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Ecodan thoughts please: QUHZ-W40VA takes longer than I would like to revert back to heat mode following a DHW cycle

 
Air Source Heat Pumps (ASHPs)
Last Post by Barry the bear 11 months ago
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 Barry the bear
(@barry-the-bear)
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Joined: 11 months ago
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Topic starter 14/10/2024 9:02 am  
My QUHZ-W40VA takes longer than I would like to revert back to heat mode following a DHW cycle. 
 
In the attached typical example (data from MELClound) DHW heating finishes around 1.07pm. The QUHZ leaves standby and reverts back to heat mode around 4.11pm, but only when the flow temperature (blue line) matches the 32C UFH flow temperature set (in this example) by weather compensation. The room stats are calling for heat the entire time, so the heating should return sooner.
 
I have been firmly told by Mitsuibishi that the system is behaving normally, but a delay of 3 hours returning to heat is excessive and disruptive. Heating the DHW to a higher temperature (anything more than more than 50C currently set) and or a lower UFH flow temperature exacerbates the problem, as would multiple DHW cycles in a 24hr period. My UFH will run at low flow temperatures, but not immediately after a DHW cycle. In all other respects the system is fine.

 
Any expert opinions out there!
 
1.07pm DHW cycle finishes
4.11pm heat mode starts when 32C flow temp matched

This topic was modified 11 months ago by Mars

   
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Mars
 Mars
(@editor)
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Joined: 5 years ago
Posts: 3415
15/10/2024 9:08 am  

Welcome to the forums @barry-the-bear

Your experience does seem a bit unusual, but I noticed that the unit in question is a 4kW Ecodan QUHZ. What makes this model stand out is that it uses CO2 (R744) as its refrigerant, which is quite different from other Ecodan models that use traditional refrigerants like R32. While I’m not a refrigerant specialist, CO2-based systems tend to operate differently, especially with regard to temperature management.

One key aspect is that CO2 systems generally prefer lower return temperatures, and this can impact the delta T. With a CO2 system like the QUHZ, a delta T of around 30°C isn’t uncommon, which can explain why you’re noticing specific temperature behaviours. It’s possible that the system is designed to prioritise certain flow temperatures during mode changes (for example, between heating and defrost) to optimise refrigerant performance.

It might be that when the unit switches modes, it needs to hit a specific flow temperature - like the 32°C you’ve observed - before resuming normal operation. This is likely tied to the system’s internal logic for efficiently managing CO2 refrigerant, which behaves differently compared to more common refrigerants.

It could be helpful to have a chat with a Mitsubishi support to get some insight into how the QUHZ manages these transitions. They should be able to explain the logic behind the temperature regulation more clearly.

Let us know.


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 Barry the bear
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Topic starter 15/10/2024 10:10 am  

Thank you for your welcome and reply.

You are right that the QUHZ use CO2 (R744) refrigerant. CO2 does have special properties and may operate more efficiently closer to its critical point (31°C), but I have no idea if that plays a role in the delay transitioning from DHW mode to heating mode - and I'm not sure Mitsubishi do either or at least it's not something they've bothered to explain to me. I have talked to them several times, but not once was the refrigerant mentioned.

About the specific (transitional) UFH flow temperature, there seems nothing special about 32C. That's just what it was on the day. What seems to matter is that the post DHW flow temperature matches the required UFH flow temperature. If I fixed the UFH flow at 40C for example the UFH would start fairly promptly, at 38C it take a little longer and so on. The UFH delta T is typically around 7C.

Cheers!



   
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 Anonymous 5011
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Joined: 3 years ago
Posts: 563
17/10/2024 5:33 pm  

So I suppose the real question is you house cooling down to much or are temps ok?

How is the circulation pump operated on all the time?



   
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 Barry the bear
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Topic starter 18/10/2024 11:56 am  

The house isn't cooling while for heat mode - the fabric of the house and the MVHR and so on ensures that doesn't happen, but that doesn't mean it isn't desirable to have a shorter delay. The pump doesn't run all of the time. 



   
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 Anonymous 5011
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Posts: 563
18/10/2024 4:04 pm  

Posted by: @barry-the-bear
↑

The pump doesn't run all of the time.

That may be your issue. You have hot water sitting in, I assume is an insulated pipe, so it will not cool quickly. If your pump runs when ever there is a demand for heat, the hot water in the pipe is flushed away when the diverter valve moves to heating system and cools really quickly.

The other issue could be if you have mixer valve on the UFH, it will just recycle 90% of its return water and only let a small amount of hot water through. So you see an issue, but in reality you UFH is getting hot water.

Today even without having to do DHW heating, my ASHP compressor went off 8 hrs ago, water is just circulating, sensing the return temp, if it needs the compressor to start it will.



   
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 Barry the bear
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Posts: 4
Topic starter 18/10/2024 5:24 pm  

I don't know if you are right or not!. I am sure though that the UFH doesn't receive any hot water until QUHZ sends it and UFH pump, housed in the packaged thermal store won't run until the flow temperature reduces to the required level. My thought was that the rapid cooling from minute 11 to 15 ideally should continue a little longer. 



   
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