Heat Pump Pricing versus current costs

michaels

More on deltaT - this site gives a ratio of the heat output at different (relative) flow temperatures.  When you buy a rad they typically give the heat output at a delta T of 50C (say flow at 70, room at 20). 

I am looking at installing a Heat Pump and need to trade of radiator size vs flow temp and am currently looking at a max flow temp of 45c - assuming a room temp of 20C means my delta T is 25 so I need to multiply the radiator nominal output by 0.41 to work out what it will output for me.  My current rads output the required output at about 55C at -3C outside (I know this from having a weather comp gas boiler with with 'heating curve' and this ties in with the heat loss calc I have had done).  As one example I have a vertical radiator, double panel, no fins 450 wide x 1800 tall rated 1175 watts. Output at 45C flow would be 482w - if I upgrade to a 600x1800 vertical double panel with fins rated 2450w it will output the required 1000w at 45 flow (25 deltaT)

Delta T Conversion l Explanation, Calculations & More (radiatorsonline.com)

Delta T (ΔT)	Correction Factors
75°	1.69
70°	1.55
65°	1.41
60°	1.27
55°	1.13
50°	1
45°	0.87
40°	0.75
35°	0.63
30°	0.51
25°	0.41
20°	0.3
15°	0.21
10°	0.12
5°	0.05

Reed_Richards

michaels said:

I am looking at installing a Heat Pump and need to trade of radiator size vs flow temp and am currently looking at a max flow temp of 45c - assuming a room temp of 20C means my delta T is 25 

Actually no it doesn't, because you are not taking the return temperature into account.  Heat pumps are usually set up to operate at higher water flow rates than a gas or oil boiler, aiming for a 5 C temperature difference between flow and return.  So if your flow is 45 C then your return will be 40 C, the mean radiator average temperature will be 42.5 C.  Therefore your delta T will be 22.5 C when aiming for a room temperature of 20 C.

If you want your delta T to be 25 C you would need to set your flow temperature to 47.5 C.

But thanks for reproducing that look-up table for radiator output correction factors for different delta T values.  

michaels

Reed_Richards said:

michaels said:

I am looking at installing a Heat Pump and need to trade of radiator size vs flow temp and am currently looking at a max flow temp of 45c - assuming a room temp of 20C means my delta T is 25 

Actually no it doesn't, because you are not taking the return temperature into account.  Heat pumps are usually set up to operate at higher water flow rates than a gas or oil boiler, aiming for a 5 C temperature difference between flow and return.  So if your flow is 45 C then your return will be 40 C, the mean radiator average temperature will be 42.5 C.  Therefore your delta T will be 22.5 C when aiming for a room temperature of 20 C.

If you want your delta T to be 25 C you would need to set your flow temperature to 47.5 C.

But thanks for reproducing that look-up table for radiator output correction factors for different delta T values.  

Thanks.  I suspected as much but was trying to keep things simple.  Not sure whether the heat loss calcs are accurate enough to determine the actual required flow temp to within 1c, I suspect there will be tuning of the curve required once installed.  My rads proposal puts me about 5% over what would be needed for a 25 deltaT in aggregate.

Reed_Richards

In the old days the heat pump grant you got ("RHI") depended on your maximum output water temperature for heating.  These days it's not nearly so much of an issue.  But the correction factor for delta T = 22.5 C must be 0.355 approximately, instead of 0.41 for delta T = 25 C.  So that would require your radiators to be 16% over what would be needed for a 25 deltaT in aggregate.

wrf12345

"Now all that happens with my heating is I set a fixed flow temperature (I vary it slightly at times) that uses the least electricity and is most efficient."

Do all heat pumps allow you to do this and is it via some kind of controller? 

matt_drummer

wrf12345 said:

"Now all that happens with my heating is I set a fixed flow temperature (I vary it slightly at times) that uses the least electricity and is most efficient."

Do all heat pumps allow you to do this and is it via some kind of controller? 

This is done via the controller that Daikin make for their heat pumps.

I think most or even all heat pumps will let you run at a fixed flow temperature, even if you did it with a flat weather curve.

The result I get is because I designed my system to run at 30c at the coldest outside temperatures.

