Posted by: @bones

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I am in a similar situation as you.  Have an oversized heat pump 7KW in a 2KW at -2C property with UFH.  I have selected a Vaillant  3.5KW as my replacement.  Why, right size, appears to me to be the most purchased machine, lots of service engineer options.

I also have a buffer tank on my current system.  I plan to convert this to a volumiser on my new system.  

I also looked at the new black version due to be released in the UK next month.  I managed to get hold of a spec from Germany.  I could not see much of a difference in terms of the spec to the current UK heat pump.

You would need to engage someone that is able to calculate and design your system and select the correct pump capacity to open loop a system with an 18m run from the garden (this is possible to do as we have installed them as far as 20m and they work fine) balancing rads against UFH is tricky but possible, Fire Power sell an FRV lock shields for the rads this would help as you could set the rads based on a set flow rate and calculate the required flow - capacity making sure they are the correct Kw (link below)

Use of a buffer:- Ok so this is where you get a bit of kick back, buffers are often a no no, if you can avoid a buffer its ideal but in your case what a buffer will do for you if you are willing to loose a bit of efficiency is allow to move the heat twice, once from the ASHP internal pump to the buffer and then again from the buffer to the home, the other thing having a buffer and 2nd pump will allow is for you to select another pump at the correct capacity to handle the UFH and rads rather than rely on the internal pump of the ASHP. (so options are bigger pump open loop, 2 smaller pumps with hydraulic break buffer) having a buffer can also often disguise an over sized ASHP somewhat but as you are changing the heat pump so there is no need.

There is one more option, you can choose an ASHP such as a Mitsi Ecodan that has no pump, calculate a light commercial pump (if needed) to do the duty and revert back to no buffer, of see if the pump in the Vaillant will do the duty which it just might.

Best advice, see if you can engage a company that is use to doing these kinds of upgrades and changes that minimises your costs and keeps as much of the existing equipment you have but can re-design or reverse design your whole system. We have changed 6 of these in the last year alone and it takes quite an investigation to be sure, example, if the original company installed the wrong size ASHP and could not balance the system, what else did they get wrong, the hardest thing you will find is someone willing to take on another's bad installation and offer guarantees.   

https://www.firepowerheating.co.uk/frv   

Editor's Note: use code RHHFRV10 at checkout to get 10% off.

@ashp-bobba in reading your post, I did wonder where your stated 3 to 3.5kW heat loss came from? Was this a calculated figure or derived from operation of your existing unit?  I ask because I have a 3.5kW Vaillant and it will give over 4KW at -5.  My 110sq m house was subject to a deep refurb and that included an mvhr system. Most of the time over this winter the pump has been consuming about 500W and giving a heat and DHW cop of 3.9 so 2 to 3 kW of heat.  Maybe your 3.5 kW demand is calculated but does it include an allowance for the ventilation losses?  If the figure is derived from the demand on your existing system maybe the smaller 3.5 Vaillant would be suitable?  

A related issue is of course the sizing of the emitters.  You only say UF and radiator numbers.  Do you know what they will emit at what DT.  My system was set on WC of 0.6 most of the winter.  I have both radiators and UF all on one zone.  In February I discovered the UF had a stuck valve.  Once I cleared that I could drop the WC curve to 0.4 - which is probably marginally low.

i am definitely not a heating engineer but reflecting on how our system seems to operate in practice.

@davidb I think this question is to @bones

Posted by: @davidb

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@ashp-bobba in reading your post, I did wonder where your stated 3 to 3.5kW heat loss came from? Was this a calculated figure or derived from operation of your existing unit?  I ask because I have a 3.5kW Vaillant and it will give over 4KW at -5.  My 110sq m house was subject to a deep refurb and that included an mvhr system. Most of the time over this winter the pump has been consuming about 500W and giving a heat and DHW cop of 3.9 so 2 to 3 kW of heat.  Maybe your 3.5 kW demand is calculated but does it include an allowance for the ventilation losses?  If the figure is derived from the demand on your existing system maybe the smaller 3.5 Vaillant would be suitable?  

A related issue is of course the sizing of the emitters.  You only say UF and radiator numbers.  Do you know what they will emit at what DT.  My system was set on WC of 0.6 most of the winter.  I have both radiators and UF all on one zone.  In February I discovered the UF had a stuck valve.  Once I cleared that I could drop the WC curve to 0.4 - which is probably marginally low.

i am definitely not a heating engineer but reflecting on how our system seems to operate in practice.

