What should my heat curve be?

[AliSmi]

Hi all, looking for some 101 ASHP advice...

 

I've had a Grant Aerona3 10kw R32 Heat ASHP installed this summer, the installer set the heat curve at 1.3, Grant recommends setting it between 1-1.6, I've read and watched videos where they all seem to be sub 1.0.

The heating is all radiators.  Can anyone shed any light or is it a case of experimenting?

 

Thanks in advance

AliSmi

[JohnMo]

Many variables to take into account.

 

Start at 25 flow at 20 outside, then look at your heat loss report and design report, set the other end of the curve to your design outside temp and design flow temp.

 

If house is too warm after 24 hrs reduce the cold end of curve, if to cold increase. Do 1 Deg at a time to see how it changes things.

[akjos]

As JohnMo said, many variables to take into account, which means nobody will be able to tell you what to set your heat curve to without knowing the details.

 

The simple answer is: Don’t fix it if it ain’t broken.

 

The more complex approach, if it isn’t working properly right now or you want a better performing system:

1. Figure out your heat loss. Preferably room by room at certain outside design temp. (Example: -3C).

2. Get the data sheets for your radiators and calculate what temperature needs to flow through them with a delta T of 5C to achieve the heat loss from 1.

3. Look at the Grant manual and see the heat curve lines. Find the one which gives you the needed flow temp at the design temp.

4. That’s the curve which will work best for you.

[AliSmi]

Thanks both, some stuff to dig into...

[JamesPa]

39 minutes ago, akjos said:

As JohnMo said, many variables to take into account, which means nobody will be able to tell you what to set your heat curve to without knowing the details.

 

The simple answer is: Don’t fix it if it ain’t broken.

 

The more complex approach, if it isn’t working properly right now or you want a better performing system:

1. Figure out your heat loss. Preferably room by room at certain outside design temp. (Example: -3C).

2. Get the data sheets for your radiators and calculate what temperature needs to flow through them with a delta T of 5C to achieve the heat loss from 1.

3. Look at the Grant manual and see the heat curve lines. Find the one which gives you the needed flow temp at the design temp.

4. That’s the curve which will work best for you.

The problem with this is step 1, knowing your heat loss.  Heat loss surveys are not always accurate and can be wildly inaccurate, so whilst they are a starting point it's unlikely this alone gives the optimum curve.

 

If it aint broke don't fix it does of course apply but the lowest possible flow temperature will give you the highest efficiency (roughly 2-3% change in efficiency per degree, which can soon mount up.). @JohnMo suggests an iterative approach based on experiment, which is the only one I can see can possibly work, unless your heat pump features auto adaption or you have a third party ft optimiser such homely.

 

Whether it's worthwhile depends on how much you want to chase efficiency.  

[ReedRichards]

9 hours ago, JohnMo said:

Start at 25 flow at 20 outside, then look at your heat loss report and design report, set the other end of the curve to your design outside temp and design flow temp.

 

If house is too warm after 24 hrs reduce the cold end of curve, if to cold increase. Do 1 Deg at a time to see how it changes things.

 

This seems an excellent methodology but your house will only get too warm if you aren't using a room thermostat and/or TRVs as these will act as an end stop.  You could set the room thermostat a few degrees higher than you actually want and set all the TRVs to max but then you would have to spend time balancing all your radiator flows so that each room reached the temperature you want at about the same rate. And then adjust your heat curve settings.

 

Or you just reduce the cold end of the curve until you can't get the house warm enough.

 

8 hours ago, JamesPa said:

Whether it's worthwhile depends on how much you want to chase efficiency.

 

Some heat pump owners seem to get a bit obsessed with chasing efficiency.  For me, efficiency is good but not at the expense of comfort or convenience.