Power consumption heat pump

QrizB

DeletedUser said:

QrizB said:

DeletedUser said:

Is there a nice little graph somewhere that displays ASHP v GSHP electricity required at various outdoor temperatures to bring a room up to 21 degrees?

No, there are far too many variables. That's why a heat pump quote needs to start with a proper energy survey (not just the tick-box exercise of a EPC).

Really? Other than those i've listed, what are the other variables?

OK, your question was:

Is there a nice little graph somewhere that displays ASHP v GSHP electricity required at various outdoor temperatures to bring a room up to 21 degrees?

Factors that you haven't specified and which I can't reasonably infer:

1. What is the heat capacity of the room?
2. What is the rate of heat loss from the room?
3. What is the starting temperature of your room?
   
4. What is the humidity of the outside air feeding the ASHP?
5. How do you propose correlating air temperatures (ASHP) with ground temperatures (GSHP)?
6. Are you delivering the heat at 35C (UFH or fan coils) or at 55C (radiators)?
   

QrizB

coffeehound said:

wittynamegoeshere said:

The point I'm making is that the difference between the flue and outside air temperature will be greater if the air is colder. 

I see your point, but the fact is the specific heat of air is about 1 kJ/kg/℃ whereas that of water is 4.2kJ/kg/℃, meaning that air takes very little energy to heat up.  So the OAT is basically negligible in greater scheme of things

The energy density of natural gas is around 15kWh/kg (55.6MJ/kg). A boiler running at 30kW output will be burning 2kg of gas per hour. This requires 6kg of oxygen, and as air is only 21% oxygen it will need to draw in 28.6kg of air.

Assuming a very cold British day with ambient air at -20C, and a poorly set up boiler with flue gas at 60C, the energy lost in heating the air will be (80 x 28.6 x 1) = 2.28MJ. This is roughly 2% of the 111.2MJ contained in the burned gas, or 600Wh.

(I've assumed stochiometric combustion, and the boiler varying its fan speed to match the heat demand. If the fan runs significantly faster than needed, more air will be drawn through the system and heat losses will be greater.)

tux900 said:

You are forgetting (or not aware of?) the pre- and post-burn purging of the combustion chamber - this significantly affects overall efficiency given the cooling of the hex and casing. This is one reason why short cycling is so undesirable. 

The heat lost during purging should be around (typically much less than) 600W, as calculated above.

QrizB

wittynamegoeshere said:

Has anyone put together any kind of website calculator that could make illustrative estimates?

Personally I'm not that ccomfortable with the idea that I have to get in an expert who's going to basically prescribe what heat pump I need.

Also... is there any harm in overdoing it?  If I just go a size or two bigger just in case then would it run at lower efficiency or would there be other downsides, besides the obvious one of having less money left in the bank?

If you want to claim RHI you're going to need an accredited installer who will do all the magic calcs anyway.

If you're not interested in RHI and want to DIY, it seems to me that the biggest risk is going too small rather than too big.

Just to illustrate, my 3-bed semi has an 18kW gas boiler that's definitely oversized. Looking at Mitsubishi ASHP sizes at City Plumbing, I would expect a 11kW heat pump to be adequate; an 8kW might do the job but a 5kW almost certainly won't. The prices currently are £2445 for the 5kW, £3210 for 8kW and £3670 for the 11kW (all plus VAT); the premium for 11kW over 8kW is £460.

Verdigris

Is a slightly oversized ASHP not breaking a sweat more efficient than a "correctly" sized one working close to maximum?

Are you allowed to install an slightly oversized ASHP, under RHI rules, to, say (but perhaps not out loud), allow for a future extension?

QrizB

wittynamegoeshere said:

It would be good to have some margin.  Plus, am I right in thinking that a higher-powered heat pump might not need to be on as long, given big enough radiators of course?

Back on  page 1 of this thread I shared a link to a Mitsubishi technical data book (yes I mis-spoke and typed Panasonic):

QrizB said:

Deleted_User said:

Why can I not find a simple statement of input power consumption of a heat pump?

As everyone has said, the power consumption varies. For a 12kW heat pump producing its rated output, it might only draw 2.4kW when the weather's warm but 8kW when it's brass monkey time. See section 5.2 of this Panasonic pdf for examples.

What exactly are you trying to work out?

It includes efficiency data for particular models of ASHP running at various load levels. Lower loads give higher COPs.

Eg. for the 11.2kW model, at 2C ambient and 45C water out (page 99 of the pdf):

• Max/nominal power, 11.2kW out with COP of 2.60
• Mid power, 9kW out with COP of 2.89
• Min power, 3.9kW out with COP of 3.02

I've picked these numbers as being near the middle of the table (plus the 2C lines are mostly highlighted so I'm less likely to mis-read them).

[Deleted User]

Number 10 is heated by a polluting 1937 oil fired boiler - so Boris - get your own house in order, before dictating to us how we should heat our homes. 

[Deleted User]

QrizB said:

DeletedUser said:

QrizB said:

DeletedUser said:

Is there a nice little graph somewhere that displays ASHP v GSHP electricity required at various outdoor temperatures to bring a room up to 21 degrees?

