30% less heat delivered than the Sontex.”
That discrepancy almost entirely due to the working fluid configuration not the temperatures. The heat pump is coded to assume glycol mix; which has a lower heat capacity than water.
“The Sontex has Class 2 accuracy guarantee while the manufacture’s inbuilt monitoring has no accuracy guarantee, therefore I’m more inclined to believe the Sontex.”
I’d say not quite.
The Sontex is type approved to MID Class 2. In the real world it will only be as accurate as the installation. For pure water systems it will indeed be good. For glycol mixtures it’s anybody’s guess as to how closely the actual mix matches the working fluid configured on the heat meter calculator. There is no “guarantee” of accuracy though.
I’m not a Sontex employee or rep so can’t add much more than I have already.
If the old meter body “did have some dirt” that is definitely a problem for your system in general. (there shouldn’t be any)
I would waegr that it’s probably also a problem for the Sontex meter in particular as these rely on vortex-shedding, vortex-shedding relies on sharp edges and strong reflections amongst other things, and dirt in our meter is probably gong to soften all those / reduce vortex shedding frequency / cause under-reporting of flow.
If it were mine I would isolate, pull the thing to bits (you can dismantle the flow sensing element on these), and physically inspect for muck in the pressure ports to rule that out.
Here’s what one looks like inside: (the little holes are the pressure ports to the pressure transducers that are looking to sense the vortices as these are shed from the assembly designed to create/reflect these - see Sontex Superstatic 749 Fluidic Oscillator Heat Meter - YouTube for the flow patterns that crate pressure changes that the meter picks up)
Vs an ultrasonic meter of the “external transducer and mirror” style: (signal shoots across the pipe, hits the mirror, along the pipe, hits the mirror, then to the other ultrasonic transducer) Very important on these that the transducers are on the SIDE of a pipe if mounted horizontally not on the top (air) or bottom (dirt). There’s really nowhere for any dirt to settle here.
Or an ultrasonic meter of the “inline transducer” style: (signal shoots along the pipe between the two transducers) Orientation is less critical for this style and again there’s really nowhere for any dirt to settle here.
Hello @marko those are really interesting cut through pictures and insights on dirt vs air, thanks for sharing!
We did initially think that this was the case, so I will try and explain why we think the error is due to the surface mounting of the temperature sensors, or perhaps an issue with one of the Samsung sensors.
Taking a snapshot of the data in a steady state period:
The samsung in this example is reporting 1- (2203/3097) = 30% lower than the sontex, exactly in line with the differences Glyn is seeing from the energy reports on the samsung display. If the samsung was measuring the same DT as the sontex but had a calibration for 20% propylene glycol at 4020 J/kg.K that would only present a 3.8% error.
Glyn also has a couple of DS18B20 temperature sensors attached carefully to the surface of the pipes:
The samsung DT seems to be around 0.7K lower than the other two DT’s, that’s going to vary a bit of course but seems to be fairly consistent and appears to explain the 30% discrepancy that Glyn is seeing
No worries on cut throughs. Sometimes it’s easier to take the 4.5" electric spanner to something than to describe it in words.
(the day job is heat metering and much time is spent on the various ways the real world differs from lab conditions - should write a cheat sheet at some point as time allows)
Thanks for the explanation on the Samsung. I think you’re closer to right than I am on this one.
Differences in heat capacity are not in fact large enough to explain what you’re seeing.
Watch the units though. 15.66 litres/minute is not 15.66 kg/second. You ought to correct for density when converting volume at a temperature to a mass flowrate.
Perhaps the error caused by their measuring temperatures on the refrigerant side is also more significant when operating at deltaTs near the bottom of the expected range. (nits probably designed to operate at 5 or 8K dT rather than 3K?)