I have calculated the heat loss using heatpunk as 3kw, our neighbour who has the same house had a few calculations which came in around 3-3.5kw. We do have MHVR in our house but this also doesn't work so hasn't been calculated into heat loss, something I will have to look into. From my understanding the 3.5kw and 5kw vaillant are the same, just the smaller unit is software locked to a max output but the minimum modulation is the same so I don't think it is worth going for the smaller unit other than upfront savings which is minimal.

For the emitters I have no idea what the UFH emits, we have had flows of 35-40 this winter and it has kept the house warm. For the rads I have calculated them as sufficient using heatpunk for a flow temp of 40. 

The FRV's you mention look good, I may have to invest in some of these to help balance the system.

So if I go for set the buffer up in 2 port I would need to use either the ASHP internal pump or a single larger pump rather than 2 pumps?

We have had a few companies come out and look at the system who just won't go near it, the company we are using have suggested improvements to our system but have admitted that they cannot guarantee any fixes as they are not 100% sure what the issue is, if they rip it out and start again they will give us a guarantee that the system will work correctly.

Posted by: @trebor12345

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Posted by: @bones

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I am in a similar situation as you.  Have an oversized heat pump 7KW in a 2KW at -2C property with UFH.  I have selected a Vaillant  3.5KW as my replacement.  Why, right size, appears to me to be the most purchased machine, lots of service engineer options.

I also have a buffer tank on my current system.  I plan to convert this to a volumiser on my new system.  

I also looked at the new black version due to be released in the UK next month.  I managed to get hold of a spec from Germany.  I could not see much of a difference in terms of the spec to the current UK heat pump.

I am thinking the same, I would prefer Vaillant as there is a lot more info out there regarding the arotherm, there controller and app seem much better than the Ideal as well. I have looked at the spec sheet for the new arotherm and I agree it doesn't look like any major improvements, slightly better COP figures on the spec sheet - The installer has said it should be the same price as the current unit so I figure I may as well wait for the new unit as I won't need the heating over summer anyway.

Posted by: @bones

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The FRV's you mention look good, I may have to invest in some of these to help balance the system.

So if I go for set the buffer up in 2 port I would need to use either the ASHP internal pump or a single larger pump rather than 2 pumps?

We have had a few companies come out and look at the system who just won't go near it, the company we are using have suggested improvements to our system but have admitted that they cannot guarantee any fixes as they are not 100% sure what the issue is, if they rip it out and start again they will give us a guarantee that the system will work correctly.

 

Hi, If you mean 2 port (only using 2 ports of the buffer) this would be a volumiser and yes only 1 pump, if you use it as a buffer (ASHP F+R on one side and Home F+R on the other) you would need a second pump for the house side.

I hear this often about companies hesitation, if not nearly all the time, if you are in Kent I would be happy to help, unfortunately we only cover Kent due to the volume of work here is overwhelming. It takes a very confident company in their abilities to offer improvements and take the risk. 

We installed a new system for a client that had been turned down by multiple bigger companies Inc one of the best companies in the country* for design as they said the existing underground pipework from their oil boiler looked 20mm and cannot carry the 8kW required to heat the home and as the home was 100 years old and the fact that the customer did not want to dig up 20m of garden or change their cast iron rads it would not work, it was also unknown what level of insulation was under the ground. We attended, to investigate, calculated the Rads, calculated the runs and explained to the customer about the risks but said we can prove if it will work or not for £100 before installation, the client engaged.

We 1st inspected with an IR camera on the main pipe work under the ground just to make sure there was no extreme leakage, then we disconnected both ends and measure the maximum flow rate we could achieve through the pipe in the ground, it turns out we could get 11.1kW through the pipe and it was in fact 25mm with 100mm insulation, we then heated the water in the test loop to 50 Deg C and measured the flow to see if there was any drop in tempersature, the drop was 0.7 Deg C so not bad. We then calculated like in your case what index PA was needed which led to a light commercial pump at 8M head.

As a result the customer engaged us to install, the cast rads were around 5-9% under sized in 2 rooms either side of the hall way, the hall way had an old stelrad so we upgraded this to 140% for that area that then supported the 2 short areas as it was between them.