No, there are far too many variables. That's why a heat pump quote needs to start with a proper energy survey (not just the tick-box exercise of a EPC).

Really? Other than those i've listed, what are the other variables?

OK, your question was:

Is there a nice little graph somewhere that displays ASHP v GSHP electricity required at various outdoor temperatures to bring a room up to 21 degrees?

Factors that you haven't specified and which I can't reasonably infer:

1. What is the heat capacity of the room?
2. What is the rate of heat loss from the room?
3. What is the starting temperature of your room?
   
4. What is the humidity of the outside air feeding the ASHP?
5. How do you propose correlating air temperatures (ASHP) with ground temperatures (GSHP)?
6. Are you delivering the heat at 35C (UFH or fan coils) or at 55C (radiators)?
   

Sorry, i don't see how any of that is relevant to the question asked.

QrizB

DeletedUser said:

QrizB said:

DeletedUser said:

QrizB said:

DeletedUser said:

Is there a nice little graph somewhere that displays ASHP v GSHP electricity required at various outdoor temperatures to bring a room up to 21 degrees?

No, there are far too many variables. That's why a heat pump quote needs to start with a proper energy survey (not just the tick-box exercise of a EPC).

Really? Other than those i've listed, what are the other variables?

OK, your question was:

Is there a nice little graph somewhere that displays ASHP v GSHP electricity required at various outdoor temperatures to bring a room up to 21 degrees?

Factors that you haven't specified and which I can't reasonably infer:

1. What is the heat capacity of the room?
2. What is the rate of heat loss from the room?
3. What is the starting temperature of your room?
   
4. What is the humidity of the outside air feeding the ASHP?
5. How do you propose correlating air temperatures (ASHP) with ground temperatures (GSHP)?
6. Are you delivering the heat at 35C (UFH or fan coils) or at 55C (radiators)?
   

Sorry, i don't see how any of that is relevant to the question asked.

Nevertheless, it is.

Here, read this:

https://www.bbc.co.uk/bitesize/guides/zwrxsbk/revision/1

BikingBud

Reed_Richards said:

QrizB said:

Your 30kW gas boiler took 2 hours to warm your house, so your house needed 60kWh to warm it. If you had a 12kW heat pump you would have had to turn it on 5 hours befofe you arrived home to get the same effect.

In the morning, your boiler takes 30 minutes so your house needs not more than 15kWh to warm from overnight. With a 12kW heat pump you'd need to turn it on at 0545 not 0630.

None of this is rocket science.

I agree, but don't forget about the radiators.

I have a 12 kW heat pump but 9.6 kW of radiator output at the specified operating temperature.  Before that I had an oil boiler with 12.7 kW of radiator output, assuming 75 C flow and 65 C return.  Even in a cold house the only way to boost the radiator output is to boost the central heating water temperature and previously I had no way of doing that except by removing the facia of my oil boiler (which was external to my house) and turning the temperature control up.  So even though the oil boiler had a 24 kW output capability I could not use all of it to heat my house quickly.

Gas boilers are smarter than heat pumps or oil boilers and a modern one can probably raise it's flow temperature beyond normal if there is a big demand ("load compensation", a feature of Opentherm).  This would raise the heat output capacity of your radiators beyond their rated values.  Even so, you might not be able to match the heat output capacity of a 30 kW boiler so if it took two hours to warm your house from cold, the likelihood is that the house needed less that 60 kWh to warm it.
         

How are gas boilers smarter than oil boilers or heat pumps? It's an energy exchange mechanism, it follows the laws of physics. And we all know you canna change them!

The controllers are the smart element, some will have simple timers, turn heating on and off. Others may be configured and able to learn; to provide a room temperature of say 21degc in the bathroom at 07:30 they determine they need to switch on at 04:30 and others will be zoned for different rooms, have different temp curve for the increase in heat required for the drop in temp and further outside air temp compensation curves etc.

However smart controllers can be dumbed down significantly by people without the knowledge "tweaking" any one of the multiple control curves away from the optimum. This then gets you into inefficient territory and the horror stories about how poor the heat pump systems are begin to spread.  

There are several considerations when you assess heat pumps as a solution, efficiency, install cost and running cost, they will change over time and too often people conflate these considerations.

shinytop

wittynamegoeshere said:

Has anyone put together any kind of website calculator that could make illustrative estimates?

Personally I'm not that ccomfortable with the idea that I have to get in an expert who's going to basically prescribe what heat pump I need.

Also... is there any harm in overdoing it?  If I just go a size or two bigger just in case then would it run at lower efficiency or would there be other downsides, besides the obvious one of having less money left in the bank?

Yes, the MCS one required for RHI is available here.  It's a bit fiddly but not too bad.

https://mcscertified.com/mcs-launch-new-improved-heat-pump-calculator/

If you overdo it there is the danger it will 'cycle' too much; i.e. it will not be able to modulate down enough.  They work better when running more constantly.