The move to heat pumps

Martyn1981

QrizB said:

Martyn1981 said:

JKenH said:

Why not just use an immersion heater for the DHW element using either a solar diverter or cheap overnight electricity? If the daytime rate is 14p and night time 5p you have the same savings as a COP of 2.8 and no loss of heat in the pipe work transferring the water from the heat pump to the tank (and back again). You can then run your heat pump at lower temperatures. 

Yeah, I was wondering that too, but I wasn't sure if you could get a tank big enough, and how much the power would have to be to heat that much water in just 7hrs in the winter.

The power required is pretty easy to work out.

A cubic metre of water weighs a tonne. The specific heat capacity of water is 4.2 joules per gram per degree C, 4.2 megajoules per tonne per degree C. If the water starts at 10 deg. C and is heated to 70 deg. C that's a temperature rise of 60 degrees and an energy requirement of 252 MJ.

A kilowatt-hour is 3.6 MJ, so heating the water will take 70 kWh. To heat it in 7 hours will require a 10kW immersion heater. Big by domestic standards (and drawing ~42 amps), but no larger than some modern instantaneous showers.

Thanks, cool (pun intended) numbers for me to play with.

From there, can I assume it's safe to half that number for baths and showers, on the basis that the water temp will be closer to 40C(?), so a mix, for example, of 10C cold feed and 70C from the tank?
So, for those 12 baths and showers per day it would take 35kWh, which I think ties in with the number he posted a long time back of DHW gas consumption being around 12MWh pa ...... if my memory serves me correctly.
So just need a 500lt hot water tank.

Starting to think that a heat pump, and one bath/shower per person per day, would be a lot more easy, affordable, green and sustainable.

Reed_Richards

Cardew said:

On the other hand, those having paid good money to have a system fitted tend to be far less critical. This could be because some take more care to operate the system correctly, or are more realistic in their expectations.

A cynic would say it's because we don't like to admit that we made a mistake but speaking for myself it;s both of the above.  

QrizB

Martyn1981 said:

From there, can I assume it's safe to half that number for baths and showers, on the basis that the water temp will be closer to 40C(?), so a mix, for example, of 10C cold feed and 70C from the tank?

All other things being equal* it would be better to heat 1000 litres of water to 40C rather than 500 litres to 70C that you blend down:

• Your heat pump will work at a higher COP when producing lower-temperature DHW; and
• Heat loss through the lagging will be lower, despite the increased surface area.
  

* They never are, of course.

shinytop

QrizB said:

Martyn1981 said:

From there, can I assume it's safe to half that number for baths and showers, on the basis that the water temp will be closer to 40C(?), so a mix, for example, of 10C cold feed and 70C from the tank?

All other things being equal* it would be better to heat 1000 litres of water to 40C rather than 500 litres to 70C that you blend down:

• Your heat pump will work at a higher COP when producing lower-temperature DHW; and
• Heat loss through the lagging will be lower, despite the increased surface area.
  

* They never are, of course.

Aren't we answering a question that shouldn't really be asked?  The average consumption of water for a 6 person household is about 600 litres/day.  That's total water, not hot water.  Getting an ASHP and then wondering how to heat 1000 litres of water a day is like carefully separating all your recyclable waste then chucking it all on a bonfire.  

Finding more sustainable ways of producing domestic heating and HW is one part of the solution but we also have to use less of it.   

silverwhistle

shinytop said: Finding more sustainable ways of producing domestic heating and HW is one part of the solution but we also have to use less of it.   

:-)  'ang on, 'ang on. A slightly different perspective from the point of view of a single person household. Three bedroom terrace and electric shower, PV and hot water diverter and wood burner. This last winter was a cold one and so on target for a high 2,500 kwh annual gas bill, and  as I'd moved to Octopus from Ebico and because of the standing charge and lower unit rate was somewhat profligate.. My lounge was sometimes down to <=16c in the morning which I didn't mind as I don't sit around there at that time and own vests and woolen jumpers..

I'd find it even more difficult to justify the investment in a HP than some of the figures used above. It might be worth my while to put some more west facing PV on my shed to extend the day in that regard, and I've certainly considered a cheap MHRV unit for the bathroom, but a full blown system would be a difficult call. In the shoulder months I already use surplus PV to heat an oil-filled radiator, so in my case it might be worthwhile to cover any gaps with direct electric heating and possibly using a storage rad at cheap times using IFTTT or similar.

