HeatPumpMonitor - DHW efficiency

Was having a nose at the graphs for Heat Geek’s Mytchett heat pump and noticed that for yesterday (1st Jan 2023), it’s showing “Water Heating” as 5.62 but there do not appear to be any DHW sections on the graph.

Might it have been too short a period to render the light blue background for? If their DHW was already at desired temperature perhaps?

If you click on the Water Heating COP you will be able to see the power used and generated,

As you can see this is literally 120w so may have just turned on for a moment and then off again.

Adam’s tank is quite unique (custom design and extremely high efficiency but I believe at a price point of ~ £1.5 - £2k) and something I hope gains a lot more traction in the Heating world very soon.


Thank you. I had not noticed that you could click on those values for more detail.

I am looking forward to the Heat Geek video on the subject. The stats someone shared in this group showed their Newark partnership cylinders featured a 6m² surface area coil.

[Here is the Newark cylinder - RW]

Yep, the larger of the new HG cylinders do have a 6msq coil.

But as I’ve alluded to in many DHW threads. It’s not always about the coil, it’s how you deliver the heat to the coil or plate.

Investigations I’m doing now show there isn’t much between a 6msq coil, 3msq plate, a 3msq coil and a 1.5msq coil.

On the Vaillant, no matter what transfer medium you’ve got (coil or plate), you need to be using Eco mode for best efficiency. Low compressor mode, but it’s much slower. That’s the trade off.

If you use Normal you are giving up around 0.6 COP per hot water run. (versus Eco)

And heating from 45C to 50C loses you about 0.4 COP too. It soon all adds up.

This is why Adam hot water is flying, he’s using both Eco and only heating to 45C. Plus it’s always slightly warmer ‘down south’. :rofl:

If you’re using Normal and heating towards 50C and beyond, you’ll never match his efficiency. 6msq coil or no 6msq coil.

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Following your guide, I dropped my heat curve to 0.5 yesterday from 0.6 but back up to 0.55 when our guests noticed it being cooler. Will give 0.5 another try later this week as the outside temperature is forecast to drop a few degrees.

Our current cylinder is only 150L. On Normal mode it took an hour last night to heat the water from 20 to 48. I have just amended the “Max cyl. charging time” from 60 min to 120 min and changed the “mode on setback” from Normal to ECO. :crossed_fingers:

If you put max charging time to 120 AND anti cycle time to 0 you can then get unlimited DHW run times.

The water wasn’t as cold as the previous night so I’m not able to really compare the outcome yet but it looks like it still ran for an hour, despite me increasing the charge time setting to 120 mins and amending the schedule in the weekly planner from 03:30-04:30 to 03:00-05:00.

In some respects, the outcome was worse as the COP was 2.46 heating from 34°C to 48°C, with the previous night COP of 2.41 heating from 18.5°C to 48°C.

  1. Starting colder will always yield better results as the heat pump spends more time in colder flow/return.

If you click on instantaneous COP you should see the effect of this.

  1. You say you’re heating to 48, but looks like your flow finishes past 60+ and return close to that. You’ll lose a lot of COP once you get past 50C flow/return.

What is your cylinder charge offset setting? This setting dictates how much your flow temp goes above your target temp.

The 120 charge time setting is a maximum… not a target length.

“Max.cyl charging time”

If your hot water run only takes an hour, it only takes an hour. it finishes when your target temp is reached.

I was being optimistic the heat pump might set the flow rate, etc. based on the schedule / max charge time.

I was also forgetting the outside temperature was colder last night so the heat pump would have had to work harder.

Cylinder charge offset is on 5K. What would you suggest reducing it to, please?

Yes, cold plays a part

Nope. I’ve got my flow rate high to it keeps the DT narrow so the heatpump doesn’t have to work as hard.

If you’re only wanting 48C, you might as well put it down to 1K. ie, end temp should only creep a little over 48C (offset + DT), especially if you keep the flow rate high and the DT narrow.

Might be worth just making small changes each night and seeing what happens.

One final thing, are you sure you’re in Eco? Cos the graph looks like Balance to me (ie, Normal to 45C and then Eco beyond that).

Thank you.

I agree with your observation that it is not on Eco for DHW yet.
The setting I found was for “set-back mode” (under Circuit 1) which I switched from Normal to Eco. I did not spot anything similar in the DHW menu.

It’s not on the sensocomfort, it’s on the heat pump controller.

I detailed where on here

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Perfect, thank you.
I’ve dropped the Cylinder Charge Offset to 3K and switched the DHW to Eco, it was on Balance.
Will see how that goes tonight.

The most obvious difference with having the “set-back mode” set to Eco, was the flow rate dropping to zero, rather than sitting at 8.8 l/m

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How does keeping dT narrow mean less work for heat pump?

Far more gentle power consumption this morning. Peaked at 1226 W towards the very end of the DHW cycle. Previous nights would see power consumption peaking around 2800 W after the first ten minutes.

Flow temp peaked a little below 60°C. Will see how these settings fair for a few days before trying a lower offset.

Flow goes out 40C and comes back 30C, heat pump has gotta raise the water by 10C before sending it back out.

Flow goes out 40C and comes back 35C, it’s only gotta raise the temp by 5C, so less energy / work required?

My understanding anyway. :man_shrugging:

If the controller is able to adjust the flow rate to target a specific dT, then it’s able to do this without changing the power it uses. Same if you’re fixing the pump speed to achieve a certain dT. Faster flow = lower dT for same heat produced.

What’s more efficient, with respect of consumed energy?

If the water is flowing slower, is there more opportunity for heat to transfer into the cylinder?

I’ve learned here that the pump uses significantly less power than the compressor but, pumping water that’s already warm (low dT) repeatedly to save the compressor working as hard feels like it might be a waste of energy.

Of course, there’s long term wear and tear to consider. If the compressor is an expensive part to service/replace then perhaps the high flow rate and low dT is the better option.

This makes no difference. It was highlighted on the heat geek mastery course.
It was recommended to have faster flow. Will need to go back and check what his reason was.

The can be tons of combinations of DT x SHC x flow rate to get to the same heat output.

My theory is that the combination with the lowest DT may stress the controller the least?

Perhaps a test is required (if you/we are able to track compressor speeds/power).

ie, one run with wide DT and one run with narrow DT

I’m running mine at just over 2 DT (with 100% pump speed) and i’m getting the best DHW performance i’ve seen.