ASHP - Advice on DHW & CH flow temperatures

akirkby

Hi,
I'm looking for any advice/guidance as to the way to determine the best temperature settings for a LG Therma V Split ASHP. Specifically those settings for the Domestic Hot Water and the Central Heating.
The original installed had set them to 55 for the hot water and 50 for the central heating...? Reading on other forums, some people suggest setting the central heating to around 30 degrees?
The system is a standard radiator install - they were replaced/upgraded when the ASHP was installed.
Any comments/advice would be welcomed.
Andrew

matelodave

ASHP are running at their optimum at the lowest temperature you can get away with which means luke warm radiators which are on for most of the day and night.

Heatpumps dont lend themselves to short time runing like a high temperature gas or oil boiler, ie they wont heat the house for an hour in the morning and then back on in the evening. ideally dont let the place get cold, just set the room temp back by 2-3 degrees during the day and overnight

The COP for most ASHP is specified at 7 degrees outside witha flow temp of 35 degrees, some will also be speced at 0 and 35 degrees which will give you some idea of how the efficiency (and running costs) vary depending on the external and flow temperatures.(it's reckoned that every degree higher than 35 will increase energy consumption by around 2.5%, so running at 45degrres could cost 25% more than 35 degrees)

TBH 30 degrees is a bit low for a radiator system and even low for underfloor, so I'd be inclined to try it at around 40 and leave them running to see how you get on - it's not really the time of year to try doing too much tweaking as we are vitually just coming out of the heating season. But try it at 35 -40 in spring and autumn and perhaps wind it up a bit by a degree or so if it's not warm enough. Low temperature heating is slower to react so only do small tweaks and give it a day or so to see what the effect is.

I have underfloor heating and mine runs at an average of around 35 degrees but is weather comensation controlled, so when its really cold the flow temp increases to around 40-42 and when its not so cold it drops to between 30-35, but most of the time its around 33-37

We only heat our hot water to 45 degrees which we find is hot enough for showers and hairwashing straight from the hot tap.

The main thing is to avoid the backup/boot or immersion heater kicking in

Have a look at this thread which has a lot of info
 https://forums.moneysavingexpert.com/discussion/1464827/updated-air-source-heat-pumps-air-con-full-info-guide-is-it-cheaper-to-run-than-mains-gas#latest

and theres a lot of discussion on this one about settings as wel,l despite the title 
https://forums.moneysavingexpert.com/discussion/6219931/most-efficient-way-to-run-underfloor-heating/p1 - the principles of setting and tweaking are quite well explained with quite a few examples together with the resulting effects.

If you've still got questions after trawling through that lot then come back and ask

Reed_Richards

Your system is very similar to mine except that my LG Therma V is not split.  My system was set up for 50 C central heating (where 50 C is the temperature you set on the control) and 50 C DHW (which means the flow temperature has to be about 55 C).  My house has upgraded/replaced radiators which are sized for an average temperature of 45 C (50 C flow and 40 C return).  During a warmer spell in March I reduced the central heating setting to 40 C and have left it there ever since without noticing any obvious difference.  But it will be taking longer to raise the temperature as necessary (my heating is "off" at night).  When winter draws in again I will put the central heating temperature back up to 50 C.

My heat pump cycles almost all of the time; it heats the output water to the specified temperature, the return water temperature gets too high so it goes off for a while.  If I turn down the central heating set temperature it will use less energy per cycle but will require more cycles for the same amount of heating.  I don't know where the optimum point is; ideally it would do load compensation but I am not aware of any heat pump with that capability.

I respect @matelodave's opinion but he has underfloor heating and that conditions you to believe that ASHPs are slow to give heat, which I don't think is actually true if your radiators have been sized correctly.           

matelodave

have a llok here as well https://forums.moneysavingexpert.com/discussion/6240076/i-bought-a-heat-pump/p1 itsgot lots of info from R_R above who has a system more like yours

Cardew

The first line of matelodave's post sums it up 'ASHP are running at their optimum at the lowest temperature you can get away with'

edited

Reed_Richards

Cardew said:

The first line of matelodave's post sums it up 'ASHP are running at their optimum at the lowest temperature you can get away '

..with.  But that doesn't help because the question is, in effect, "What is the lowest temperature you can get away with?"  The trade-off is between economy and responsivity and it's really a personal choice.  

Cardew

Reed_Richards said:

Cardew said:

The first line of matelodave's post sums it up 'ASHP are running at their optimum at the lowest temperature you can get away '

..with.  But that doesn't help because the question is, in effect, "What is the lowest temperature you can get away with?"  The trade-off is between economy and responsivity and it's really a personal choice.  

We are down to semantics(again) . The OP asked for guidance on the 'best temperature' for his unit, so clearly it is his personal choice.

Reed_Richards

So let's be scientific.  Suppose your system is designed to provide sufficient heat when the outside temperature is 0 C, the room temperature is 20 C and the average water temperature in the central heating is 45 C.  Now the outside temperature is, say, 10 C on average 10/(20-0) = 0.5.  Given that heat loss will be fairly linear with temperature difference your house will lose heat at now at 0.5 x the rate it would have done with an outside temperature of 0 C.  Heat gain from the radiators should also be linear with temperature difference.  You only need half as much so your average radiator temperature needs to be 20 + 0.5(45-20) = 32.5 average.  So if your system is designed for a 10 C difference between flow and return you need to set the flow temperature to 37.5 C to achieve a 32.5 C average.

Obviously your system won't be based on exactly the temperatures I have chosen but the maths can be applied to your numbers in exactly the same way.