New member - Arotherm ASHP newbie

[Nev]

Hello all,

Looking forward to learning from the collective expertise of the group. I am low carbon home convert and committed to reducing the carbon foot print of our home. Recently invested in an Arotherm Plus heat pump 5kW unit with 200L uvc. We have a fairly modern home (2001) 3 storey (120m²), semi detached maisonette, cavity wall insulation, DG, etc so not a leaky old barn conversion by any stretch of the imagination. Radiators (15) sized base on heat loss calcs. The system design, heat loss calcs indicated the 5kW Arotherm Plus could do the job with a design flow temp of 45°C. Quote from Mitsubishi Ecodan came back with 7/8kW unit. System was working fine but when we hit the first cold snap the pump is struggling to get to the 45°C flow temperatures with 100% modulation all the time. Subsequently indoor temps fall the colder it gets. Set for down to -5 this week! Unit has frequent defrost cycles (every 30 - 50 minutes) when the temp falls below 2°C. It seems to me to be a pump undersizing issue, but would welcome thoughts and experience of others.

Having spoken with the installers they are adamant the heat loss calcs and design are correct. One proposed solution is the addition of the 40L buffer tank to the system. Anyone know how a buffer tank will overcome the pump not hitting the design flow temperatures?

 

Thanks

[ProDave]

Hi and welcome to the forum.

 

I am sad to say my gut feeling is whoever sized your system got the heat loss calculations wrong and the fact is, 5kW is not enough to heat your property when it is really cold.  A 2001 build could be really good, or ot could be pretty mediocre. A lot of it will be how well it was detailed.

 

Did you buy it as a design and install package?  Have you gone back to the supplier to tell them it is not performing adequately?

[Gone West]

12 hours ago, Nev said:

Having spoken with the installers they are adamant the heat loss calcs and design are correct.

Have you asked to see the calculations and what indoor and outdoor temperatures were used to do the calculations.

[jack]

13 hours ago, Nev said:

Anyone know how a buffer tank will overcome the pump not hitting the design flow temperatures?

 

If it's usually only during short periods (eg, when it's cold overnight) that it's struggling, a buffer tank could help by allowing the ASHP to use its excess capacity to heat the buffer during warmer periods, for use during the colder period where it would otherwise struggle. However, this only works when there's enough time for it to catch up during warmer periods. During a long cold spell, it likely won't help much if at all.

 

 

[ProDave]

13 hours ago, Nev said:

Having spoken with the installers they are adamant the heat loss calcs and design are correct. One proposed solution is the addition of the 40L buffer tank to the system. Anyone know how a buffer tank will overcome the pump not hitting the design flow temperatures?

 

Thanks

Then call them round NOW while it is cold and task them with making it work.  you really don't care how they do that as long as it ends up working in very cold weather.

[Nev]

9 hours ago, ProDave said:

Then call them round NOW while it is cold and task them with making it work.  you really don't care how they do that as long as it ends up working in very cold weather.

Design and install package and the installer are due here tomorrow morning. It's -3 tonight and does not get above 0 tomorrow so ideal time for them to see the issue. My feeling is the system is definitely undersized for really cold weather. Works fine above 4°C.

 

Design temps used were -2.5°C for 6 months of the year (or that was possibly the Ecodan) or -3°C for 12 months. I have had Vaillant out to the pump twice and as far as they are concerned the pump is working fine. They have referred me back to the installer, which tells me they believe it is a design issue.

 

I remain to be convinced a 40L buffer tank will make the difference required, but as you say the only thing that matters is they do whatever it takes to get the system working.

 

Thanks for the comments shared this far.

[ProDave]

I can't see a buffer tank will change anything. That won't make it produce more heat.  A buffer tank even outs the heat load for when it is running at low power when not much heat is needed.

 

I suspect smoke and mirrors here, by the time they can get around to installing this buffer it will have warmed up, they will claim success and sweep it under the carpet, so I would only accept them trying that with a written undertaking that if it still fails to perform for the entire next winter they will look at it again.

 

Can I ask what heating you had before and do you still have any old winter heating bills?

[SteamyTea]

9 hours ago, ProDave said:

I can't see a buffer tank will change anything. That won't make it produce more heat.

It won't make the ASHP produce more power, but it can produce more energy (energy is the modern term for heat).

By allowing the ASHP to run for longer, there is less of a cooling cycle as the ASHP is on more.

This is why I am of the opinion that all heat pump systems should be fitted with a buffer.  Just establishing what size, and to what temperature, is needed becomes the issue for greatest efficiency.

[Dave Jones]

you have found the Achilles heel of ASHP, when its cold they don't work.

 

Hopefully they will be phased out once Worcester Bosche get their hydrogen combi boiler up and going.

[SteamyTea]

Just now, Dave Jones said:

you have found the Achilles heel of ASHP, when its cold they don't work.

The do work, but they have to be sized correctly.  I could claim that your car does not work when it is cold, and if you had bought one to deal with ice and snow, you would be fine,  for those few days a year when we have snow (actually got some here this morning).

 

They are used, in the UK, to help get our CO_2e emissions lowered, even if supplementary is sometimes needed.

[ProDave]

30 minutes ago, Dave Jones said:

you have found the Achilles heel of ASHP, when its cold they don't work.

 

Hopefully they will be phased out once Worcester Bosche get their hydrogen combi boiler up and going.

Not true.  My 5kW ASHP has been heating my house nicely here in the Highlands where we are now into the 7th consecutive week of average temperature below 0 and -16 one night (a new record in the time we have been here)  Last night was -7.5

 

Where they struggle is in poorly insulated houses and where they have been specified too small as the designer failed to correctly calculate or measure the actual heat load needed.

