Heating Degree Days (HDD) reality check please
Discussion
I've downloaded a HDD spreadsheet for my local area (Culdrose airbase).
I used the default 15.5 degrees, as it aligns with the periods where our old oil heating was switched off (HDD below 1).
I've summed a year of numbers, divided by 366 (leap year) to get an average of 4.014.
On New Year's eve, the HDD number was 4.3, and the ASHP cost me £2.12p.
The average number, 4.014, is 93% of 4.3, so the average day would cost £1.97.
Multiply by 365, annual bill would be £720.
Obviously weather patterns vary, so last year doesn't necessarily equal next year, and being on a dual rate tarrif complicates things, but am I in the right ballpark?
Trying to get my head around ASHP running costs, as yesterday's number (£4.66, but it did drop to 1 degree) was quite scary. With oil, it was just pay once, done for the year (900 litres on average)
I used the default 15.5 degrees, as it aligns with the periods where our old oil heating was switched off (HDD below 1).
I've summed a year of numbers, divided by 366 (leap year) to get an average of 4.014.
On New Year's eve, the HDD number was 4.3, and the ASHP cost me £2.12p.
The average number, 4.014, is 93% of 4.3, so the average day would cost £1.97.
Multiply by 365, annual bill would be £720.
Obviously weather patterns vary, so last year doesn't necessarily equal next year, and being on a dual rate tarrif complicates things, but am I in the right ballpark?
Trying to get my head around ASHP running costs, as yesterday's number (£4.66, but it did drop to 1 degree) was quite scary. With oil, it was just pay once, done for the year (900 litres on average)
clockworks said:
I've downloaded a HDD spreadsheet for my local area (Culdrose airbase).
I used the default 15.5 degrees, as it aligns with the periods where our old oil heating was switched off (HDD below 1).
I've summed a year of numbers, divided by 366 (leap year) to get an average of 4.014.
On New Year's eve, the HDD number was 4.3, and the ASHP cost me £2.12p.
The average number, 4.014, is 93% of 4.3, so the average day would cost £1.97.
Multiply by 365, annual bill would be £720.
Obviously weather patterns vary, so last year doesn't necessarily equal next year, and being on a dual rate tarrif complicates things, but am I in the right ballpark?
Trying to get my head around ASHP running costs, as yesterday's number (£4.66, but it did drop to 1 degree) was quite scary. With oil, it was just pay once, done for the year (900 litres on average)
Hmmm - few pointersI used the default 15.5 degrees, as it aligns with the periods where our old oil heating was switched off (HDD below 1).
I've summed a year of numbers, divided by 366 (leap year) to get an average of 4.014.
On New Year's eve, the HDD number was 4.3, and the ASHP cost me £2.12p.
The average number, 4.014, is 93% of 4.3, so the average day would cost £1.97.
Multiply by 365, annual bill would be £720.
Obviously weather patterns vary, so last year doesn't necessarily equal next year, and being on a dual rate tarrif complicates things, but am I in the right ballpark?
Trying to get my head around ASHP running costs, as yesterday's number (£4.66, but it did drop to 1 degree) was quite scary. With oil, it was just pay once, done for the year (900 litres on average)
You need to exclude water heating - HDD data only relates to space heating - even if you say I used and average of X in summer and apply that to winter with a 10% uplift to compensate for the higher losses (lower cold water temp in and higher tank losses in the house)
If you have data on usage keep a track of it and then you can use the regression tool to work out what your target base line should be
Exclude the period (days) where you don't heat - you will get days above and below one but in the spring, summer and autumn months the house has some thermal mass that is not going to be affected on a daily basis by swings in temps
It's not total HDD divided by number of days in the year it's Energy used divided by HDD total for the heating period
What you need to measure is Energy per HDD
For me my target is anything with a 4.
My base is just below 4 but that was set with scheduled heating rather than 24/7
B'stard Child said:
Hmmm - few pointers
You need to exclude water heating - HDD data only relates to space heating - even if you say I used and average of X in summer and apply that to winter with a 10% uplift to compensate for the higher losses (lower cold water temp in and higher tank losses in the house)
If you have data on usage keep a track of it and then you can use the regression tool to work out what your target base line should be
Exclude the period (days) where you don't heat - you will get days above and below one but in the spring, summer and autumn months the house has some thermal mass that is not going to be affected on a daily basis by swings in temps
It's not total HDD divided by number of days in the year it's Energy used divided by HDD total for the heating period
What you need to measure is Energy per HDD
For me my target is anything with a 4.
My base is just below 4 but that was set with scheduled heating rather than 24/7
DHW usage is pretty constant - couple of bowls for washing up etc. Old system was an immersion for water (not plumbed into the boiler). Less than 30 minutes to heat up during the cheap rate, so around 1.5kWH a day. Showers are electric. Appliances are cold fill.You need to exclude water heating - HDD data only relates to space heating - even if you say I used and average of X in summer and apply that to winter with a 10% uplift to compensate for the higher losses (lower cold water temp in and higher tank losses in the house)
If you have data on usage keep a track of it and then you can use the regression tool to work out what your target base line should be
Exclude the period (days) where you don't heat - you will get days above and below one but in the spring, summer and autumn months the house has some thermal mass that is not going to be affected on a daily basis by swings in temps
It's not total HDD divided by number of days in the year it's Energy used divided by HDD total for the heating period
What you need to measure is Energy per HDD
For me my target is anything with a 4.
My base is just below 4 but that was set with scheduled heating rather than 24/7
Presumably the heat pump will be less efficient when heating the cylinder, so I'll knock off 1kWH each day for the DHW.
I've got 6 days of accurate data (Shelly EM on the garage CU, monitoring the main feed to the ASHP).
Dividing daily kWH by the relevant HDD number gives me 1.75 as the lowest, and 2.3 as the highest.
I think solar gain probably accounts for the lowest number (coldest day, but the only one with any actual sunshine).
The 2 days with high numbers are probably because the outdoor temp dropped suddenly, and the ASHP would've ramped up quickly.
For 3 days, the numbers were 1.95, 1.97 and 1.96. Stable outdoor temps, min to max around 2 degrees.
If I work off a figure of 2.1 kWH per HDD "number", does that sound reasonable?
Looking again at the HDD data, I can probably exclude the period where the number is below 3 for several days in a row. Like you say, thermal mass will take care of the odd day with a higher number. I used to switch off the oil boiler around the middle of May, and turn it on again at the end of September.
Excluding that period, I get 1312.3. Multiply that by 2.1 gives 2755 kWH for the year.
I think my numbers are pretty close to what the original heatloss calcs came up with.
The one mistake they made was using Plymouth as the HDD base, which is generally colder than where I am. HDD number of 1858 listed on the heatloss calcs.
Their calcs suggested around 3300 kWH input for heating (assuming the rated COP of 3.2)
The one mistake they made was using Plymouth as the HDD base, which is generally colder than where I am. HDD number of 1858 listed on the heatloss calcs.
Their calcs suggested around 3300 kWH input for heating (assuming the rated COP of 3.2)
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