ASHP water leaving temperature control

[JohnMo]

Many heat pump suffer short cycling or fairly short run times in shoulder months, mine included.

 

So in an attempt to understand this more clearly and hopefully developing a coping strategy for all I am after some information.

 

Water leaving temperature will have a target temp let's say 30 degrees, the heat pump has hysterisis for stopping and starting around this temp.

 

So mine (Maxa) it is 

To start heating CH water the return water needed to be target delta T (between flow and return) + and adjustable 2 degs. So at 30 Deg the flow has to of reduced to between 22-23 Deg.

 

To stop heating the overshoot is allowed to to 0.2 degs over so 30.2 degs.

 

Grant/Chofu heat pumps are 

Start 8 degs below target (22 Deg)

Stop 1 Deg above target (31 degs)

 

The above are adjustable settings 

 

What is yours?

Make

Heating type - radiators, UFH or mixed.

 

Another interesting feature is circulation pump option mode.

Run always when there is a heating demand and sniffer mode, with only periodic running of circulation pump, one for a minute off for 10 mins.

 

Do you know how yours operates?

[IanR]

20 minutes ago, JohnMo said:

What is yours?

Make

Heating type - radiators, UFH or mixed.

 

Another interesting feature is circulation pump option mode.

 

Do you know how yours operates?

 

Nibe F2040 with SMO40 controller

4P Buffer

Zoned UFH + Wet Duct heat exchanger on MVHR

 

Nibe uses Degree Minutes for space heating rather than straight temp hysterisis. Factory set at 60 DM (range 2 - 2000)

 

My ASHP typically runs for an hour and switches off for an hour or longer depending when DM 60 is met.

 

The circ pump works in sniffer mode.

[Dave Jones]

from my talks with panasonic, the largest factor in causing short cycling was incorrectly sized heaty pump followed by zoning.

 

Our was only turned on march, with flow temp set to 25 to give a too warm 21 inside (no loft insulation at time) it was getting COP of 8+ and never really turned off. We have a setback now so wil be interesting come November time to see how it performs now we are fully insulated and air tight.

[PhilT]

4 hours ago, JohnMo said:

Many heat pump suffer short cycling or fairly short run times in shoulder months, mine included.

How would you define short cycling in this case?

[JohnMo]

25 minutes ago, PhilT said:

How would you define short cycling in this case?

Anything from a 1 min to about 10 min. Once you get to around 10 mins your heat pump has started modulation and you are getting a reasonable CoP. So the longer run you can get the better.

[sharpener]

4 hours ago, IanR said:

Nibe uses Degree Minutes for space heating rather than straight temp hysterisis. Factory set at 60 DM (range 2 - 2000)

I believe Vaillant Arotherm plus are similar, default also -60. Not sure about max, ?-120 DM.

[PhilT]

14 minutes ago, JohnMo said:

Anything from a 1 min to about 10 min. Once you get to around 10 mins your heat pump has started modulation and you are getting a reasonable CoP. So the longer run you can get the better.

Do you have an auto mode? If so might be worth trying. On mine it makes a HUGE difference to the operation in the shoulder months. Although my HP is at least 50% over capacity, in the shoulder months the compressor always runs continuously at no less than minimum power for no less than 20 minutes, so typically, in 40 minute cycles the flow temp curve rises and falls between 30 and 40, with the pumps running continuously. Last year I changed from WC to auto mode; in Sep last year in WC mode the COP average was 3.6, and in Oct in auto mode the COP was 4.2. Previously in WC mode I could see the compressor switching on and off every few minutes trying to maintain whatever constant flow temp was determined by the WC curve.

[JohnMo]

1 hour ago, PhilT said:

2 hours ago, JohnMo said:

Anything from a 1 min to about 10 min. Once you get to around 10 mins your heat pump has started modulation and you are getting a reasonable CoP. So the longer run you can get the better.

Expand  

Do you have an auto mode

Unfortunately not. So may have to set something up manually. But luckily have lots of concrete to soak up the heat.

 

Interesting to see the different schemes in place from various manufacturers.

