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Sensors positioning for Ecodan 8.5kw ASHP with pre-plumbed cylinder and buffer

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(@tadpole)
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My first post, from a novice. We have just moved into a highly insulated and well-sealed (fabric first) timber frame home.

We have a Mitsubishi Ecodan PUZ-WM85VAA, EHPT30X-UKHDW1L 300l pre-plumbed cylinder, Telford 4 port buffer (120l?) in the garage loft with insulated pipes underground into the house.

Our plumber has installed Ecodan's before but never with a buffer. The buffer was at my insistence based on views from another forum - and I already acknowledge that this may have been misinformed.

Our (he is with me in all this) first challenge is just the DHW which we have on a recirculating loop with pump. I'm sure I'll be back with the heating once this is solved.  I've attached an example of what is happening from our Melcloud data.

PastedGraphic 1

In my words, the heat pump runs for about an hour then stops, sometimes with a U1 error and sometimes not. The flow temperature as reported never climbs above 34. The flow from the heat pump into the buffer is too hot to hold. The flow from the buffer into the cylinder assembly never gets hot - I'm guessing 34..... I realise the settings all need to be explored and tweaked. I've tinkered so much that I've considered a factory reset as a start point, but not gone there yet.  I "think" the first port of call is to move the flow sensor from it's current position on the pre-plumbed flow to the buffer flow. At least then the heat pump and cylinder electronics can make decisions on correct data.

Is this a sensible place to start?  Plus of course all other views will be welcome. 

I'll be back I'm sure for help with integrating our heat pump, UFH, Givenergy battery, Eddi, Solarwatt panels etc.

thanks in advance for any and every contribution.

Derek


   
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(@derek-m)
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Posted by: @tadpole

My first post, from a novice. We have just moved into a highly insulated and well-sealed (fabric first) timber frame home.

We have a Mitsubishi Ecodan PUZ-WM85VAA, EHPT30X-UKHDW1L 300l pre-plumbed cylinder, Telford 4 port buffer (120l?) in the garage loft with insulated pipes underground into the house.

Our plumber has installed Ecodan's before but never with a buffer. The buffer was at my insistence based on views from another forum - and I already acknowledge that this may have been misinformed.

Our (he is with me in all this) first challenge is just the DHW which we have on a recirculating loop with pump. I'm sure I'll be back with the heating once this is solved.  I've attached an example of what is happening from our Melcloud data.

PastedGraphic 1

In my words, the heat pump runs for about an hour then stops, sometimes with a U1 error and sometimes not. The flow temperature as reported never climbs above 34. The flow from the heat pump into the buffer is too hot to hold. The flow from the buffer into the cylinder assembly never gets hot - I'm guessing 34..... I realise the settings all need to be explored and tweaked. I've tinkered so much that I've considered a factory reset as a start point, but not gone there yet.  I "think" the first port of call is to move the flow sensor from it's current position on the pre-plumbed flow to the buffer flow. At least then the heat pump and cylinder electronics can make decisions on correct data.

Is this a sensible place to start?  Plus of course all other views will be welcome. 

I'll be back I'm sure for help with integrating our heat pump, UFH, Givenergy battery, Eddi, Solarwatt panels etc.

thanks in advance for any and every contribution.

Derek

I assume that your DHW is being heated by your solar PV system via the Eddi, but obviously that will not be the case during the Winter months.

Rather than removing the buffer tank, I would suggest that you convert it to perform the duty of a volumiser. This can be achieved by removing the primary inlet and secondary outlet pipes located near the top of the buffer tank and connecting these pipes together using a bypass pipe. Then remove the secondary return pipe from near the bottom of the buffer tank and connect it to where the secondary outlet pipe was removed. Then plug the two open connections. The buffer tank should then have one inlet connection and one outlet connection and be located in the return pipe to the heat pump.

It may also be possible and/or necessary to remove the secondary water pump.

 

 

This post was modified 11 months ago by Derek M

   
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(@tadpole)
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Joined: 11 months ago
Posts: 4
Topic starter  

Thank you both.

I think I need to be patient as I am between our installer "firing it up" and Mitsubishi commissioning the system.

I'm pretty sure that I will be back.

thanks

Derek


   
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(@harriup)
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Posts: 83
 

I only know the odd bit that I have read about buffers, and that they are a source of divergent opinions. My understanding is one would provide some hydraulic separation between the primary circulation and the heating circuits. You haven't specified how your system is laid out but from what you describe I might speculate that the buffer is installed between the heat pump and the tank. Is this right?

As the primary pump that pushes the water through the heat pump is installed on the tank then that needs to work on the flow and return to/from the pump, but if you have a buffer on this primary circulation pipework then that will allow the water to take a shortcut, so the water from the pump arrives hot at the buffer, but is mixed with colder water from the return before reaching the flow thermistor on the pre-plumbed pipework, and then not being hot enough to transfer heat into the tank.

As the pressure from the primary circulation pump is divided within the buffer tank this will have an effect on the water flow through the heat pump itself, reducing it, and it is this low flow rate through the heat exchanger between the refrigerant and water circuit that I believe might be giving you the U1 errors.

In addition as the flow thermistor is registering a lowish temperature the heat pump is probably being pushed much harder to increase the amount of heat transferred into the water circuit, which is why your buffer inlet is too hot to touch.

As Derek described, removing it entirely or converting it to a volumising tank on the return pipe is the answer.

