Heat Pump Monitor & emonPi advice

Super, thank you. I will post back in a few weeks once (fingers crossed), we’re all up and running. Much obliged.

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Looking forward to hearing the results of your install as well. Especially that temperature drop across the heat exchanger as @johncantor mentioned. Will you be measuring the flow temperature to the radiators coming off the heat exchanger or are those emonTH points at the end of long pipe runs?

Interestingly the Midea installation manual seems to insist on either a heat exchanger or a buffer… The -2C output of 13kW suggests that it’s maybe closer to your heat loss when it’s really cold.

Maybe fit one of the EmonTH’s temporarily on the flow from heat-exchanger. This would give you an assessment of the heat-exchanger dt, also an idea of any pipe loss, not that it matters if its useful to the house…


Thanks. The external emonTH sensors are placed just before the last radiators in each zone (our plumber was good enough to put thermowells into the 21mm flow pipe under floorboards) with the emonTH’s hidden in cupboards nearby. Would you recommend also placing sensors on the radiators themselves somehow?

Thank you, I will do that - and I appreciate the advice on putting the lower tank sensor at the same point as the ASHP probe - I had not considered the potential benefit of matching these.

I wondered if there was any advice or experience on how to create semi-permanent seal for the DS18B20 probes within the larger thermostat pockets of a typical water tank? The pocket diameters are significantly wider than the sensors which I’m conscious might skew readings.

Imagined I might use following in order:

  • thermal paste on sensor
  • alu foil ‘plug’ to help reflect heat back inside the pocket
  • final bubble wrap wad for insulation

Any more professional methods that have been tried/tested? Apologies if I missed previous articles on this. Tku

I think I might have confused you (and myself) with my final comment. You already have a sensor right by the heat exchanger. ignore my suggestion of temporarily using an EmonTH for anything

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Sounds sensible to me. I’d be tempted to fill the pocket with the grease and push the sensor into that. Then cover with your alu foil and whatever insulation takes your fancy.

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Happy New Year everyone.

By way of an update, the system is largely installed now and providing excellent visibility of the ASHP performance generally. Still, some things to fix, namely some unexplained dropouts from the SDM120 meter and work to integrate the Sontex heat meter which I’ll address separately.

While not directly related to the emonpi setup, I’d greatly appreciate your thoughts on the data being obtained by it in the meantime given the heat pump experience within the forum…

We’re seeing a consistent 7-8 °C loss between the primary and secondary flow temperature within the plate heat exchanger. Our radiators are unable to break 38-39 C as a result - a long way off the 50 C max design temp. The two flow sensors are in close proximity to one another and am confident they are reading fluid temps accurately (all genuine, tested DS18B20s from OEM with each one plumbed into a brass sensor pocket).

While the ASHP itself is working very well, the data points to a significant temperature loss at the source. I am told the heat exchanger is there to maintain hydraulic separation but wondering what value this is adding besides some protection for the ASHP.

Is this normal for other ASHP installations across the group and data others may have obtained? Any insight is hugely appreciated…


There will obviously be some loss across the heat exchanger. How much depends on the flowrate and the design of the heat exchanger. A greater flowrate gives less time for heat to move from the primary to the secondary. An exchanger with less area provides less capacity to move heat, and there are other design paramters that affect the exchange. The simplest way to reduce the difference would be to reduce the pump speed but that affects the mass of water delivered and so the total heat transferred. It all depends what’s most important to you. Changing the heat exchanger would offer a greater range of possible performance combinations. Your temperature difference is normal, but on the high side, AIUI.

A quick google shows lots of hits on the subject of which one is Heat Exchanger Selection and Sizing Guide

Thanks for sharing @thedawnbefore wow that’s really interesting and quite a temperature drop across the heat exchanger! Do you have the spec/model number of the installed heat exchanger, it would be interesting to understand what the options might be for reducing that temperature drop as @djh mentioned. @johncantor would be interested in your latest update as well.

Do you know if they decided on a heat exchanger rather than low loss header or just direct connection because of the wish to have glycol in the heat pump loop but not the rads? or was it just for hydrolic separation?

Can you take a pic of the heat exchanger for interest? Is that a normal cylinder with coil for DHW?
Anyhow, plate heat exchangers are not common (I dont know of any), and 7-8decrees is what you might expect. one advantage is you don’t need much glycol in the system.
So, the dt across the HP flow and return looks like 7 deg to me. can you try increasing the heat pump flow a little to see what happens? would 4deg between heat pump flow and heat pump return help a bit?

