J1mbo

Thanks for the reply Peter. I did have the loft tank changed for a new one with a lid and the vent pipe passed in through a tight hole. Better than the large open bowl thats sat in the loft for the 40 preceeding years! It was disgusting actually. Will keep the weekly cycle, actually it's quite useful on a Monday morning when everyone is late trying to get through the shower back-to-back to have more energy stored anyway.

Here: arotherm-plus-spec-sheet-1892564.pdf (vaillant.co.uk) See page 6 for max outputs. e.g. the max compressor current listed on page 8 (e.g. 23.3A = 5.4kW for the 12kW HP), at 0°C, 55°C flow max output is listed as 12.3kW. 12.3/5.4 = COP 2.3 at max output in that scenario. By observation, the ratio does improve at lower loads, especially between approx. 35% and 60%.

That wasn't really my point , rather that electricity has gotten a lot more expensive and is likely to continue to do so for another 5 years according to the EU forecasts. It does depend on the window as it's volatile, according to table 211/212, CAGRs are: 4% from 1996 to 2020 - 1.5x each decade 8% from 2005 and 2019 - 2.15x each decade 5% from 2010 to 2020 - 1.6x each decade

It’s all about the flow temperature - the less is needed, the more efficiency the HP will give. In other words, radiators need to be as least hot as possible. For comfort the HP performs best plodding along 24x7. This also makes the efficiency ratios better because the flow temperatures can be reduced. This is because extra energy for heating up isn’t needed. It might not be cheaper ultimately than using timers etc but it will be more comfortable, and you might find the flow temperature can be quite dramatically reduced. Example - system I have was designed for 50C flow. I kept the timers and smart stuff from previous oil system, where rooms were heated only as they were used. However, the HP was looking expensive as you are finding. Solution has been to increase the size of the main living area radiators, and program the thing for much more level temperatures, increasing the lounge by 1.5C in the evenings and keeping elsewhere pretty level 24x7. These changes have taken a full 10C off the flow temps; it’s currently 0C outside and radiator surface temp is 40C. Search up Faral Tropical 95 radiators. There are cheaper clones. These will have more output at similar sizes as pressed steel and help reduce the flow temperatures needed. Also look fancy IMO. Also: don’t use any controls that are designed for gas or oil systems (including smart systems that aren’t for heat pumps) as these are likely to include TPI programs that promote short cycling, which is another enemy of efficiency. hope that all helps.

Seems to vary where you look - the data here: https://www.statista.com/statistics/418126/electricity-prices-for-households-in-the-uk/ shows 13 to 22 but it’s in Euro cents so there is forex in the mix too I suppose. That site also has projection data which is worth a look.

How does the installers calcs compare to the EPC?

Ordinary Telford heat pump cylinder, just plumbed up with 50 gallon header in a vented config.

My particular installation retains the header tank, and I’ve always been nervous about legionnaires after it was found in school showers (many years ago)

Reduce the impact of having no heating for two hours, I mean.

Also interesting that the MCS numbers include provision for legionaires programme.

Point is, if I set the tank to 65°C, heating it the last 10°C in particular takes ages, because the COP is very low for that part of the cycle (observed specifics, power drawn 5.4kW and cooling capacity <5kW). However, setting the tank lower means the flow temp is lower where the COP is higher. At 40°C it shows a total transfer of about 17kW. Which means the recharge is shorter even though more water is drawn from the DHW cylinder.

interesting, so the manual is erring on the side of caution then. I will adjust accordingly.

HP are typically configured for DHW storage below 60°C (significantly below in my case), so a weekly sterilisation process is needed. Traditionally this was done with an immersion but with R290 the HP can do it itself and with COP > 1, so more efficiently.

