Air Source Heat Pump direct replacement for oil central heating boiler - the payback can be very different…
This year I replaced my 25 year old oil fired central heating boiler with a Samsung 8kW Mono HT Quiet as a direct replacement reusing all the existing plumbing and tanks. I would be interested to know if anyone else has tried it. Running at slightly higher temperatures may be less efficient but it is still be much lower cost than heating oil. I found the low cost, ease of installation and the much reduced disruption made up for the loss of the boiler upgrade scheme.
The house is a 4 bedroom detached brick built in an exposed location in the south of England dating from 1890 with a modern extension. It is fully double glazed with loft insulation to around 300mm and cavity wall filled in the extension only. I would need to upgrade wall insulation for a BUS grant unless I could argue the old building is not suitable. The MCS certified quotes wanted to replace the DHW tank with a plant room and all new pipework to give the guarantee and the quotes started at £15k.
To test the suitability of my heating last winter I set my 18kW Wallstar boiler to 48C flow temperature. Set like this the daily fuel use on the coldest day was 12 litres. The 25 year old oil burner never quite reached the 18C house target temperature in all rooms on sub zero days but it was comfortable enough with the log burner when really cold. I was using 1500 litres of heating oil a year at a cost of £1200 at 80p/litre. From this trial I calculated the peak heat loss.
The replacement ASHP reused the existing indirect DHW cylinder and all the pipe work reducing costs and labour. I made no changes to the radiators except to fit fans to two radiators in the kitchen and living room. I did not fit a buffer tank. A Samsung approved speed controlled Grundfos pump was fitted in place of the old one. All the existing pipe work is 15mm minimum with 28mm to the DHW tank and whilst old it is all still in a good serviceable condition.
The Samsung supplied controller was wired up and set to use the LCD wired room thermostat with water law settings 15c,-5c : 35c,48c. All other settings are as factory. The consumer unit wiring needs to be completed and checked by an electrician. The header tank was flushed, filled and checked for leaks. The system was then switched on to test. After few minutes the new Grundfos pump started, the controller booted up and the heat pump started working first time with no fault messages.
The first test was to select DHW and it reached a 60 degree flow temperature in about ten minutes and heated the existing tank to a 45 deg C in 60 minutes. The COP based on the display was 2.4 so cheaper than heating oil when using Octopus Cosy tariff.
I opened all the thermostatic radiator valves and zones and found that with a flow temperature of 38C it keeps the whole house at 19 deg C with typical outside temperatures of 10 deg C. The COP is estimated at around 4.
I had to wait for freezing days in November for a real test. Temperatures were below zero all day with snow on the ground, and the heat pump performing the defrost cycles every hour or so. The flow temperature rises to 48deg and all rooms were achieving 18C. It is clear that you must run the ASHP continuously for this comfort level. I never turn it off but there is a set back temperature at night. Comparing oil of 12 litres a day maximum for the space heating and DHW I now use 40kWh and the COP is 2.5, at my average unit cost of 18p the daily cost is £7.20 compared with £9.60 for oil on the coldest days, i.e 25% cheaper. The comparable running costs will be even better on milder days due to the improved COP.
I found it was an easy job for anyone with plumbing knowledge but it does, of course, require a qualified electrician to complete the wiring. The official manuals are rather over detailed and difficult to understand but Midsummer Energy Ltd supplies everything you need and has a good summary of how to install this type of heat pump. Any qualified plumber could do this work. I notified the DNO on line after installation and there was no problem getting the approval to connect. In my case no planning consent was required provided I followed the local planning guidelines and building regulations.
Manufacturer’s guarantee may be affected if you do it yourself, of course. I registered for the extended warranty with Samsung on line as there may still be some recourse if a manufacturing defect is found at a later date. Samsung UK will respond if you need help with their products.
There are 1.5 million homes with oil fired central heating in the UK. If the pipework is serviceable with an indirect DHW tank then they could benefit from ASHP without needing to replace the internal plumbing systems. For my simple installation example the additional costs of ASHP compared to a replacement oil boiler can be expected have a payback of less than ten years even without the BUS scheme and MCS certification.