Nev

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 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! 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. 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.

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.

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