[JoeBano]

Grant aerona3 10kw 

3 port buffer 3 zones, 2 Ufh and 1 rad’s, main pump on sniffer mode. 
Sure it’s running 5c hysteresis but having messed with that setting for awhile. Normal cycle time in the mild months with 1 zone calling for heat on for 10 mins then off 20 mins ish. All zone calling for heat then on for 45 mins to hour. Do I have noise reduction mode on 80% as my heat loss is around 5.8-6kw at -2 and just to keep the hot water quiet as the heat pump points at my neighbour patio and he’s retired out there all day. 

 

[ReedRichards]

I have an LG Therma V heat pump with a Drayton Wiser controller.  When a cycle stops, the controller will not allow a new cycle to start until 20 minutes after the start of the previous cycle (it would do the same for an oil boiler).  I suppose there is still potential for each individual cycle to be short but there will be at most 3 cycles per hour.  And my house maintains the set room temperatures  (two zones) so it cannot be short cycling and failing to provide enough heat.

 

All radiators, 4-port buffer built-in below DHW cylinder.

Heating control by LWT only (there are alternatives that I haven't tried).

I'm not aware that I can control the overshoot temperature; I have observed a degree or two overshoot in winter.

On is via the controller which immediately starts the internal circulation pump.  Heat pump spends several minutes in "start-up mode" before it reports "internal unit" in operation.

Heat pump starts at low input power and ramps up (stepwise) as it sees fit.

WC settings : -14,55 20,22 (determined by experiment).

 

 

[JohnMo]

Had a trial last night

 

have been playing with this myself. LWT from the heat pump is controlled a couple of parameters. To start the heat pump the LWT has to be set delta temperature below target flow temperature. To stop the heat cycle it has to be a set delta temperature above target.

 

The factory settings on my heat pump for the compressor starting dT plus 2 degs and for stopping the compressor 0.2 above target. I did change these last year to 0.4 above. But always noticed a reasonably short cycle time so at 7 OAT would run for 15 minutes and off the 20. Which in it's self is ok, but not the most efficient. With a gas boiler I could run at 25 degrees flow temp, but to get the same heat into the house I now need to run at 29.

 

So last night, set the target flow temperature to 26 and overshoot to 4 degs and the heat pump ran for 20+ before I shut it off. It would have run longer, was at 29.1 when I stopped it.

 

So can get a decent cycle time easy enough, a low target flow temp will ensure the compressor doesn't restart too quickly. But can be manipulated with the circulation running in intermittent or sniffer mode

 

My run, data from the battery so is whole house. It's in 5 min snap shots, but you can see the drop in load at about 5 mins after it's hit target temp. Flow target was 26 degs, end temp when I shut down was 29.1.

 

We have a wet dull cool day again today, will will let the heat pump do a couple of cycles to see what it does.

[billt]

Samsung 8kW gen 6, radiators and 120L vented cylinder.

 

The Samsung system controller is left to its own devices. Weather compensation set to 27C flow at 15C outside, 37C flow at -4C outside with a room temperature sensor. Pump runs whenever there's a demand for heat but flow rate is adjusted by the controller to maintain about 5C flow/return differential.

 

Short cycling / short runs aren't an issue and I don't have to agonise over the minutiae of control algorithms.

[JohnMo]

3 hours ago, JohnMo said:

We have a wet dull cool day again today, will will let the heat pump do a couple of cycles to see what it does.

Started my new trial. Overshoot set for 3.0 and flow target set for 26, OAT is 12. Wouldn't normally have the heating on, so a good test for how cycling is affected. First run, ran for 1.25 hrs. Then sat idle for 1.5 hrs. On the next start I reduced the overshoot to 2.5 degs. But DHW heating kicked in.  Currently on a thermostat override to keep on to see how it does. Will leave to run the rest of the day.

[JohnMo]

Made some adjustments based on information provided here and on Renewable Heating Hub.

 

Prior to stating 

Below Target start delta was 8 degs

Above Target stop delta was 0.4. 

Target flow temp 30.2. with a small WC curve rise to 35 flow at -3.

 

This morning it was close to 11 degs, which is above where I would normally heat. After some tuning got to 2 starts per hour and a 15 to 20 min run time. The sun came out at lunch and the heat pump hasn't restarted, living room at 21.6. ASHP return temp is sitting at 21.1.

 

So setting are quite different 

Below Target start delta is now 5.8 degs

Above Target stop delta is now 2.0 degs. 

Target flow temp 25 at 14, with a WC curve to -5 and max flow temp of 32.6 degs.