This home has now been running from November 2025-today and the SCOP is 3.9 on an 8Kw Ecodan several meters down the garden, the customer reported that even on the -6 days in January the system held a perfect 21 Deg C, the system was 40% open loop and TRV's every where else. 

* No Names and not the best at everything 

@ashp-bobba on the FRVs, I’m finding them a bit annoying and not sure if you’ve experienced this. On rads where we have to reduce flow to under 0.5 l/min/m², I find that the valve is not very accurate, and, more often than not, it just shuts the valve off completely and the rads are stone cold. I’ve now had to put all those valves up to 1 l/min/m² just to make sure they remain open.

Posted by: @editor

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@ashp-bobba on the FRVs, I’m finding them a bit annoying and not sure if you’ve experienced this. On rads where we have to reduce flow to under 0.5 l/min/m², I find that the valve is not very accurate, and, more often than not, it just shuts the valve off completely and the rads are stone cold. I’ve now had to put all those valves up to 1 l/min/m² just to make sure they remain open.

I have only used a couple so far and that was at 2-l/pm as i needed around 750 watts out of the rad, that said i did DT clamp and IR test as well, it just helped me set them much quicker.

Are your rads with these on very small heat requirement? 1ltr is only 350W, also they do a low flow insert that goes with the valve but I don't know at what point (what is low flow) you would use this, I assume from below 1-ltr/pm 

Perhaps it may be better if @bones waits for IMI neo flow and change all of the UFH and TRVS to match thats another way that might be good for a mixed rads and UFH balance, as soon as these come out I am going to offer them to a client at cost just to get some data back. 

I am myself looking for all market solutions for room control and mix rad / UFH solutions that offer a 90% close or programmable part close so I can replicate our preferred style of commissioning which is having 30% open loop system while being able to control most rooms as I find this is what most clients would like. If I was to programme all of or the main rooms to close 90% I no longer need to have an open loop as all of the 90% closed rads will equate to a prat open loop, I am sure this is one future design that will be common practise. I think this would reduce nearly all offthe miss control flow failures and some good news for many on here that have that issue.

Self balancing is coming too, lets have some of that please.

Posted by: @bones

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When we have had the system working we have had it in WC using the thermostats as a limiter but have had difficulty balancing the upstairs radiators to be comfortable. 

 

I presume this means that upstairs was too hot?  If so and given the FT art which you were operating, no need to worry too much about radiator sizing and you may wish to try turning FT down even further and operating the UFH open loop.  Take note of the min FT setting, this is frequently set to 35 or even 37, which is far too high in many cases bus avoids call outs on account of the radiators not 'feeling hot'.

Posted by: @bones

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Posted by: @trebor12345

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Posted by: @bones

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I am in a similar situation as you.  Have an oversized heat pump 7KW in a 2KW at -2C property with UFH.  I have selected a Vaillant  3.5KW as my replacement.  Why, right size, appears to me to be the most purchased machine, lots of service engineer options.

I also have a buffer tank on my current system.  I plan to convert this to a volumiser on my new system.  

I also looked at the new black version due to be released in the UK next month.  I managed to get hold of a spec from Germany.  I could not see much of a difference in terms of the spec to the current UK heat pump.

I am thinking the same, I would prefer Vaillant as there is a lot more info out there regarding the arotherm, there controller and app seem much better than the Ideal as well. I have looked at the spec sheet for the new arotherm and I agree it doesn't look like any major improvements, slightly better COP figures on the spec sheet - The installer has said it should be the same price as the current unit so I figure I may as well wait for the new unit as I won't need the heating over summer anyway.

 

I have the Vaillant 7kW and confirm that I am very pleased with it.  Both App and control interface provide everything you need.  For the Arotherm Plyus the 3.5kW is just the 5kW downrated in firmware, which cuts the max output without affecting the min.  I dont know if thats also true of the Arotherm pro.  If so might be worth getting the 5kW because DHW reheat will be quicker

@bones based on what you have said I cant think of a reason why you would need a buffer or a secondary circulation pump.  No harm in fitting a 2 port volumiser though and, given your low loss, it would likely be a good addition.  It needs to be either in the flow (arguably marginally preferable) or the return, but not between the two.  It should be plumbed so it is NOT engaged during a DHW run.