Heat Pump Pricing versus current costs

wrf12345

"Running at 65c will not get you more heat" Running a 5kw heat pump at 65 degrees for a given time won't get you more heat than running at 30 degrees for the same time???

matt_drummer

wrf12345 said:

"Running at 65c will not get you more heat" Running a 5kw heat pump at 65 degrees for a given time won't get you more heat than running at 30 degrees for the same time???

That's it exactly.

5kW of heat is 5kw of heat.

If you fitted it to your house and got 5kW of heat at 65c and then brought it round to my house I would get 5kw of heat running it at 30c.

That is because my radiators are big enough to deliver 5kW of heat with a flow temperature of 30c.

If I set that heat pump to run at 65c it would never get there, it would only ever get to low 30's because my radiators absorb all the energy a 5kw heat pump can deliver at 30c or so.

If I ran water through my radiators at 65c my heat output would be over 50kW, ten times what they 5kW Samsung heat pump can produce.

That heat pump in my house would use somewhere between half and a third of the electricity it would use in your house.

This is all of course at the coldest temperatures.

When it is warmer you wouldn't need to run at 65c.

And that is what these heat pumps are based on, most of the time you won't be running at anything like 65c but when you need it you can.

This allows you to fit it with no modifications to your radiators and run at a reasonable cost most of the time.

But on the worst days it will still be able to heat your house using small radiators suited to a gas or oil boiler.

The downside is that the cost on those really cold days will be horrific and I suspect some people will find it pretty scary watching their heat pump's electricity consumption.

It will work and it will feel like the gas boiler you are used to because the radiators will be hot like they are now.

But it won't be as efficient as something that heats the water to a much lower temperature.

Maybe you now understand?

If you flow water around your radiators at 10lpm, 65c in and 55c back to the heat pump you will get the same amount of heat as somebody flowing 10lpm at 35c in and 25c back to the heat pump.

In both cases 10 litres per minute of water has cooled down by 10c. That is where the heat comes from, it is the amount of water flowing and how much it cools down by, not how hot it is to start with.

You could even flow 20lpm at 30c in and 25c back out, that is twice the amount of water cooled down by half the amount. The same heat as the other two examples.

The drawback is that radiator output falls dramatically as the difference between the mean flow temperature and the room temperature approach each other.

At dT 50c a 1kw radiator puts out 1kW of heat. That same radiator at dT 10c only puts out 120W of heat and at dT 5c only 50W.

That is why the radiators need to be so big to run very low flow temperatures.

That doesn't mean they need to be eight times what you have now as the radiators in most houses are already bigger than they need to be for the gas or oil boiler.

To run at 30c I would imagine that most house need to double or triple the radiator output.

But somewhere between 35c and 40c is a good place to aim for.

matt_drummer

wrf12345 said:

"Running at 65c will not get you more heat" Running a 5kw heat pump at 65 degrees for a given time won't get you more heat than running at 30 degrees for the same time???

wrf12345 said:

"Running at 65c will not get you more heat" Running a 5kw heat pump at 65 degrees for a given time won't get you more heat than running at 30 degrees for the same time???

Another way to look at it.

It will, of course in the same house.

In your house with your radiators you will get more heat at 65c than 30c.

But if you need 65c to get heat then that heat pump will never even run at 30c in your house. The water will go back to the heat pump at virtually the temperature it left at and the heat pump will stop.

When the radiators are too small you will never even get the heat pump to run at low temperatures because the radiators can't get rid of very much heat. The heat pump needs to see a difference in the leaving and returning water temperature in order to operate.

stripling

@matt_drummer

When the radiators are too small you will never even get the heat pump to run at low temperatures because the radiators can't get rid of very much heat. The heat pump needs to see a difference in the leaving and returning water temperature in order to operate. 

Thank you @matt_drummer you have just given me a short clear way to make my case with the tentacled one about why, if they are insisting on a 4kw heat pump for a 5.6kw (de Podesta calc not MCS) heat loss, they must upgrade all my rads to enable low flow temps (the only way to get more than 4kw out of the Daikin). They design for 50º which is not so good and they know my house is leaky as a sieve... 

matt_drummer

stripling said:

@matt_drummer

When the radiators are too small you will never even get the heat pump to run at low temperatures because the radiators can't get rid of very much heat. The heat pump needs to see a difference in the leaving and returning water temperature in order to operate. 