IIRC MCS only allows certification to 55°C for heating. However the Arotherm Plus can and does provide 75°C flow if so configured. It is used for the weekly legionaires program for DHW, if enabled (which it should be). The SCOP numbers assume weather compensation. it does not deliver 55°C at a COP of 3.4. It does provide 44°C at 0°C at about 3.2. The max output tables give the actual COP at 55°C, which IIRC is about 2.4. The SCOP is higher as most the year it will be running lower, although I don't know the parameters against which it is calculated. The system has various options of DHW. I've ended up with a 250 litre tank running in 'normal' mode with 5 persons that like showers and a tank set point of 48°C. This allows the HP to put more kW into the cylinder as it's being used as the temperature is lower, leading to shorter recharge times. At 65°C it's probably 1:1 so max of about 10 or 11kW. At lower temps it can produce 15kW and more. HTH

@Fenton H - Vaillant have a plate heat exchanger for the Arotherm Plus, so you just have glycol in the primary circuit between the HP, heat exchanger, and DHW cylinder coil. It provides protection to -18°C at the required concentration. The HP also has a frost protection feature so it will run anyway even if no call for heat.

air to air will also achieve quicker warm up and can achieve COP of 6. DHW is the killer to your config @eandg, reheat time with a small HP will be extended and you'll have no heating whilst it does it. Thermal mass of UFH downstairs will help though.

For the benefit of the search: Vaillant controls calculate energy integral from a starting point of -180° minutes. As each minute passes, the average °C that the flow temperature deviates from the target for that minute is added to the running total, subject to a floor of -180° minutes. i.e. once the target flow is reached, and the HP starts to overshoot, it it were 1°C above the target for 1 minute, the energy integral would then read -179° minutes. Therefore the set range controls the cycle length mid-season, where the ASHP minimum output exceeds thermal load. The default is to turn on compressor at -60° minutes and off at 0° minutes. If that were to be changed to say -90° minutes, then the HP will allow the circuit to cool more leading to a longer cycle period.

Correct, and electricity is likely to continue to go up (it's basically doubled in the last decade) according to some EU study until 2025 at least. However, government will surely increase tax on heating oil in order to meet their CO2 targets and also electricity doesn't run out like a tank of oil. I would also say, anecdotally my house (with similar thermal load) is much more comfortable with the ASHP as I just leave it on all the time and have increased radiator sizes to allow for a lower temperature, and of course CO2 impact is reduced by probably 70% - depending on your location.

Cost £14k. RHI payments £12k.

I don't know the Ecodan system, but I do have a Vaillant Arotherm plus installed. Its DHW has three modes - eco, balance or normal. Essentially normal runs flat-out until cylinder is satisfied, eco heats the tank at 45% compressor modulation, and balance (I think) aims for a target flow temperature whilst heating the tank, so reduces compressor load as flow temperature increases. The DHW side is worth getting right. My household is 5 and we have pumped showers. In Eco mode, the system can be plodding along doing DHW for hours during which time there is no heating. I've more recently lowered the tank temp (to 48°C - which is counterintuitive, but bear with) and changed the mode from Eco to normal. Because the tank is cooler, running at max the HP can deliver more KW to the cylinder (12-18kW in my case, as it recharges). Although this mode sacrifices efficiency - DHW is averaging about 2.4 - it greatly reduces the recharge time to the point that the DHW isn't really noticed in terms of impact on heating. DHW load in my case is small compared to heating load. Tank is 250 litres. Re oversizing the HP. System volume is a key factor. The challenge is mid season. So the HP modulates down to 30% or so, however when it's warmer the efficiency is also higher as outside is warmer and the flow temp target is lower. An 11kW system running with 30% modulation at 30°C flow target might still produce 6kW or more. This means it will not be able to match the fabric loss, so will cycle, which needs to be minimised. I found this study an interesting read, it basically says 11kW HP needs 250 litres system volume, so a small buffer like 50 litre specified basically for defrost may not be sufficient. Hope that helps.

Sorry I mean Armaflex. Thanks all.

Water pipes feeding ASHP are insulated using Climaflex or similar of course but how should lever valves and strainers be insulated?

Thanks, HP control interface manual is available here: 0020291573_00.pdf (vaillant.co.uk) e.g. "Compr.start heat. from" on pg 26

Vaillant controls - compressor starts at -60°min and stops at 0. DHW maintains -5°min.

Would love an explanation of this term. Seems to be critical to heat pump operations.