[SteamyTea]

6 minutes ago, ProDave said:

Where they struggle is in poorly insulated houses and where they have been specified too small as the designer failed to correctly calculate or measure the actual heat load needed.

Not the levels of insulation, or airtightness, that is the problem.  It is all down to the sizing for the heat losses.

If you have two houses of identical design, in the same weather regime, but one has an overall heat load of say 1 kW.K^-1, and the other has a heat load of 1.5W.K^-1, it is a sizing issue.

Just claiming that they only work best in well insulated houses is missing the real issue of bad design execution.

(there ay be other reasons to not have one, but it is not thermodynamics)

[ProDave]

My point was, a lot of old poorly insulated and leaky houses have a heat demand that is higher than any ASHP made and yet installers still specify one with a large degree of hope. That is where you see failed installations that cannot get a house warm enough.

[SteamyTea]

7 minutes ago, ProDave said:

My point was, a lot of old poorly insulated and leaky houses have a heat demand that is higher than any ASHP made and yet installers still specify one with a large degree of hope. That is where you see failed installations that cannot get a house warm enough.

No, you can get heat pumps of just about any size, Carrier makes a 45 kW model, and you can always have several of them.

It is a design problem, not a technology problem.

25 minutes ago, SteamyTea said:

(there ay be other reasons to not have one, but it is not thermodynamics)

 

[ProDave]

Okay there is a practical limit of about 10kW electrical power for a single phase supply, so to get the mosters, or a bank of them, you would need a 3 phase supply.

 

If you need that much heat your money would be better spent reducing the amount of heat the house needs.

[Dave Jones]

1 minute ago, ProDave said:

Okay there is a practical limit of about 10kW electrical power for a single phase supply, so to get the mosters, or a bank of them, you would need a 3 phase supply.

 

If you need that much heat your money would be better spent reducing the amount of heat the house needs.

 

western power will only install 3 phase on new builds, you can have a single phase meter but the supply will be 3 phase.

[Nev]

On 08/02/2021 at 21:58, ProDave said:

I can't see a buffer tank will change anything. That won't make it produce more heat.  A buffer tank even outs the heat load for when it is running at low power when not much heat is needed.

 

I suspect smoke and mirrors here, by the time they can get around to installing this buffer it will have warmed up, they will claim success and sweep it under the carpet, so I would only accept them trying that with a written undertaking that if it still fails to perform for the entire next winter they will look at it again.

 

Can I ask what heating you had before and do you still have any old winter heating bills?

I had a Potterton Condensing Gas Boiler (set at 19.5kW). I have tracked energy consumption and billing for years so have a very good handle on energy consumption and costs to heat our home and hot water. The HPS performance estimate predicted a slight increase in annual cost, but does n't take into consideration the fact we have solar PV, which I believe would offset the increase. The running cost was not the primary motivation for changing although I did not want a significant increase in running costs. With the recent cold snap (0 to -5°C for a week) I thought the performance estimates were a work of fiction much like VW emission figures with figures of 50-60KWh/day and £8-9/day even taken into account that you really need to view running costs over a full year.

Following on my original post and two visits from Vaillant engineers I have learnt quite a lot. Hopefully the comments below will prove useful for others

1. Buffer tank fitting - the 40L buffer tank did reduce cycling significantly. Although the maximum flow temperature achieved did not improve significantly not having to de-ice every 40-60 minutes allow longer periods of heating to get the house up to temperature. Of course this increased energy consumption!
2. Set up - It is clear that the installer and Vaillant did not take time to set the pump correctly. Early on I informed Vaillant the pump does not appear to be modulating or responding to set point temperatures on the controller. I was basically fobbed off, but after further investigation the set point issue was a simple setting in the controller. Change from Active to Expanded. More alarming is following a visit yesterday the pump does now appear to be modulating correctly and using about 60% less energy than before the change was made. The problem appears to have been the pump in the HP was set at a fixed speed rather than auto-control.
3. Pump size - This is the first winter the new Arotherm Plus (3, 5, 7 ,10 ,12 kW) has been tested. The older Arotherm (5, 8, 11, 15 kW) is still available. The installer has informed me that the smaller models (3, 5) of the new Plus appear to be experiencing more 'niggles' than the larger models (7+). They have fitted approx. 100 across the range. The issue is the other way round with older Arotherm. The 5kW is the most reliable model in the range with larger models being less reliable. Obviously anecdotal information, but the installer has fitted a lot more of these and they have decided going forward to fit buffer tanks as standard to all 5kW Arotherm Plus (if space permits).

With the  set up corrections made it is still an unknown of how it will perform in cold weather, as the weather is now mild. Vaillant and the installer agree the heat loss calculations and radiator sizing is correct, but Vaillant insist the pump is working correctly. The installer has now offered to exchange the pump for a larger 7kW new Plus model (which will increase running costs slightly) or a 5kW more 'robust' older Arotherm (running costs the same). I am minded to go up in size to give some head room.

 

Overall although it is an experience I could have done without, if the pump operates correctly (modulates as required), is sized correctly I am confident that it will provide enough heating and DHW for the home at a similar cost to our gas central heating system (caveated that we have solar PV to offset potential increase in cost) and I think over the lifetime of the pump the price differential between gas and electricity will narrow.

 

Many people on this forum have said it already, but the lessons learned. Make sure the sizing calculations are correct to the best of your ability and ask lots of questions. Make sure the pump is set up correctly. Get the pump fitted in autumn definitely not spring. For your consumer rights you need to have the pump tested in winter within 6 months of fitting. If things are n't quite right it is the installers responsibility to prove there is no issue in the first 6 months. After 6 months it becomes your responsibility.