But I did the same with my old heat pump with radiators that were too small and it worked, just at 43c instead of 30c

The key to this is the monitoring as it visualises what is going on with the heat pump. This allows you to make adjustments and see instant results.

I could write about why I think this works with Daikin heat pumps in my house but I don't think it would be particularly helpful. My situation is probably not normal, we like it warm, I have lots of solar panels and large battery storage that enables me to always run from the cheapest electricity.

I do think it is a viable strategy to run a heat pump, find the lowest you can run it at and try and keep it there for as long as possible.

If you remember that heat produced is the product of the change in water temperature and the amount of water, then if you can control either or both of those you can control the heat produced with a fixed flow temperature. It is what I do.

I set a dT that I can't normally achieve (between flow and return) and the circulation pump runs at the lowest volume of water to try and achieve the dT I set. As it gets colder outside the dT gets bigger because the water goes back to the heat pump cooler as the radiators can now cool the water more (which delivers more heat). If or when the set dT is reached then the heat pump has to do something to keep it at the set value. It does this by tell the circulation pump to move more water. As more water is flowing then the heat output rises further.

As long as I stay within the ability of my radiators to deliver the heat produced I never need to touch the flow temperature.

My limit is about 5.5kW at a flow temperature of 30c and my heat loss is about 4.5kW at -3c

This is why I never need to go above 30c or so but I can if I want or need to

I am controlling the heat firstly with the dT, secondly the flow rate, and finally the flow temperature if I ever need to.

I am always at the least costly flow temperature and the electricity use varies for two reasons, I am producing more heat by heating the water more (that is a bigger dT, not to a higher temperature) and heating more water if I ever needed to .

I think this is the cheapest and most efficient way to run that heat pump in my house and the only way to make it cheaper is to turn it off but then we would be cold.

To put it into context though. We are heating a four bedroom detached house with an glass conservatory to 22c at all times. It is difficult to be precise because I have had a change of heat pumps but I expect to use about 2,500kWh a year for heating and dhw. Even at the svr that is only about £500 a year and you save the gas standing charge. That makes the heat pump pretty cheap to operate whatever you think.

I could probably use a bit less if I really tried.

So in summary, if I was a heating engineer designing heating systems and installing heat pumps this is what I would do/choose

Insulate as much as possible before doing anything, less heat required is better!
A heat pump with a maximum output to suit the heat loss of the house at the lowest outdoor temperatures
A heat pump with the biggest range of modulation
A heat pump with a low minimum electrical input
A heat pump with the most customisable controls.
Design a system to run at the lowest possible flow temperature within physical and cost restraints
Design a system (if retrofitting especially) to run at low flow rates
Try and run at a fixed flow temperature, ideally where the heat pump uses the least electricity and is most efficient
Design the system so that heat output is controlled by dT and/or flow rate ensuring that the heat pump is at the most efficient and cheapest flow temperature for as much time as possible
Do not fit any zones, trvs or buffer tanks unless absolutely necessary
Size radiators so that rooms do not get to hot or too cold, that is balancing the heat output relative to the heat loss and desired room temperatures
Fit a monitoring system, getting the best out of a heat pump is not that difficult if you know what is going on. Monitoring is worth every penny in the long run.


I apologise for rambling a lot, I am at work so not so much time to construct a better reply.

Reed_Richards

wrf12345 said:

"Now all that happens with my heating is I set a fixed flow temperature (I vary it slightly at times) that uses the least electricity and is most efficient."

Do all heat pumps allow you to do this and is it via some kind of controller? 

When my heat pump was installed (December 2020), my installer set its controller to run at a fixed flow temperature and advised me to try weather compensation when I had built up confidence in the fixed flow option.  But in my case the fixed flow temperature was 50 C, rather than Matt's 30 C, which is phenomenally low/good for heating with radiators.

Nowadays I use weather compensation very effectively because my maximum flow temperature is 50 C and so there is plenty of "room" to make this lower.  Matt, on the other hand has very little room, the flow temperature has to be higher than the room temperature or you will end up cooling instead of heating.  So the fixed flow temperature that works for him isn't what works for me.

Matt worked very hard to end up with the super-efficient heat pump system he has now.  The other way you might be able to achieve that level of efficiency is to have underfloor heating throughout, and even then the heating pipes need to be closely spaced to enable them to be run at lower temperatures with sufficient heat output.

