Posted by: @andrewj

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Can anyone explain in a bit more detail how load and weather compensation work together to ensure a comfortable internal environment?

I can't tell you how manufacturers do it, but I can tell you how I do it using a DIY approach on a Midea heat pump. I have weather compensation on all the time when the heating is on, and have tuned the curve so that most of the time it gets it about right. I then have a python script running all the time (I use python to monitor my system over modbus, which can set (write) as well as get (read) parameters) which checks the actual room temp in a central room against the desired room temp once an hour. If the actual room temp is more than one degree out from the desired room temp, then the script moves the whole weather compensation curve up or down a bit, meanin I don't have to faff about changing the leaving water temp, the weather compensation curve still does that. If the actual room temp is 1-1.99 degrees below the desired room temp, then raise the weather compensation curve end points each by one degree, if it is 2-2.99 below, up the WCC end points by two degrees, same thing for 3 degrees (but no more), and same thing in reverse for if the room is over-temp. It works well, and comfort is improved rather than adversely affected. I should add the most of the interior doors are open all the time, so air does move freely around, no doubt aided by the fact it is an old leaky building that is most definitely not hermetically sealed.    

Definitely not like a thermostat

Load compensation takes room temperature and target temp and modulates system flow temp to correct. The change in temperature is weighted based on difference. Do not use this with UFH as system is slow to respond and you get very big temperature swings.

Hi Andrew,

I will try to keep it simple.

If you use a stat in the house as a straight forward stop and start it causes the ASHP to cycle more and effectively cost more money to run and repair in its lifetime.

If you have a correctly set up system and each room's emitters match their room load at peak then you can ask the ASHP to just supply the required load to each room via the flow temp adjustment performed by the curve. (an emitter of any given value changes its output if the flow and return temp changes, the higher the input flow the higher the output from the emitter to the room) so we can use this to match the loss which also rises and falls with the external temperature, the colder outside the higher the heat loss and the reverse.

Assume a room in your house looses 250w when external temp is 10 Deg c it also could mean the room will loose 350w when outside is -3 Deg C. If you have a fixed system with a stat you would need to set the flow temp to a fixed 45 or 50 regardless of external temp to match the highest demand and rely on the stat to switch the system on and off, this is not energy efficient.

Its probably better to explain in % although I could be at risk of over simplifying:

If I design your system at 100% when the peak is -3 Deg C, any temperature above -3 is less heat loss and less demand so something like below:

External temp = heat loss load of calculation % = flow temp to match (assumes desired target is 21/22)

-3 = 100% = 45 flow

-1 = 90% = 43 flow

+3 = 80% = 40 flow

+7 = 70% = 38 flow

+10 = 60% = 36 flow

+15 = 40% = 32 flow

+18 = 30% = 30 flow

+20 = 10% = below 30 

The above is just an example of the cascade effect and not actual calculated for accuracy.

The WCC will try to track this and keep each room at the desired balanced input which = desired temperature. You can also think of it like a bucket with hole in it, you must keep the water level at 4l (comfort level) but the hole gets bigger or smaller subject to external temp (heat loss) varies, water goes in to fast or to slow to match what it is loosing you move over or under your 4l mark causing discomfort. Fixed speed is you turning on and off the water full speed, WCC is the flow is varied you slowing and speeding up the water keeping an even balance and maintaining that 4l level all the time.

I hope this helps.

@johnmo you get big swings using the curve or using fixed stat?

@johnmo I have just seen that Mars has uploaded another podcast titled to your question, I more or less answered but I have not watched this cast yet, I am going to watch it now and I will come back to you once I see what was not covered.

I didn't realise you question was in relation to that specific podcast.

Posted by: @ashp-bobba

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@johnmo you get big swings using the curve or using fixed stat?

If you have UFH you get big swings in room temperature using load compensation. Unless you use very thin screed. So really more like a radiator than UFH.

You can also get big swings in room temperature using a standard thermostat. Found using a +/- 0.1 or a plus 0 and minus 0.1 hysterisis one can work well, with careful balance of flow temperature. Useful for time of use tariff and using floor as a storage heater. These DO NOT work well with radiators or houses with UFH that are not generally airtight and well insulated.

But back to the OP questions, depending on controller you may have just weather compensation, weather compensation and load compensation as well as thermostat control. You can normally use one or multiples of these control strategies. Weather compensation on its own is generally good for most houses. Load compensation needs careful selection of where the temperature sensor is located, a draft could cause ALL rooms to overheat.

Nothing is really that straight forward, but generally WC is, once tuned to your house.

this is the actual answer to yur question and they both aim to provide the same thing and are related, this means both effect the other in some form.

🔹 Weather Compensation

• What it does: Adjusts the flow temperature of the heating system based on outdoor temperature.

• How it works: Uses an outdoor sensor. As it gets warmer outside, the system lowers the water temperature it sends to the radiators or underfloor heating.

• Goal: Maintain indoor comfort while using the lowest possible flow temperature, increasing efficiency (especially important for heat pumps).

🔹 Load Compensation

• What it does: Adjusts the system's output based on the internal heat demand (i.e., the difference between the actual indoor temperature and the set point).

• How it works: Uses indoor sensors or thermostats to measure current indoor temperature and modulate output accordingly.

