Weather compensation just means the flow temperature to the heat emitters is hotter the cold it is outside.

it’s been mandated on boilers in Germany since 1985. It is mature technology. This is relevant because this is where most heating technology is researched and developed. 

A heat curve reflects the rate of change of the water supplied to the emitters..floor or radiators

It also reflects the thermal performance of the building, and in case of the floors their structure and coverings 

a starting point is this at 20c outside, 20c inside, the flow temperature will be 20c

But of course 20c flow temp is no good when it is less than 20c ouside or as low as 6c

-6c outside, 20c inside flow temperature 65c

And for a floor in screed with ceramic tikes

-6c outside, 20c inside, flow temperature 29c

and for a floor made of wood with carpets

-6c outside, 20c inside and 50c flow temperature

Weather compensation is important for the efficient use of energy as  it matches heat loss to heat generated

for both heat pumps and gas boilers the lowere the flow temperature the lower the return the higher the efficiency.

indoor compensation is a necesary level of additional control  even with weather compensation in most cases 

this reduces the flow temperature as the room temperature rises. If the room temp rises above the set point the heat source is removed, but the pump not shut off

Many parameters exist to tailor a heat pump to a building, but these are only useful if the controls integrated with the heat pump.

Not much more to add but some thoughts based on my experience.

As @alec-morrow says, weather compensation is good because when done properly it matches heat loss to heat generated.  When it's set up properly it also uses the lowest possible temperature to heat the house, which is good for an ASHP.  But because heat loss is matched to heat generated then it's not very good at warming the house up from cold.  If you let the house cool down then it will take a long time to warm up or you'll have to temporarily boost the flow (or let the system do it for you).  Because of this, a properly calibrated weather compensated ASHP system is best left running all or most of the time. 

My installer set the system up with weather compensation and external thermostats.  The weather compensation temperatures were far higher than they needed to be; probably 5C higher than they are now.  This meant the thermostats, set at 21C, controlled the ASHP, switching it on and off as required.  This was better than using a constant flow temp but not really using weather compensation properly.   

Now our external thermostats are set at 30C so are always calling for heat.  The flow temp is about 42C at -3C outside and 30C at 20C outside. This has kept the house at 20-21C at outside temps from -2C to 14C.  Some have questioned this approach, including my installer, but it works for us.  I may have to adjust at extremes but so far so good.  

I tried running my ASHP without weather compensation for a couple of weeks.  I used a modest 45C constant flow and set the house to 21C using the thermostats.  This used 20-25% more electricity to achieve the same result.  I'm now back to weather compensation.

The next thing I want to do is to try using Mitsubishi's Auto Adaptation, which integrates room temperature control and weather compensation. 

The bivalent controls on our heat pump don't allow us to run the boiler on weather compensation which is a pity. When we get off the HVO trial, we'll switch our heat pump over to weather compensation because our heat pump manufactures have made some software tweaks which will make it feasible for us to use now, so that experiment will probably start in April.

@derek-m has already found all the relevant settings in our manual so it'll be a quick switch over.

Excellent topic @editor - thanks for starting.

I wonder can a formula be used to calculate what a specific flow temp will be at an outdoor temp based on the curve values used?

@adaml ask the manufacturers, for sure they will have a diagram or table

 but be careful understanding these things is a bit if a rabbit hole especially for a heat pump due to the algorithms that operate the pump.

users Comoensated heating systems should just focus on room temperature.. not excessively what the equipment is doing..that will just frustrate you

@alec-morrow 
would you say that installers choose not to set up weather compensation because they find it more straightforward for the user to use a fixed flow temperature and room thermostat ? 
 

Posted by: @adaml

Excellent topic @editor - thanks for starting.

I wonder can a formula be used to calculate what a specific flow temp will be at an outdoor temp based on the curve values used?

Hi,

The required water flow temperature varies with the heat loss and hence heat demand. The heat loss is dependent upon the delta T between indoor and outdoor temperatures in conjunction with how well the property is insulated and draft proofed. How well the heat demand is met is dependent upon the size and capacity of the heat emitters.

If you vary the above parameters in the modelling tool that I produced, it is quite easy to see how this affects the required water flow temperature.

@derek-m please could you post a link to your modelling tool for me. 

Posted by: @heat-pump-newbie

@derek-m please could you post a link to your modelling tool for me. 

Hi Newbie,

Are you still making smoke?

I have attached a copy of the spreadsheet. Please read the notes on sheet 1, which should help.

Thanks Derek. Wow, a lot of work's gone into that!

No more smoke for the moment sadly - I've used up all my supply of pellets. I'm also sorry to admit to you that I've taken down the little shed. However, having experimented with a roll of weed control fabric, I'm going to try and rig up something to deflect the cold air out through the back gate and away into the garden, where it won't be able to return to the back of the 'pump' (which I'm now learning to call the Compressor). 

@derek-m 

Thank you for getting back.

Does a formula exist to calculate the flow temp for a given outside temp? For example, what would the flow be at an outdoor temp of 8°C with a curve of X? I seem to remember you posted a formula some time ago, I cannot find it though.