Thank you @matt_drummer you have just given me a short clear way to make my case with the tentacled one about why, if they are insisting on a 4kw heat pump for a 5.6kw (de Podesta calc not MCS) heat loss, they must upgrade all my rads to enable low flow temps (the only way to get more than 4kw out of the Daikin). They design for 50º which is not so good and they know my house is leaky as a sieve... 

Just ask for the 8kW, it is what I have and my heat loss is around 4kW at -3c

The 4,6 and 8kW are the same heat pump, the `smaller' ones are just software limited.

There is no point having anything other than the 8kW.

As for running at 50c, well it's bad news as I think you know.

wrf12345

Thanks, I realised my mistake almost as soon as I posted. So a high temp heat pump can still run optimally as a low temp heat pump if you "double up" on the radiators and there are no efficiencies in going for a lower rated heat pump as a more powerful one will just adjust its temp and flow rate to run in a similar manner? 

Reed_Richards

Wrong and wrong, I believe.

A high temperature heat pump uses a different refrigerant to a standard ("low temp") heat pump.  Running at the same temperature this can cause the high temperature heat pump to be a bit less efficient.  Like-for-like comparisons tend to show this.

In general a more powerful heat pump than you need will not be able to achieve as low a power output as a lower rated one.  This will cause it to cycle more when trying to achieve a low temp and this can reduce its efficiency.  BUT within a range of heat pumps from the same manufacturer this may not be true; sometimes it turns out that the less powerful heat pump is just a software-limited version of a more powerful model so their minimum power draw is the same.  And the modulation range of a heat pump can differ from one manufacturer to another.

I'm told my 12 kW heat pump (LG Therma V) is a software-limited version of the 16 kW model.  But last evening it was 13 C outside and my heat pump was keeping the house warm at 20.5 C by running intermittently at around 2 kW output.  This is superbly economic for me because that meant my whole house was drawing 2.4 kW and my battery can (just) provide that power output.  I had charged up my battery the previous night so I was running my heat pump on electricity costing me 6.9 p per kWh.

 

  

smallblueplanet

matt_drummer said:

wrf12345 said:

The drawback is that radiator output falls dramatically as the difference between the mean flow temperature and the room temperature approach each other.

At dT 50c a 1kw radiator puts out 1kW of heat. That same radiator at dT 10c only puts out 120W of heat and at dT 5c only 50W.

That is why the radiators need to be so big to run very low flow temperatures.

I got most of your post, but I don't understand what the bold sentence is saying because of the 'technical' terms ie dT

 But somewhere between 35c and 40c is a good place to aim for. 

I presume that the only way to see if you can run your system of rads and HP efficiently at those flow temps is to try it and to see how much electricity you use?

matt_drummer

wrf12345 said:

Thanks, I realised my mistake almost as soon as I posted. So a high temp heat pump can still run optimally as a low temp heat pump if you "double up" on the radiators and there are no efficiencies in going for a lower rated heat pump as a more powerful one will just adjust its temp and flow rate to run in a similar manner? 

As Reed says, a `high temperature' heat pump uses a different refrigerant R290 compared to mainly R32 these days.

But it just means they can heat the water slightly hotter, otherwise no difference.

Vaillant Arotherm+ use R290 but most people run them really low and get top performing heat pumps. They are favored by many Heat Geek installers.

These heat pumps are not designed to be as efficient as running at low flow temperatures, they just exist because the R290 is more environmentally friendly and it gives options to people who want it for the reasons already outlined.

Reed is also correct in explaining to you the restrictions in heat output you get with a heat pump.

You don't have massive modulation like you do with a gas boiler and you also can't oversize.

So in general, at best, you could expect something like 25% to 100% of the rated output.

Many heat pumps are worse than this because the manufacturers make one heat pump and then software limit the upper end whilst the bottom end stays the same.

So with my 9kW Daikin the lower end was 4kW and the top 9kW. That is because it is really a 16kW heat pump that has been derated to only 9kW. The 16kW has a range of modulation from 4kW to 16kW, that is the 25% to 100%

You also need to understand that the amount of heat you get per unit of energy consumed varies massively depending upon how you use the heat pump.

Your gas boiler will be 90% (say) whatever you do, on for 10 minutes, off for 30 minutes, on again for 10 minutes etc doesn't harm the efficiency much.

Heat pumps don't work like this, some are better than others.