• Goal: Prevent overshooting the target temperature and reduce cycling.

🔄 How They Relate

• Both aim to modulate heat pump output to improve efficiency and comfort.

• They can be used together: Weather compensation adjusts based on external conditions, while load compensation fine-tunes based on internal conditions.

• When both are implemented properly, the system can maintain steady temperatures with minimal energy use.

📈 Role of the Internal Stat (Thermostat)

When the internal stat is active while using a weather curve, it modulates or overrides the weather compensation in real time based on actual indoor temperature.

Here's what it does:

1. Feedback Loop:

   • If the room is too warm, the internal stat can limit or reduce the flow temperature—even if the weather curve calls for a higher one.

   • If the room is too cold, it can allow the system to boost the temperature, sometimes outside the preset weather curve limits.

2. Comfort Control:

   • Prevents overheating on sunny days when solar gain raises indoor temps despite cold outdoor conditions.

   • Improves responsiveness in real-world scenarios that the outdoor sensor alone can’t account for.

3. Efficiency Optimization:

   • Keeps the system from running harder than needed, which improves Coefficient of Performance (COP) and saves electricity.

🧠 Summary:

When a Mitsubishi ASHP is using a weather compensation curve, the internal stat acts as a real-time corrective tool, fine-tuning the heating output based on actual indoor conditions. It ensures both comfort and efficiency by adjusting or overriding the weather curve’s output.

If you're configuring such a system, you usually want to:

• Enable internal stat feedback if occupant comfort is a top priority.

• Rely purely on the weather curve if the building has stable thermal performance and you want maximum energy efficiency with fewer system interventions.

Posted by: @ashp-bobba

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this is the actual answer to yur question and they both aim to provide the same thing and are related, this means both effect the other in some form.

 

🔹 Weather Compensation

• What it does: Adjusts the flow temperature of the heating system based on outdoor temperature.

• How it works: Uses an outdoor sensor. As it gets warmer outside, the system lowers the water temperature it sends to the radiators or underfloor heating.

• Goal: Maintain indoor comfort while using the lowest possible flow temperature, increasing efficiency (especially important for heat pumps).

---

🔹 Load Compensation

• What it does: Adjusts the system's output based on the internal heat demand (i.e., the difference between the actual indoor temperature and the set point).

• How it works: Uses indoor sensors or thermostats to measure current indoor temperature and modulate output accordingly.

• Goal: Prevent overshooting the target temperature and reduce cycling.

---

🔄 How They Relate

• Both aim to modulate heat pump output to improve efficiency and comfort.

• They can be used together: Weather compensation adjusts based on external conditions, while load compensation fine-tunes based on internal conditions.

• When both are implemented properly, the system can maintain steady temperatures with minimal energy use.

I think I understand weather compensation ok and I can see how that works by altering flow temperature such that each emitter is supplying the right amount of output to match the expected heat loss at the outside temperature it was designed for and thus maintain a set temperature in the room.  As each emitter has been sized for, say, 21c given the room's heatloss the theory is that every room in the house is matched in temperature (I guess assuming the emitters have been properly balanced.)

Where I struggle with Load Compensation is that there is a temperature sensor (I won't use the word thermostat) in a location in the house supplying temperature OF THAT ONE AREA to the ASHP controller which adjusts flow temperature to try and get that ONE LOCATION to the right temperature also taking the outside temperature into account.  Hmmm....I think maybe just writing that has helped: I would then, again assuming the emitters have been properly balanced, assume that all the other rooms follow suit.

We've just had a Cosy 9 installed with one Cosy Pod but haven't been able to try the heating yet because it's been so warm!  

Posted by: @ashp-bobba

↑

📈 Role of the Internal Stat (Thermostat)

When the internal stat is active while using a weather curve, it modulates or overrides the weather compensation in real time based on actual indoor temperature.

Here's what it does:

1. Feedback Loop:

   • If the room is too warm, the internal stat can limit or reduce the flow temperature—even if the weather curve calls for a higher one.

   • If the room is too cold, it can allow the system to boost the temperature, sometimes outside the preset weather curve limits.

2. Comfort Control:

   • Prevents overheating on sunny days when solar gain raises indoor temps despite cold outdoor conditions.

   • Improves responsiveness in real-world scenarios that the outdoor sensor alone can’t account for.

3. Efficiency Optimization:

   • Keeps the system from running harder than needed, which improves Coefficient of Performance (COP) and saves electricity.

---

🧠 Summary:

When a Mitsubishi ASHP is using a weather compensation curve, the internal stat acts as a real-time corrective tool, fine-tuning the heating output based on actual indoor conditions. It ensures both comfort and efficiency by adjusting or overriding the weather curve’s output.

If you're configuring such a system, you usually want to:

• Enable internal stat feedback if occupant comfort is a top priority.

• Rely purely on the weather curve if the building has stable thermal performance and you want maximum energy efficiency with fewer system interventions.

 

Thing is, when explained like this it sounds very much like a thermostat, albeit not on/off!  What is happening to the other rooms as the room with the sensor is now controlling things?  

just stick with weather compensation, get that correct first. Only enable load compensation if really needed. Reasons are solar gain, drafty house and it gets windy and stuff like that.