We'd like to remind Forumites to please avoid political debate on the Forum... Read More »
We're aware that some users are experiencing technical issues which the team are working to resolve. See the Community Noticeboard for more info. Thank you for your patience.
📨 Have you signed up to the Forum's new Email Digest yet? Get a selection of trending threads sent straight to your inbox daily, weekly or monthly!
Heat Pump Sizing?
Options
Comments
-
michaels said: I know that although we have a 12kw heat pump our rads can not emit more than about 7.5-8kw of heat even with a 55C (max) flow temp and it is taking a while for the temperature to recover even though that output is fine to maintain the temp even with -5 outside temp as we have now.Is that 7.5-8kW based on a ∆T50°C (pretty standard when looking at radiator specs) ?A ∆T50°C equates to a flow temperature of 70°C, and when running at 55°C. your radiator output would drop to ~6kW. At 45°C, the output would only be 4kW..Her courage will change the world.
Treasure the moments that you have. Savour them for as long as you can for they will never come back again.1 -
FreeBear said:michaels said: I know that although we have a 12kw heat pump our rads can not emit more than about 7.5-8kw of heat even with a 55C (max) flow temp and it is taking a while for the temperature to recover even though that output is fine to maintain the temp even with -5 outside temp as we have now.Is that 7.5-8kW based on a ∆T50°C (pretty standard when looking at radiator specs) ?A ∆T50°C equates to a flow temperature of 70°C, and when running at 55°C. your radiator output would drop to ~6kW. At 45°C, the output would only be 4kW..
TBH the pipe size also means that I would struggle to transfer more than 7 to 8 kw at a reasonable flow rate. Plan is to upsize 9 of 16 rads to give the same heat transfer at 40CI think....0 -
QrizB said:michaels said:Random question, but how much more energy does it need to raise the temperature as opposed to maintain the temperature?
...
So it would appear that increasing the temp takes a fair bit more power than maintaining the temp and maybe this needs to be factored in alongside the heat loss calcs?It depends on the effective heat capacity of your house.It might be easier to imagine a hot water storage tank (the maths is easier). A tank will have a rated heat loss. Let's assume we've got a 100-litre tank with a rated heat loss of 1.2kWh/day when filled with water at 80C. 1.2kWh/day is equivalent to a continuous power of 50 watts. So it'll only need 50 watts of heat to keep the tank at 80C once it's hot.Now imagine we've used all the hot water and it's been refilled from the mains, and that cold mains water is only at 10C. We need to heat the water up from cold to 80C, that's an increase of 70C.Heating a litre of water by 1C takes 4200 joules (ie. watt-seconds), or 1.167 watt-hours. We've got 100 litres to heat by 70C, so that's 70 x 100 x 4200J which is 29.4 million joules or 8.167 kWh. you need to add that much energy in addition to the continuous 50 watts (arguably you could get there with 50 watts but it would take weeks).So you'll bump your 50 watts up by an extra kilowatt and have a new taankful of hot water after eight hours or so.---It's not quite the same with houses; their heat loss is higher (mine's around 5kW right now, per my Octopus Mini I'm using less than 6kWh of gas an hour) and the change in temperature is smaller, but also their heat capacity is much higher. Bricks are around 800J per kg C, and a house might need 40 tonnes of bricks. That's 32MJ (almost 9kWh) per degree C, and that's just for the bricks.Does that help at all?I think....0 -
michaels said: I wonder what the typical ratio of maintenance power to raise by 1c power is or alternatively typically how much extra power you would typically need to raise temp by saying 2c per hourHer courage will change the world.
Treasure the moments that you have. Savour them for as long as you can for they will never come back again.0 -
QrizB said:
Let's assume we've got a 100-litre tank with a rated heat loss of 1.2kWh/day when filled with water at 80C. 1.2kWh/day is equivalent to a continuous power of 50 watts. So it'll only need 50 watts of heat to keep the tank at 80C once it's hot.Now imagine we've used all the hot water and it's been refilled from the mains, and that cold mains water is only at 10C. We need to heat the water up from cold to 80C, that's an increase of 70C.Heating a litre of water by 1C takes 4200 joules (ie. watt-seconds), or 1.167 watt-hours. We've got 100 litres to heat by 70C, so that's 70 x 100 x 4200J which is 29.4 million joules or 8.167 kWh. you need to add that much energy in addition to the continuous 50 watts (arguably you could get there with 50 watts but it would take weeks).So you'll bump your 50 watts up by an extra kilowatt and have a new taankful of hot water after eight hours or so.