Mitsubishi EcoDan 8.5 kW ASHP - radiators on a single loop
210l Mitsubishi solar tank
Solar thermal
3.94kW of PV


   
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(@sunandair)
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Posts: 345
 

Posted by: @tadpole

My first post, from a novice. We have just moved into a highly insulated and well-sealed (fabric first) timber frame home.

We have a Mitsubishi Ecodan PUZ-WM85VAA, EHPT30X-UKHDW1L 300l pre-plumbed cylinder, Telford 4 port buffer (120l?) in the garage loft with insulated pipes underground into the house.

Our plumber has installed Ecodan's before but never with a buffer. The buffer was at my insistence based on views from another forum - and I already acknowledge that this may have been misinformed.

Our (he is with me in all this) first challenge is just the DHW which we have on a recirculating loop with pump. I'm sure I'll be back with the heating once this is solved.  I've attached an example of what is happening from our Melcloud data.

-- Attachment is not available --

In my words, the heat pump runs for about an hour then stops, sometimes with a U1 error and sometimes not. The flow temperature as reported never climbs above 34. The flow from the heat pump into the buffer is too hot to hold. The flow from the buffer into the cylinder assembly never gets hot - I'm guessing 34..... I realise the settings all need to be explored and tweaked. I've tinkered so much that I've considered a factory reset as a start point, but not gone there yet.  I "think" the first port of call is to move the flow sensor from it's current position on the pre-plumbed flow to the buffer flow. At least then the heat pump and cylinder electronics can make decisions on correct data.

Is this a sensible place to start?  Plus of course all other views will be welcome. 

I'll be back I'm sure for help with integrating our heat pump, UFH, Givenergy battery, Eddi, Solarwatt panels etc.

thanks in advance for any and every contribution.

 

Hi Tadpole

I think you’re wise to let the installer and manufacturer fully install and commission before trying to fix anything. 

However, your 4 port buffer tank is a monster. At 120 litres your system is carrying around 18 radiators volume of extra water which will need to be heated up just to be stored this cannot, IMO, be efficient. I’m not sure how this was intended to be used from info so far but if you end up using it as a return flow volumiser then this is an excessive amount of surplus hot water since an 8.5 only requires around 12 litres to service a defrost cycle. 
Regarding the MELCloud report it’s clear this shows a very tiny temperature difference between Flow and Return which can be caused by high flow rate but also if the Flow thermistor is mistakenly on the secondary flow pipe (example). And as you appear to know this might be because the flow thermistor is not positioned on the Right pipe and may not even be placed on the Primary side at all. 
It also looks like the Primary Flow and Primary Return have a dominant flow rate, returning too much hot water straight back to the heat pump… If this is the case, this is known as kettling where the primary loop temp gets hotter and hotter. Possibly leading to cycling or error message. This might also point to why the DHW pipe is relatively cooler. 

It would be useful to understand what the room heating system is/ or is going to be. and if you are considering modifying  this system with a single primary circulating pump and a separate preinstalled DHW pump for the cylinder. 

I hope you can let us know.

👍🏻👍🏻

 

 


   
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(@tadpole)
Active Member Member
77 kWhs
Joined: 11 months ago
Posts: 4
Topic starter  

Hi SUNandAIR,

All good comments - thanks.

 

Without wishing to bore with our long back-story, we started out on this custom build around 12 months ago.  I found buildhub to be a great resource for understanding many things. The topic of buffer tanks surfaced quite often and the prevailing opinion appeared to be "buffer tanks good" to stop heat pumps cycling when servicing a small UFH loop. I asked our plumber to include one and he chose the size. I suspect a Mitsubishi "representative" (own customer service, own sales rep, distributor sales rep, who knows?) actually chose. Probably the same Mitsubishi "representative" that specified a 13.5kW ASHP when I had calculated that at -5 outside and 23 inside, the house should need around 2.5kW.  I even supplied the spreadsheet for them to reconsider.

In the event, I specified 8.5kW as that was the smallest R32 "quiet" model with a 300l DHW tank. I agreed to take the risk if it was not enough. [I know the spreadsheet is pretty accurate and our air test came in at 1.39.]  My primary concern was having plenty of hot water as inexpensively as possible.  We have wet UFH downstairs and upstairs with individual wireless stats. There are 2 manifolds, each with their own pump - all configured by the UFH distributor.

There's only us two retirees here permanently but we have many visitors, so built 3 double bedrooms with 3 en-suites.  All plant is in a detached garage loft with insulated pipes underground (to minimise penetrations), and hot water recirculation which I haven't yet configured timing for.

I'm still waiting (3 weeks now) for "Mitsubishi" to commission the system which I'm told is in the price I paid.  Your diagnosis seems  sound. Having the primary flow thermistor on the cylinder rather than buffer is ridiculous. But now you've highlighted that the buffer tank is huge, it seems to me that a small buffer on the return would be a much better fit for our system. 

Thank you for your support and sharing your considered views. I definitely don't feel alone with this.

 

 

 


   
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(@sunandair)
Prominent Member Member
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Joined: 2 years ago
Posts: 345
 

@tadpole 

hi. Thanks for the further information. I can see that the reservoir of hot water may have been proposed to feed the two underfloor heating manifolds. This is outside my detailed knowledge base.

Regarding The thermistor placement; when the commissioning engineer arrives perhaps you could ask for clarification where the thermistors should be positioned. 

I hope you get a good briefing on hand over.

 


   
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