The standard heat pump installation from Octopus, Heat Geek, BG or whoever is likely to end up more like mine than like Matt's.  If you aspire to a system like the one he has then you may have to fight to get larger radiators than you are originally offered.  And that's if you have the space to fit them.

        

smallblueplanet

Reed_Richards said:

wrf12345 said:

"Now all that happens with my heating is I set a fixed flow temperature (I vary it slightly at times) that uses the least electricity and is most efficient."

Do all heat pumps allow you to do this and is it via some kind of controller? 

When my heat pump was installed (December 2020), my installer set its controller to run at a fixed flow temperature and advised me to try weather compensation when I had built up confidence in the fixed flow option.  But in my case the fixed flow temperature was 50 C, rather than Matt's 30 C, which is phenomenally low/good for heating with radiators.

Nowadays I use weather compensation very effectively because my maximum flow temperature is 50 C and so there is plenty of "room" to make this lower.  Matt, on the other hand has very little room, the flow temperature has to be higher than the room temperature or you will end up cooling instead of heating.  So the fixed flow temperature that works for him isn't what works for me.        

So are your radiators at about 100% efficiency (according to Octopus heat loss) and what enables you to lower the flow temp to raise efficiency? If that is what you are implying.

Reed_Richards

When you say "efficiency" I don't think that is what you mean.  The heat output of a radiator should just depend on its design and the average temperature of the water inside the radiator.  If a radiator became clogged with debris that could reduce its efficiency but as almost all of my radiators were new and almost all of the pipes feeding them were renewed I presume they continue to give the heat output they were designed to give. 

Fundamentally what enables me to lower the flow temperature is the temperature outside.  According to calculations if it's -3.7 outside and I want it to be 21 C inside then I need water leaving my heat pump to be 50 C to make the heat output from my radiators to be big enough to match the calculated 8.95 kW heat loss from my house.  But if it is only 8.5 C outside then the heat loss from my house would only be about 4.5 kW so I only need my radiators to match that with 4.5 kW of heat output.  If they can do 9 kW at 50 C input, 45 C out, average 47.5 C so delta t = 47.5 - 21 = 26.5 C then for half that heat output they should be able to run at about delta T = 15 C, that would be 36 C average so 38.5 C in, 33.5 C out.  That's the whole fundamental of Weather Compensation.

Everyone with a standard heat pump system will want to use weather compensation to boost their efficiency and reduce their running costs.  It's just that @matt_drummer has such a super-efficient system already that he hasn't got the "headroom" to use weather compensation as well.            

matt_drummer

Reed_Richards said:

When you say "efficiency" I don't think that is what you mean.  The heat output of a radiator should just depend on its design and the average temperature of the water inside the radiator.  If a radiator became clogged with debris that could reduce its efficiency but as almost all of my radiators were new and almost all of the pipes feeding them were renewed I presume they continue to give the heat output they were designed to give. 

Fundamentally what enables me to lower the flow temperature is the temperature outside.  According to calculations if it's -3.7 outside and I want it to be 21 C inside then I need water leaving my heat pump to be 50 C to make the heat output from my radiators to be big enough to match the calculated 8.95 kW heat loss from my house.  But if it is only 8.5 C outside then the heat loss from my house would only be about 4.5 kW so I only need my radiators to match that with 4.5 kW of heat output.  If they can do 9 kW at 50 C input, 45 C out, average 47.5 C so delta t = 47.5 - 21 = 26.5 C then for half that heat output they should be able to run at about delta T = 15 C, that would be 36 C average so 38.5 C in, 33.5 C out.  That's the whole fundamental of Weather Compensation.

Everyone with a standard heat pump system will want to use weather compensation to boost their efficiency and reduce their running costs.  It's just that @matt_drummer has such a super-efficient system already that he hasn't got the "headroom" to use weather compensation as well.            

Both ways work.

If you had the monitoring that I have we would be able to try different ways of running to find the best, but you don't have that. What you have works for you. It may be the best anyway. You are happy and that is all that matters.

But I would still try to run at a fixed flow temperature that is low as I can get and then vary the heat output using flow rate.

The more I think about it the more sense it makes.

Why increase flow temperature to get more heat when you can increase flow rate and leave the temperature low to get the same result?