If you want to heat your house as quickly as possible, turn off weather compensation and set the flow temperature to the highest your heat pump is capable of. This will give you the least efficient operation of your heat pump but the fastest restoration of the normal house temperature.Reed2 -
Reed_Richards said:QrizB said:
Let's assume we've got a 100-litre tank with a rated heat loss of 1.2kWh/day when filled with water at 80C. 1.2kWh/day is equivalent to a continuous power of 50 watts. So it'll only need 50 watts of heat to keep the tank at 80C once it's hot.Now imagine we've used all the hot water and it's been refilled from the mains, and that cold mains water is only at 10C. We need to heat the water up from cold to 80C, that's an increase of 70C.Heating a litre of water by 1C takes 4200 joules (ie. watt-seconds), or 1.167 watt-hours. We've got 100 litres to heat by 70C, so that's 70 x 100 x 4200J which is 29.4 million joules or 8.167 kWh. you need to add that much energy in addition to the continuous 50 watts (arguably you could get there with 50 watts but it would take weeks).So you'll bump your 50 watts up by an extra kilowatt and have a new taankful of hot water after eight hours or so.
N. Hampshire, he/him. Octopus Intelligent Go elec & Tracker gas / Vodafone BB / iD mobile. Ripple Kirk Hill member.
2.72kWp PV facing SSW installed Jan 2012. 11 x 247w panels, 3.6kw inverter. 34 MWh generated, long-term average 2.6 Os.Not exactly back from my break, but dipping in and out of the forum.Ofgem cap table, Ofgem cap explainer. Economy 7 cap explainer. Gas vs E7 vs peak elec heating costs, Best kettle!0 -
I'm wondering if the rate of cooling with the heating off might be used as a proxy for the loss of stored heat. Obviously a bit simplistic as potentially the air (measured temp) probably cools quicker than the fabric.
However to what extent is it fair to say that if the heating is off for an hour that would have needed 6kwh to maintain inside temp then you would need an extra 6kwh on top of the normal maintain temp input to return to the original temp?I think....0 -
Very informative post QrizB
Should I in that case have my heatpump offset to say +4c whilst the IOG hours are in play?4.29kWp Solar system, 45/55 South/West split in cloudy rainy Cumbria.0 -
michaels said:I'm wondering if the rate of cooling with the heating off might be used as a proxy for the loss of stored heat. Obviously a bit simplistic as potentially the air (measured temp) probably cools quicker than the fabric.michaels said:However to what extent is it fair to say that if the heating is off for an hour that would have needed 6kwh to maintain inside temp then you would need an extra 6kwh on top of the normal maintain temp input to return to the original temp?For a short outage (an hour or two) that's probably a reasonable approximation.For a longer one, as the building cools down the delta-T gets smaller and the rate of heat loss will decrease (like R_R described, and the same phenomenon you see with your radiators). So the energy needed to warm back up will be less than simply multiplying 6kWh by the time it was off.Spies said:Very informative post QrizB
Should I in that case have my heatpump offset to say +4c whilst the IOG hours are in play?
N. Hampshire, he/him. Octopus Intelligent Go elec & Tracker gas / Vodafone BB / iD mobile. Ripple Kirk Hill member.
2.72kWp PV facing SSW installed Jan 2012. 11 x 247w panels, 3.6kw inverter. 34 MWh generated, long-term average 2.6 Os.Not exactly back from my break, but dipping in and out of the forum.Ofgem cap table, Ofgem cap explainer. Economy 7 cap explainer. Gas vs E7 vs peak elec heating costs, Best kettle!0 -
For those who wish to get into the weeds, this bloke has been writing some interesting stuff some of which is relevant to the discussions above. Check the articles in the menu on the right hand side
https://protonsforbreakfast.wordpress.com/
He is Michael de Podesta, who worked for the National Physical Laboratory for 20 odd years and so has a pretty good grounding in data collection and analysis, He has been measuring and monitoring of his heatpump, solar and battery systems and trying to make sense of some of the data.Never under estimate the power of stupid people in large numbers2
Confirm your email address to Create Threads and Reply

Categories
- All Categories
- 350.9K Banking & Borrowing
- 253.1K Reduce Debt & Boost Income
- 453.6K Spending & Discounts
- 243.9K Work, Benefits & Business
- 598.8K Mortgages, Homes & Bills
- 176.9K Life & Family
- 257.2K Travel & Transport
- 1.5M Hobbies & Leisure
- 16.1K Discuss & Feedback
- 37.6K Read-Only Boards