@jamespa — one of the big problems with historical oil consumption is you probably only have scrappy records (if that) of past oil deliveries that may not show any consistent regularity. There was a time when I tried to fill the ~1100L tank once a year in the summer when prices were lower, but that didn't always work out. The bottom line is the 1000L per annum is a very rough estimate. I certainly wouldn't bet my life on it!

We did discuss the oil to heat pump jump in apparent heat delivery some time ago in this thread. We didn't really reach a firm conclusion (i was still very new to heat pumps back then), more something along the lines that a lot of small things can add up, plus, crucially, the different use pattern, timed vs continuous running.

What I am reasonably sure about is that the overall picture (mostly my financial records are good enough to mean that i probably did include all oil purchases over the years, what is less clear is how and when the oil was used, and of course with the heat pump I have much better records) over several years is that energy delivered to the house did show a huge jump when I had the heat pump installed, and I think most of the explanation lies in the timed vs continuous heating pattern of use. My house probably did cool overnight and during the day when I used oil, I vaguely recollect it was only on for a small number of hours in the morning, and few more in the evening during the week, maybe more at the weekend. We were younger and fitter back then!   

@cathoderay

Noted.  In fairness some people only have scrappy records of gas consumption, its geeks like us that now keep everything!

Clearly if you dont have records and dont have much idea how the house was heated its going to be difficult.  You still have an enormous increase in consumption however, you must have been living in something close to a freezer!

Posted by: @jamespa

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you must have been living in something close to a freezer!

The years between oil use and the heat pump (roughly 2019 to 2021) when I used standard mains electricity to heat only the room(s) I was in were like living in a freezer in the colder months. The kitchen could get down to 7°C overnight. Maybe that put a rosy glow on the oil years, when in fact the oil years were also rather chilly at times. Plus as I said, we were hardier when younger!   

Posted by: @jamespa

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@cathoderay

Noted.  In fairness some people only have scrappy records of gas consumption, its geeks like us that now keep everything!

Clearly if you dont have records and dont have much idea how the house was heated its going to be difficult.  You still have an enormous increase in consumption however, you must have been living in something close to a freezer!

@jamespa This does make me wonder though, how much more energy does the typical household use to heat a house constantly at 21C (heat pump) versus the conventional boiler approach of turn it on for an hour in the morning before you get up and go to work, and again for 3h in the evening when you get home. Does this make heat pumps more (or only) suitable for those who are home all day and would otherwise have had the heating on anyway.

If we are struggling to get heat pump installs to compare favourably on a price per kWh basis (given the 'spark gap'), if we are then using twice the amount of energy in providing low and slow heating to keep the house at a constant 21C (when it otherwise might have spent half it's time at 15C), no wonder people notice their bills have doubled, and then try to turn down the heat pump to save electricity and then complain their house is permanently cold. I love my heat pump, but I'm not sure I'd love it so much if we were out at work all day.

Posted by: @old_scientist

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@jamespa This does make me wonder though, how much more energy does the typical household use to heat a house constantly at 21C (heat pump) versus the conventional boiler approach of turn it on for an hour in the morning before you get up and go to work, and again for 3h in the evening when you get home. Does this make heat pumps more (or only) suitable for those who are home all day and would otherwise have had the heating on anyway.

Remember that the house continues to lose energy when the heating is off so the answer to the question depends how much it cools when its turned off.  If it doesn't cool too much (which many dont) then the reduction in energy lost to the outside world is small, so the reduction in energy required to replace the energy lost (which is what you have to do) is also small.  You can easily do the very rough maths.  So for example if a house is heated half time to 20, and at OAT=0 drops to a min of 14 when its not heated, the average temperature of the house is roughly 18.5 (using linear ramp up and down, an approximation of course).  So the saving in this example is 18.5/20 ie 7.5%.

Posted by: @old_scientist

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If we are struggling to get heat pump installs to compare favourably on a price per kWh basis (given the 'spark gap'), if we are then using twice the amount of energy in providing low and slow heating to keep the house at a constant 21C (when it otherwise might have spent half it's time at 15C),

The typical saving is nothing like that much as the calculation above shows.  Even if, when its 0 outside, it magically goes from 20 to 15 and back to 20 instantaneously (which happens only if it has the thermal characteristics of a tent) the average temp is still 17.5 in this case for a saving of 12.5%.  Obviously if it cools this much when its warmer outside then the savings are more, but most houses wont do this because of the high heat capacity (often called 'thermal mass').  For example the heat capacity of my house is very roughly 14KWh/ degree C and the loss at -2 is 7kWh.  This means that it takes 2 hours to cool by 1 degree C when its -2 outside.

When I had my gas boiler I changed from on/off heating in the 'normal' pattern to heating 24x7 and at the same time reduced the flow temperature, resulting in a reduction in the amount of gas I used annually.  This was, I am pretty certain, the slight extra energy required being offset by the increased efficiency because the boiler was actually condensing, and also not cycling as much because I was heating at lower temperatures