In its most basic terms, a heating system consists of some form of thermal energy generator pouring thermal energy into your home, which in turn leaks out. Just as water flows down a slope (unless prevented from doing so), thermal energy will flow from a higher temperature to a lower one.
A home heating system can therefore be represented as a number of open vessels, each connected together using pipework of varying diameters, with the heat source pumping thermal energy into the first vessel representing the water within the heating system.
That is really clear - it gives me a picture - thank you.
But why do some heat pumps struggle - cycle lots - when they only need to top up the heat flow a little bit - when we have these English blah days, sort of damp, 5-10º ish maybe a bit windy? The modulation problems?
Is this because the 'minimum' power is too big? So they keep firing up more than you would wish to top up the heat loss using lots of Kws instead of just ticking over constantly at a low powered rate?
Is this where the modulation range counts? Being able to operate efficiently with not much power to just keep the chill off? Because this is what bothers me about running costs.
How do you balance that mid range level of heat loss via flow temps and radiator sizes? Do you need to make your emitters bigger? Does that solve it or help it? In terms of keeping running costs down by preventing the pump from firing up repeatedly?
Can you compensate for a not-very-good modulation range by making more emitter space available? Do you have to change the flow temperature? I just don't want to end up with a heat pump that costs me a lot to run during the most common type of Spring/Autumn weather we have on this soggy little rock in the sea.
This post was modified 8 months ago 2 times by Lucia
In its most basic terms, a heating system consists of some form of thermal energy generator pouring thermal energy into your home, which in turn leaks out. Just as water flows down a slope (unless prevented from doing so), thermal energy will flow from a higher temperature to a lower one.
A home heating system can therefore be represented as a number of open vessels, each connected together using pipework of varying diameters, with the heat source pumping thermal energy into the first vessel representing the water within the heating system.
That is really clear - it gives me a picture - thank you.
But why do some heat pumps struggle - cycle lots - when they only need to top up the heat flow a little bit - when we have these English blah days, sort of damp, 5-10º ish maybe a bit windy? The modulation problems?
Is this because the 'minimum' power is too big? So they keep firing up more than you would wish to top up the heat loss using lots of Kws instead of just ticking over constantly at a low powered rate?
Is this where the modulation range counts? Being able to operate efficiently with not much power to just keep the chill off? Because this is what bothers me about running costs.
How do you balance that mid range level of heat loss via flow temps and radiator sizes? Do you need to make your emitters bigger? Does that solve it or help it? In terms of keeping running costs down by preventing the pump from firing up repeatedly?
Can you compensate for a not-very-good modulation range by making more emitter space available? Do you have to change the flow temperature? I just don't want to end up with a heat pump that costs me a lot to run during the most common type of Spring/Autumn weather we have on this soggy little rock in the sea.
Most heat pumps will cycle when the OAT is 10ºC or above, the frequency is cycling may be dependent upon quite a number of parameters.
If I have time I will produce a diagram tomorrow, that will hopefully explain heat pump operation a little clearer.
In its most basic terms, a heating system consists of some form of thermal energy generator pouring thermal energy into your home, which in turn leaks out. Just as water flows down a slope (unless prevented from doing so), thermal energy will flow from a higher temperature to a lower one.
A home heating system can therefore be represented as a number of open vessels, each connected together using pipework of varying diameters, with the heat source pumping thermal energy into the first vessel representing the water within the heating system.
That is really clear - it gives me a picture - thank you.
But why do some heat pumps struggle - cycle lots - when they only need to top up the heat flow a little bit - when we have these English blah days, sort of damp, 5-10º ish maybe a bit windy? The modulation problems?
Is this because the 'minimum' power is too big? So they keep firing up more than you would wish to top up the heat loss using lots of Kws instead of just ticking over constantly at a low powered rate?
Is this where the modulation range counts? Being able to operate efficiently with not much power to just keep the chill off? Because this is what bothers me about running costs.
How do you balance that mid range level of heat loss via flow temps and radiator sizes? Do you need to make your emitters bigger? Does that solve it or help it? In terms of keeping running costs down by preventing the pump from firing up repeatedly?
Can you compensate for a not-very-good modulation range by making more emitter space available? Do you have to change the flow temperature? I just don't want to end up with a heat pump that costs me a lot to run during the most common type of Spring/Autumn weather we have on this soggy little rock in the sea.
You may be getting a bit over-concerned about cycling. Pretty much all central heating systems cycle at some point, its absolutely inevitable when the heat loss from the house is less than the minimum that the heating source can turn down to. Gas boilers tend to have a much greater modulation range than heat pumps so you would think would cycle less. But its normal to install a gas boiler that is 2-3 times the size actually needed, so in practice they are likely to cycle just as much.
You cant compensate (for any given model of boiler/heat pump) by making more emitter space available, because fundamentally its the house loss that is driving everything and that doesn't change. The only exception is if making more emitter space (and thus lower flow temperature) happens to allows you to use a smaller heat pump (most heat pumps have a maximum output that increases with reduced flow temperature) and thus puts you into another range, but that's a very borderline case which wont apply to many properties
The cycle on/off ratio in any given situation is determined by the ratio of heat demand (ie heat lost from the house) to minimum output. You cant change that. All you can change is the frequency of cycling. It gets larger if you have a larger system volume (one reason why people argue for buffers), and you can also set a minimum on time on the heat pump control. But the on/off ratio wont change.