Complicated, isn't it? :-)

Reed_Richards

QrizB said:A

ll other things being equal* it would be better to heat 1000 litres of water to 40C rather than 500 litres to 70C that you blend down:

As I have solar panels that's pretty much what I am doing at the moment, using the panels to power my immersion heater.   In fact I can get to about 75 C before activating a safety cut-out.  Granted in my case its 300 l rather than 500 l.  The really bad thing is that my immersion heater is at least half way up the cylinder.  Anybody know any tricks for easy destratification?  

michaels

Martyn1981 said:

QrizB said:

Martyn1981 said:

JKenH said:

Why not just use an immersion heater for the DHW element using either a solar diverter or cheap overnight electricity? If the daytime rate is 14p and night time 5p you have the same savings as a COP of 2.8 and no loss of heat in the pipe work transferring the water from the heat pump to the tank (and back again). You can then run your heat pump at lower temperatures. 

Yeah, I was wondering that too, but I wasn't sure if you could get a tank big enough, and how much the power would have to be to heat that much water in just 7hrs in the winter.

The power required is pretty easy to work out.

A cubic metre of water weighs a tonne. The specific heat capacity of water is 4.2 joules per gram per degree C, 4.2 megajoules per tonne per degree C. If the water starts at 10 deg. C and is heated to 70 deg. C that's a temperature rise of 60 degrees and an energy requirement of 252 MJ.

A kilowatt-hour is 3.6 MJ, so heating the water will take 70 kWh. To heat it in 7 hours will require a 10kW immersion heater. Big by domestic standards (and drawing ~42 amps), but no larger than some modern instantaneous showers.

Thanks, cool (pun intended) numbers for me to play with.

From there, can I assume it's safe to half that number for baths and showers, on the basis that the water temp will be closer to 40C(?), so a mix, for example, of 10C cold feed and 70C from the tank?
So, for those 12 baths and showers per day it would take 35kWh, which I think ties in with the number he posted a long time back of DHW gas consumption being around 12MWh pa ...... if my memory serves me correctly.
So just need a 500lt hot water tank.

Starting to think that a heat pump, and one bath/shower per person per day, would be a lot more easy, affordable, green and sustainable.

So to me the answer is to make high users (me) face the true economic (including environmental costs) in the marginal prices they face for gas and electricity and then they will be able to take sensible decisions on what heat source to use and how much heat altogether.

In theory the benefits system could be used to protect those on low and even middle incomes from the impact of this on their standard of living.  The slight problem being is that many are poor at budgeting and would not use an increase in benefits to meet increased energy costs nor understand how to limit usage to reflect the cost and would scream incredibly loudly about how the high energy costs were 'killing granny'.

It would help this conversation if we were able to come up with gas and electricity prices that would apply if they also reflected their environmental (Co2) costs.  Can anyone take a stab at this?

JKenH

michaels said:

Martyn1981 said:

QrizB said:

Martyn1981 said:

JKenH said:

Why not just use an immersion heater for the DHW element using either a solar diverter or cheap overnight electricity? If the daytime rate is 14p and night time 5p you have the same savings as a COP of 2.8 and no loss of heat in the pipe work transferring the water from the heat pump to the tank (and back again). You can then run your heat pump at lower temperatures. 

Yeah, I was wondering that too, but I wasn't sure if you could get a tank big enough, and how much the power would have to be to heat that much water in just 7hrs in the winter.

The power required is pretty easy to work out.

A cubic metre of water weighs a tonne. The specific heat capacity of water is 4.2 joules per gram per degree C, 4.2 megajoules per tonne per degree C. If the water starts at 10 deg. C and is heated to 70 deg. C that's a temperature rise of 60 degrees and an energy requirement of 252 MJ.

A kilowatt-hour is 3.6 MJ, so heating the water will take 70 kWh. To heat it in 7 hours will require a 10kW immersion heater. Big by domestic standards (and drawing ~42 amps), but no larger than some modern instantaneous showers.

Thanks, cool (pun intended) numbers for me to play with.

From there, can I assume it's safe to half that number for baths and showers, on the basis that the water temp will be closer to 40C(?), so a mix, for example, of 10C cold feed and 70C from the tank?
So, for those 12 baths and showers per day it would take 35kWh, which I think ties in with the number he posted a long time back of DHW gas consumption being around 12MWh pa ...... if my memory serves me correctly.
So just need a 500lt hot water tank.

Starting to think that a heat pump, and one bath/shower per person per day, would be a lot more easy, affordable, green and sustainable.

It would help this conversation if we were able to come up with gas and electricity prices that would apply if they also reflected their environmental (Co2) costs.  Can anyone take a stab at this?