Thanks all for the very helpful feedback and queries and apologies for the slight delay in responding while travelling. I have attached photos and details of the heat exchanger, which seems to be the following model


Thank you @djh, i have a meeting with the installer later today to run through the OEM data and this guide will provide some helpful talking points. I could imagine a ~2-3C temperature drop but have difficulty with the current loss.

Thanks @TrystanLea , i’m told the heat exchanger is there to protect the ASHP (a Midea 16kw) and reduce the need for glycol in the radiators. For further context, it’s a fairly conventional well-insulated 1970 4-bed brick-built detached house, 15 rads and ~2k sqft of heated space in total.

I’m thinking any glycol saving may quickly prove false economy however and my private research suggests most ASHP’s work fine without full separation or a low-loss header instead.

Thanks @johncantor , photos attached of the Heat Exchanger specs. I will do as you suggested and run at 50 degrees today and report back. My slight fear is that due to the power consumption of the Midea ASHP, higher flow temps may not be a long-term solution for me.

And forgive the temporary OEM setup and spaghetti wiring while we test it all, still to be plumbed in! More than can be said for the pipe lagging…

I think I misled you… sorry, I meant increase the flow-rate so that the temperature difference between HP flow and return temperatures reduces a bit. It seemed to be at least 7degrees. it would be interesting to see your graphs with say 4 or 5degrees td. e.g. your 47c/39c might become 45c/40c
The plate HE looks a good type, it is a very common design. You might find selection software, but not sure that would help a lot. Hard to say if your best plan is to simply make the best of what you have, or make some changes. proceed with caution I would say

Hello Ben

@Timbones tells me that you are in the process of uploading your heat pump data to emoncms.org for the public dashboard! Look forward to seeing the data for your system!

Im interested to learn more about your setup with the heat exchanger. Did you manage to reduce that temperature drop? Do you know what the flow rates are either side of the heat exchanger?


Hi Trystan, thanks for circling back on this and looking forward to participating in HeatPumpMonitor - super initiative.

As for the heat exchanger issue, yes quite a journey of discovery.

In short, we are up and running and warm, and only thanks to OEM’s Heat Pump Monitor and the advice we received from this community.

When we installed OEM’s Heat Pump monitor alongside our ASHP last year, we expected to use it for general resource monitoring and optimisation. However, after OEM’s temperature sensors indicated the significant loss across the plate heat exchanger on our new install (pleased I bought genuine DS18B20s from you guys), and weeks of arguments with the installer, we were able to clearly pinpoint the root of the fault. This also meant demonstrating that the loss was not caused by poor lagging in our buried CH circuit which the installer was arguing.

In fact, the installer had plumbed the heat exchanger upside down. :man_facepalming:

The installer was rather blown away by the data available from OEM, After rectifying the issue (and legal action averted), we now have a moderate ~2C loss between the two circuits (see dashboard below, work in progress still). While still a source of inefficiency I am more satisfied with where we’re at. We are now working with another competent local firm to service our unit, who, interestingly, tell us they’re see the heat exchanger being used more often. They assume due to some performance risk and glycol savings from installer perspective?

I am not sure what we would have done without the data from OEM - a belated thank you to everybody who helped us with this. I am now recommending to others in the village looking to go the same route that they invest in the same capability and monitoring upfront. It has saved us a world of pain!

Best wishes,


Thanks Ben, great to hear! I’m glad this has been useful for you! Did you need any help with uploading the data to emoncms.org? I’ve created an account for you which you are welcome to use for this free of charge, I think @Timbones has sent you the details?

It would be interesting to put your figures through a heat exchanger calculator to see what it suggests you are getting for a heat transfer coefficient with those temperatures Preliminary Heat Exchanger Design | Excel Calculations. A 2C drop seems pretty good!

Hello @thedawnbefore I’ve created a little web tool to calculate expected system temperatures for a heat pump system with a heat exchanger: https://openenergymonitor.org/heatexchanger/2.html. I have written more about this over on an other thread here if you haven’t seen it already: https://community.openenergymonitor.org/t/incompetent-installers-incompetent-regulation-do-i-need-a-mim-b19n/21411/214.

It would be interesting if you could compare your measured results to those that this calculator suggests.

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Hi @TrystanLea sorry for delay coming back. The tool looks wonderful - thanks for your efforts here. Well outside my comfort zone but would like to contribute data to this once i have the sync running.

I’m continuing to have trouble getting synced to emoncms.org unfortunately, having to manually trigger the uploads from the Pi - i’m guessing it’s an issue with my feed updater.

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