@jml Thats good about their reviews, I was looking on Trustpilot. The first two images seem to be the same, but I couldnt see anything untoward about what I could see.

If the Vaillant cylinder has a relatively small coil it might be worth just asking if they fit any alternatives? 3m2 is the minimum suggested for heat pumps, or an external heat exchanger is an alternative. The heat geek cylinder has between 4-6m2.

A small coil means that heat transfer will be less efficient and slower.

I dont think my house is anywhere near as well insulated as yours. Some of it is 1850s stone walls so I think you will be fine. You can check out my blog (in my footer) if you are interested.

As you say being 100% renewable is a good feeling 🙂

Hi @JML, it's great to see these discussions, and the help many experienced and knowledgeable people here will provide. There are a lot of variables that determine how much energy you will use and what it will cost. I'm going to step back from crunching numbers and share key takeaways from our journey from ageing oil boiler to ASHP. It's a journey that I'm glad we made, and it's worked out really well for us:

*  How do you heat your house now? Twice daily, morning and evening? All day? With an ASHP and a 'low and slow' heat source, you'll be ideally heating all day. Not necessarily 24/7, but say around 18hrs a day with a night time set-back, to get that low and slow efficiency and stable running. You can't do short burst heating with a correctly sized ASHP, nor should you be cycling it on/off on room thermostats. How does that fit your lifestyle? If you're home a lot - it's ideal. If you are out during the day, maybe less so.

* Hot water produced from an ASHP is very efficient. A modern system tank can be heated early afternoon once daily, and will retain heat well into the following day for morning showering. I would ditch the electric shower and go mains pressure unvented mixer shower.

* Go for the lowest system design flow temperature. You'll read a common quote that every 1degC increase in design flow temperature adds 2.5% running cost/energy consumption to an ASHP. Panel radiators are fairly cheap to upgrade in the grand scheme, and are always money well spent. In hindsight, my design flow temp of 50degC was too high. Luckily, my house is better insulated and more airtight than the system heat loss calcs estimated, so I now have the max flow temp at 45degC, it usually sits between 35 & 40degC under weather compensated control. The house sits at 20-21degC all day, and it's efficient to run & lovely!!

* Prepare for a massive seasonal change in electricity consumption with an ASHP. It catches many people out when they experience the first really cold winter weather. They then think something is wrong with their ASHP and this forum goes into melt down! An ASHP will work hard in the winter months, and in cold damp UK winter weather when routinely defrosting can consume alarmingly high kWh per day. It's not uncommon for a relatively small change in the winter weather to almost double your daily kWh consumption if the ASHP starts defrosting frequently. In such weather all ASHP COPs drop through the floor. That may be so for 4 weeks of a 52 week year however, and in context of a very efficient and cost-effective heating appliance for the rest of the heating season. Build up the credit on the electricity account in the summer (or set it aside if you object to being a utility company's credit facility!) as the credit can disappear surprisingly quickly in the coldest winter weeks.

* Will an ASHP be cheaper to run? On standard tariffs compared to oil, maybe, but marginally so based on typical real world SCOPs. The spark gap cost differential per kWh between oil and electricity is considerable. With no sign of the Government legislating to do much about that in the short term. You can buy oil at seasonal prices once or twice a year, and bunker it in the tank, and there's no standing charge. Oil is often assumed to be more expensive than mains gas, but I found that not to be the case when I moved in 2013 from a city to an off-grid oil heated property. Fast forward to 2022 and my ASHP install: initially, my ASHP was not cheaper than oil, it was costing more and it was badly commissioned (more below). What saved big for me was proper commissioning and system control & solar PV+battery storage + a flexible time of use tariff. 85% of my electricity consumption is low rate saving me around £600 in the winter heating season compared to a standard tariff.

* Electricity consumption will increase considerably with an ASHP, you'll be an all-electric house. If you can't or don't want to add solar PV and battery into the mix, think of ways to save by getting a flexible time-of-use tariff, e.g.: an ASHP bespoke tariff, and/or smart tech like Homely that can leverage the advantage. 

* Commissioning: It's absolutely key to getting ASHP efficiency up where it should be. There's a lot of variables and an ASHP has to be fine-tuned to the property and the occupant comfort requirements. Every system is unique. In my naivety I didn't think about this. I assumed the SCOPs shown in my MCS performance estimate would be realised from the get-go. They weren't - nothing like, but it was all settings, schedules and a room thermostat that were at fault, not hardware or system plumbing. The installers had programmed my ASHP to run like a gas or oil fired boiler - no weather compensation, on in the morning, off all day, on in the evening, switched on and off by a HIVE thermostat. Hot water generation was scheduled early morning coincident with when you need space heating the most. It never stood a chance and I was waking up to a cold house. Who is the best person to commission it? You as the homeowner. You can best monitor how it is performing, and fine tune to suit you. The installers can't do that in the half day they might set aside [at best] for commissioning. It's the only aspect of the job I can criticise my installers for, the physical install was done very well, but the lads were former 'boiler bashers' who had not been trained up set up heat pumps for best efficiency. This forum was invaluable as a source of experienced-based knowledge, as was the advice of those who had experienced similar issues before me. 