 

Upside is it should run without any house thermostats and generally look after it's self.

 

Just need some warm weather to set up cooling.

 

So the take away are 

 

1. An aspect never discussed is how the heat pump responds to target flow temp.

2. What manufacturer default to may be miles away from what is needed.

3. You don't have to suffer short cycling, it fixable with a little time.

4. Base running target temp has been reduced a whole 5 degs - which should see an uplift of 0.5 in CoP.

 

[JohnMo]

1 hour ago, JohnMo said:

Below Target start delta is now 0.8 degs

Just seen a mistake and can no longer edit. The above should read 5.8 degs.

[jack]

18 hours ago, JohnMo said:

Just seen a mistake and can no longer edit. The above should read 5.8 degs.

Fixed - let me know if any issues.

[sharpener]

Why is the edit window so short @jack? Comparable sites like the Arotherm plus FB page or https://camelot-forum.co.uk/ allow indefinite editing, and it does not seem to be abused in either case.

 

[PhilT]

23 hours ago, JohnMo said:

Just seen a mistake and can no longer edit. The above should read 5.8 degs.

Apart from the power graph you showed earlier, can you see a smooth flow temp line? On mine this is the graphic which best shows the compressor either switching on and off, or running continuously

[jack]

1 hour ago, sharpener said:

Why is the edit window so short @jack?

 

We went through a period a few years ago where spammers would join, post a few inoccuous posts, then go back and edit spam into them once they'd disappeared off the front page. It was just about impossible to police.

 

We still get people posting the inoccuous initial posts, but they generally lose interest and stop posting when they realise the editing block is in place.

 

1 hour ago, sharpener said:

Comparable sites like the Arotherm plus FB page or https://camelot-forum.co.uk/ allow indefinite editing, and it does not seem to be abused in either case.

 

Facebook has its own tools for reducing the impact of spam (on top of actions of the page admins). We don't have anything like that.

 

We have over 40 times as many members as Camelot, but probably a similar number of mods.

 

If anyone wants anything edited, just let a mod know and they'll happily do it for you.

[JohnMo]

1 hour ago, PhilT said:

Apart from the power graph you showed earlier, can you see a smooth flow temp line? On mine this is the graphic which best shows the compressor either switching on and off, or running continuously

Currently no. I have a add on coming from Italy hopefully next week that ties in to the modbus allows me log running data directly. I am currently monitoring with a Shelly add on, but the graphics turn in to hourly unless you want to pay a monthly fee. Also monitor on the controller, but it has no direct output or graphics unless you tie in to the modbus.

[JohnMo]

On 12/07/2024 at 17:49, PhilT said:

Apart from the power graph you showed earlier, can you see a smooth flow temp line? On mine this is the graphic which best shows the compressor either switching on and off, or running continuously

I got this out of home assistant and is the flow temperature going in to the UFH. It has only done two cycles today. First run was at 0425, stopping at 0452, so about 27 mins run time, then off for an hour, restarting around 0550 and running 20 mins or so. It was about 10 degs outside. Now 13 deg.

 

This is the garden room I am trying to keep warm (19 to 20 degs). All being done without a thermostat on WC only. Currently getting about 1 deg swing in temperature.

 

 

This is the house also connected in the same zone so also getting the same heated water in the UFH, house floor acting as a buffer. Getting 0.25 deg Swing Temp.

 

 

 

[PhilT]

4 hours ago, JohnMo said:

I got this out of home assistant and is the flow temperature going in to the UFH. It has only done two cycles today. First run was at 0425, stopping at 0452, so about 27 mins run time, then off for an hour, restarting around 0550 and running 20 mins or so. It was about 10 degs outside. Now 13 deg.

 

 

Definitely no compressor cycling there, job done.

[JohnMo]

After the above, I did a little more fine tuning and ended up

Restart hysterisis of 4.9 and shut off hysterisis of 2.8. this suits heating and cooling.

 

Moved the heat pump on to cooling  using a target flow temperature of 14.5 at 30 degrees to 15 at 20 degrees, so cooling now also self manages, been on continuously since July, hot day the heat pumps runs 45 mins on and about 20 mins off and a cooler weather doesn't switch on at all, been off since midnight. Costs nothing to run. If it's sunny I need cooling, so PV available.