And then how would you treat oil CH? People living in the sticks are already paying a premium to heat their homes because the infrastructure isn’t in place to supply them with gas so do you then penalise them further? Should everyone not pay the same price for energy irrespective of where they live and how they heat their homes?

I have no problem with taxing oil and gas so everyone pays the same price per kWh equivalent as electricity to heat their homes but I suspect the electorate might see it differently.

shinytop

silverwhistle said:

shinytop said: Finding more sustainable ways of producing domestic heating and HW is one part of the solution but we also have to use less of it.   

:-)  'ang on, 'ang on. A slightly different perspective from the point of view of a single person household. Three bedroom terrace and electric shower, PV and hot water diverter and wood burner. This last winter was a cold one and so on target for a high 2,500 kwh annual gas bill, and  as I'd moved to Octopus from Ebico and because of the standing charge and lower unit rate was somewhat profligate.. My lounge was sometimes down to <=16c in the morning which I didn't mind as I don't sit around there at that time and own vests and woolen jumpers..

I'd find it even more difficult to justify the investment in a HP than some of the figures used above. It might be worth my while to put some more west facing PV on my shed to extend the day in that regard, and I've certainly considered a cheap MHRV unit for the bathroom, but a full blown system would be a difficult call. In the shoulder months I already use surplus PV to heat an oil-filled radiator, so in my case it might be worthwhile to cover any gaps with direct electric heating and possibly using a storage rad at cheap times using IFTTT or similar.

Complicated, isn't it? :-)

Yup.  I suppose it's like EVs; for some (e.g someone with a 100 mile round trip commute and a driveway) it's an easy econmic choice.  Others (low users, those who need to go 400 miles every weekend without stopping, no driveway), not so much.  A full ASHP suits my use quite well, yours maybe not.  I'd agree maybe direct electric or even A2A HP might work for you.  But that's very low gas usage.  I know I go on about having to use less energy but I won't put up with 16c in the lounge ....

michaels

JKenH said:

michaels said:

Martyn1981 said:

QrizB said:

Martyn1981 said:

JKenH said:

Why not just use an immersion heater for the DHW element using either a solar diverter or cheap overnight electricity? If the daytime rate is 14p and night time 5p you have the same savings as a COP of 2.8 and no loss of heat in the pipe work transferring the water from the heat pump to the tank (and back again). You can then run your heat pump at lower temperatures. 

Yeah, I was wondering that too, but I wasn't sure if you could get a tank big enough, and how much the power would have to be to heat that much water in just 7hrs in the winter.

The power required is pretty easy to work out.

A cubic metre of water weighs a tonne. The specific heat capacity of water is 4.2 joules per gram per degree C, 4.2 megajoules per tonne per degree C. If the water starts at 10 deg. C and is heated to 70 deg. C that's a temperature rise of 60 degrees and an energy requirement of 252 MJ.

A kilowatt-hour is 3.6 MJ, so heating the water will take 70 kWh. To heat it in 7 hours will require a 10kW immersion heater. Big by domestic standards (and drawing ~42 amps), but no larger than some modern instantaneous showers.

Thanks, cool (pun intended) numbers for me to play with.

From there, can I assume it's safe to half that number for baths and showers, on the basis that the water temp will be closer to 40C(?), so a mix, for example, of 10C cold feed and 70C from the tank?
So, for those 12 baths and showers per day it would take 35kWh, which I think ties in with the number he posted a long time back of DHW gas consumption being around 12MWh pa ...... if my memory serves me correctly.
So just need a 500lt hot water tank.

Starting to think that a heat pump, and one bath/shower per person per day, would be a lot more easy, affordable, green and sustainable.

It would help this conversation if we were able to come up with gas and electricity prices that would apply if they also reflected their environmental (Co2) costs.  Can anyone take a stab at this?

And then how would you treat oil CH? People living in the sticks are already paying a premium to heat their homes because the infrastructure isn’t in place to supply them with gas so do you then penalise them further? Should everyone not pay the same price for energy irrespective of where they live and how they heat their homes?

I have no problem with taxing oil and gas so everyone pays the same price per kWh equivalent as electricity to heat their homes but I suspect the electorate might see it differently.

Hopefully not causing too much offence, but if we want to get serious about climate change then we need to price energy use accordingly.  If we then decide we ant to offer subsidy to certain groups then that is fine but even those with no main gas we want to be facing the correct relative prices so they can make good decisions on the choice between better insulation and less energy use.

For example if oil prices were doubled to reflect the environmental cost but you were then compensated in cash for the increase in fuel bills I bet you would look for ways to save on the fuel bills and pocket the savings rather than just carry on using as much as you previously did.