Think of it like doing multiple short journeys in a car. The fuel consumption won't be a good as just cruising along at 50mph.

Turning on a heat pump can be like starting a car on a cold winters morning, everything needs to warm up and that uses loads of fuel compared to what it does once warm.

My heat pump is not so efficient in the first 30 minutes of a heating cycle and uses a lot of electricity. But once it has settled down it is cruising and sips electricity, many times using less electricity than a gaming console or big television.

On for 30 minutes and then off for 30 minutes uses more electricity and is less efficient than just leaving it running.

But they are all slightly different and this is the value of detailed monitoring equipment. You can see what is going on and work out the best way to run your heat pump in your home.

The last point about getting a heat pump that is too big. 

Obviously if you have a heat pump that is too big, there will be many times where its lowest output is more than you and the house can take. The heat pump won't be able to run for very long before it either has to turn itself off or you intervene and switch it off.

And I have already explained what happens when heat pumps run in short bursts.

All of this is even worse if you need to do it at higher flow temperatures.

matt_drummer

smallblueplanet said:

matt_drummer said:

wrf12345 said:

The drawback is that radiator output falls dramatically as the difference between the mean flow temperature and the room temperature approach each other.

At dT 50c a 1kw radiator puts out 1kW of heat. That same radiator at dT 10c only puts out 120W of heat and at dT 5c only 50W.

That is why the radiators need to be so big to run very low flow temperatures.

I got most of your post, but I don't understand what the bold sentence is saying because of the 'technical' terms ie dT

 But somewhere between 35c and 40c is a good place to aim for. 

I presume that the only way to see if you can run your system of rads and HP efficiently at those flow temps is to try it and to see how much electricity you use?

dT means delta T. That is the difference in temperature.

Radiator outputs are quoted at dT 50c, that is a difference between the mean flow temperature of the radiator and the room in the case.

The mean flow temperature is the average of the water entering and leaving the radiator. So let's say 75c in and 65c out, that gives a mean flow temperature of 70c.

If the room is 20c then the dT between room and mean radiator flow temperature is 50c and a 1,000W radiator will output 1,000W of heat.

As the dT between room and mean radiator temperature closes the radiator output drops dramatically.

At 30c into the radiator and 25c out then the mean flow temperature is 27.5c

If the room is 22c then dT between room and mean radiator flow is only 5.5c

That 1,000W radiator now only puts out 50W of heat.


It is quite easy to work out what temperature gives you the heat you need. You can also work it the other way round and work out what radiator sizes you need to run at a certain flow temperature.

That is what I did. I knew that the radiators already in my house were pretty well balanced in terms of the heat they delivered. That is, I had no hot or cold rooms and didn't need trvs to control anything. 

Knowing my heat loss based on my gas consumption (I always had the heating on all day and all night on the coldest days) I worked out what radiator capacity I needed to deliver that heat loss at 30c flow

I settled on 50kW of radiators (that is at dT 50c) which would supply about 5kW of heat.

Then I just worked out the size I needed in each room to maintain the balance I already had.

I actually went a little bigger as you have to compromise a little in places but it works perfectly.

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.

It doesn't really matter what the actual figure is because the heat delivered to the house is just whatever the radiators can put out at the dT between their mean flow temperature and the room temperature.

So it really doesn't matter whether my flow temperature is 28c or 34c, the heat output is more or less the same because the difference between the flow and return to the heat pump is more or less the same and my flow rate remains the same as I set a dT between flow and return that the heat pump can never achieve.

I just pick a flow temperature that uses the least electricity at the the best efficiency. If it gets a bit hot I open a window or two. If it gets a lot hot I turn it off.

Running like this is more efficient and uses less electricity than trying to get less heat or having the heat pump turning on and off with a room thermostat.

It may not suit everybody but this is what I choose to do. It arrived at this situation because it was a strategy I used to accommodate a grossly oversized/unsuitable heat pump. I really wanted to keep it so I tried everything to find a way to make it work. I still find it works really well with my new replacement heat pump.

I have tried using room thermostats that Daikin supply (the Madoka controller) but I don't like it and it is less efficient than my way of doing it.


What I would advise anybody to do is work out their heat loss, see how far you can go on radiator sizes and find a balance between cost, asthetics, and flow temperatures that suits you but try and aim for 40c flow or less at the coldest outside temperatures.