 Excellent post above

@allyfish Hi there! Thanks so much for your input and sharing your experience.

How do you heat your house now? Twice daily, morning and evening? All day? With an ASHP and a 'low and slow' heat source, you'll be ideally heating all day.

Currently there is no heating on given the summer months but we do have a log burner to assist if required.  Normally with my wife at home we turns it on when required but thankfully due to renovation I did back in 2018 the insulation helps hold the heat and obviously catches the warm weather in summer and holds it in.

I totally understand the slow and steady trickle process with ASHP and with current oil boiler would like the same process to keep it running however that just makes it very expensive as I am sure you have experienced when moving off grid to oil heating.

One thing that has helped make my decision is regardless of the winter months expense, one would still be getting a higher efficiency (based on SCOP) on returns in the long run given no boiler operates at over 100%!

Hot water produced from an ASHP is very efficient. A modern system tank can be heated early afternoon once daily, and will retain heat well into the following day for morning showering. I would ditch the electric shower and go mains pressure unvented mixer shower.

This is also good to know and will likely plumb in to existing tapes. Its only 2 of use and we are having a UniSTOR 200lt so that should cater for our needs. Tbh, I'm wondering whether that is too big for our household given its 2-3 bed house and only 2 of us. I'm thinking i would save further if if there is less water to heat??

Electricity consumption will increase considerably with an ASHP, you'll be an all-electric house. If you can't or don't want to add solar PV and battery into the mix, think of ways to save by getting a flexible time-of-use tariff, e.g.: an ASHP bespoke tariff, and/or smart tech like Homely that can leverage the advantage. 

I have just been looking at that myself and considering an array in back garden! How big are your panels and what size panels make your your array?

Is there any advice you could give, on type of panels, dedicated forums (as well as this one!) battery bank you have etc.?

Luckily, my house is better insulated and more airtight than the system heat loss calcs estimated, so I now have the max flow temp at 45degC, it usually sits between 35 & 40degC under weather compensated control. The house sits at 20-21degC all day, and it's efficient to run & lovely!!

Commissioning: It's absolutely key to getting ASHP efficiency up where it should be. There's a lot of variables and an ASHP has to be fine-tuned to the property and the occupant comfort requirements.

As per my quote/data sheet mine is set for a 45oC flow temp, but as you have done, there are ways to improve that with tweaking weather compensation etc.

Thanks again.

https://heatpumpmonitor.org/

Interestingly, I've been checking this site out after it was posted here. Some of the results others are getting from the same install I'll be getting show good stats are their home sizes are bigger than mine! 

As you say all houses & installs are different, but its reassuring to know and comforting that I am joining many like yourself in the journey to become more sustainable 100% renewable.

@jml Regarding ‘litreage’ of DHW tanks, the MCS have a lot to answer for here; because strictly speaking ours is a 4 bedroom semi (3, plus a room less than 50 square feet) the guidelines require us to have 200 litres of storage. There are two of us, no bath but a shower, cold fill for washing machine and dishwasher, we always found the previous 99 litre tank was adequate and we were never short of hot water when required. Regards, Toodles.

@jml likely a 150 litre cylinder would be ample for you, with immersion backup. The standing losses on modern cylinders are very low, so once charged there’s not much difference in running cost for various sizes. The running cost depends on how much water you use and what temperature you charge it to.

Solar: we have a small 3.6kW 9x400W array. One installer dismissed our roof over the phone having seen the address on Google earth. We persevered and found a good installer. The promised generation per annum was exceeded last year. The battery storage of 2x6.5kW has been an absolute game changer. The batteries were afterthoughts. In winter the timed charging of the batteries twice daily at low off-peak rates saves massively on our energy bill even when the solar PV is not generating. In summer we’re self sufficient and exporting enough to cover most of the annual standing charge.

Storage capacity really is dependent on storage temperature and how much you use. We have 210L. There are two of us, heat cylinder to 50 and it heats twice per day mostly. Reheat time 30 to 40mins. I find 210L just fine. 

Not a fan of the Vaillant cylinder as coil is way to small, so reheat CoP isn't the best and time to heat long.