Its actually not clear how big the penalty is. Historically the argument against 'short' (ie high frequency) cycling was compressor wear. Doubtless that is still the case, but much less so due to slow start-up of inverter-driven compressors. Perhaps someone on here has some quantified data on this for a modern compressor. There is certainly also a thermodynamic penalty (because it means that the FT when the heat pump is on must be higher to deliver the same energy on average), but if cycling is only happening at the tail ends of the heating season its not so large that you should be concerned.
One way to reduce cycling in the shoulder season is batch heating, ie heat morning and evening only, a pattern we are pretty used to! Of course this will still suffer the thermodynamic penalty due to a requirement for higher FT than the minimum theoretically possible, but that's to some extent offset by the fact that, when the heating is off, there is less standing consumption from the pump, and of course the house loss is reduced a bit because it cools down. Its actually not a bad solution all round, and a very good one if you can take advantage of ToU tarifs.
In summary, you should be probably concerned about cycling if its happening in the cold season, but if its happening in the shoulder season then its pretty normal. Getting the sizing of the pump right (ie getting the heat loss right) is the key here. However bear in mind that is itself an inexact science due to the uncertainties in the fabric, so one cannot expect perfection.
As an aside I personally believe that much more use should be made of actual consumption figures to size pumps, but the industry, with the notable exception of the heat geeks and a small number of others, hasn't accepted this. I suspect this is largely down to not wishing to rely on any data provided by the customer. I suspect this may change, there is enormous amounts of data buried in the figures from smart meters in particular, which the energy companies could mine. Id be very surprised if Octopus aren't working on this!
I don't know why Daikin are so cagey about specs - they're one of the biggest and oldest manufacturers. I might ring them tomorrow and ask...
I found Daikin to be hopeless when it comes to providing hard information. Their stock response, both on the telephone and by email, is 'ask your installer', which is a crazy answer pre-sales when you are still at the point of deciding which installer to use.
If you spend hours hunting the internet some info can be found, but its both patchy and difficult.
@lucia The EB series has a power hungry back-up heater, I have the ED series (which, in the words of the monologue “Naming of Parts”, We do not have!) I originally had the EDL model recommended to me by Octopus though they weren’t able to oblige me with an installation for reasons I have outlined elsewhere up thread. Regards, Toodles.
Toodles, he heats his home with cold draughts and cooks his food with magnets.
cycle on/off ratio in any given situation is determined by the ratio of heat demand (ie heat lost from the house) to minimum output. You cant change that. All you can change is the frequency of cycling.
One thing I am exploring on my heat pump at the moment is how to change actually run time compared to off time. So same ratio, so let's say 50% on 50% off. Bad situation is say 6 mins on 6 mins off. There are a few things you can change to change this.
Start and stop hysterisis within the control. So how many degrees above the target flow temperature the ASHP will continue running and how far below it goes before starting heating again. For mine that is set at about 7 to 8 degrees below to start and 0.2 degs above to stop. We are not discussing house thermostat!
The other is circulation pump. Normal default is to run continuously while there is a heat demand. The other is sniffer mode, so run a minute and of for 5 or more minutes.
So you can in two way make the run cycling longer, and the off time longer. But you then have to balance this with keeping a steady flow of heat to the house.
cycle on/off ratio in any given situation is determined by the ratio of heat demand (ie heat lost from the house) to minimum output. You cant change that. All you can change is the frequency of cycling.
One thing I am exploring on my heat pump at the moment is how to change actually run time compared to off time. So same ratio, so let's say 50% on 50% off. Bad situation is say 6 mins on 6 mins off. There are a few things you can change to change this.
Start and stop hysterisis within the control. So how many degrees above the target flow temperature the ASHP will continue running and how far below it goes before starting heating again. For mine that is set at about 7 to 8 degrees below to start and 0.2 degs above to stop. We are not discussing house thermostat!
The other is circulation pump. Normal default is to run continuously while there is a heat demand. The other is sniffer mode, so run a minute and of for 5 or more minutes.
So you can in two way make the run cycling longer, and the off time longer. But you then have to balance this with keeping a steady flow of heat to the house.
That all makes sense.
Many heat pumps have, I believe, a parameter which allows you to adjust the min cycle time (typically, it seems, set to 10min by default). Will adjusting this upwards (if that is possible) do the job, or do the other control features over-ride?
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
These responses are all really educational - so thanks everyone because it helps. Clearly, the closer to the mythical 'accurate' I can get my heat loss figures, the better the potential heat pump/radiator ratio and thus better efficiency.
But accurate monitoring also seems to be essential and I don't think I can find a cheap (and understandable by me) way to do that by adding a 'device' like ESPAltherma or whatever to a heat pump system. Github gives me headaches.
As I said before, my heat loss number crunching so far is higher than Octopus' (irony of ironies if you remember how I started this thread).
Today I have a visit that I've waited 9 months for under the Great British Insulation Scheme. However, I now understand these tick box projects (despite the 'rules' saying otherwise) work on being able to show big EPC jumps to make the fitter's £s... so despite everything it probably won't succeed.
The entire system - GBIS and ECO are with a few exceptions, shot to useless pieces. 🙁
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