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What is considered a decent weather curve for running weather compensation?

 
Air Source Heat Pumps (ASHPs)
Last Post by Crimson 3 months ago
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 Crimson
(@crimson)
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Topic starter 06/12/2024 8:41 am  

Hi there,

I'm trying to get my system running as close to its design of max flow temp of 40C at -3C ambient.

However one of my zones isn't getting to desired temps.

I'm adjusting max and min flow temps to try and find a balance.  Living rooms at 20C in slightly warmer temperatures feels ok, 21C in colder temperatures feels better, so really it's at the lower end of the ambient temps I want to hit 21C in living rooms.

Currently I've had to push the system up to 44C at -3C and 30C at 17C.  Living rooms are mid 20s so pretty much there this morning (suspect be warmer later in the day), however that puts my max flow temp a good 10% above design (something I'm arguing with the installers on as we've had a rad upsize to try and fix this and it's still not great).

My question is, how bad/good is that weather curve?  Is there a big cost implication over time with this?

I seem to recall people saying ideal is 35C at an ambient of 7C being a good marker.  Currently my WC puts it at 37C at 7C ambient, so a good 2C above that.  Having the WC at 35.5C at 7C ambient, living rooms are around 19.5-20C in mornings (I have a set back to avoid heat going upstairs and overheating us).  So that's not too bad, but short of really crushing the WC not sure how to get a good balance.

Here's a screenshot of what flow temps my Grant ASHP runs at with ambient temps with current WC setting.

image

Something I'm trying to do is as I change the WC, I note the ambient and flow temps and subsequent room temps so over time can map the curve against what works.  This is what I have so far:

image

 

Is that a viable approach?

It's really the 5C and below that it feels like the system needs a push.  I haven't yet got a figure of when we wouldn't want any heating, 17C feels about right but may not be 100%.

This topic was modified 3 months ago 2 times by Mars

   
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 Gary
(@gary)
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06/12/2024 9:25 am  

To me it looks like your system is pretty much meeting the design. 

You are having to increase the temp above the design as you have a setback for X hours overnight so you are having to put more heat back into the house above its heat loss to increase the temp.

40C flow temp at -3C is what it takes to maintain house temp, you would have to run hotter than that to increase the house temp which is what you are seeing in your curve.

Rule of thumb is that every 1C in flow temp is 2-3% difference in electricity consumption.

 

 


   
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 Crimson
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Topic starter 06/12/2024 9:37 am  

Thanks Gary, I'll then look at killing off the setback to see if can run it lower (I say it switches off, I have the hallway panel it go down from 23C to 20.5C between 10pm and 4:30am).

4C = 8-12% cost is not something I want to incur over time.

Main issue is upstairs performs so well and underfloor at rear of downstairs is fine, they barely call for heat and the higher flow temps makes it easy for upstairs to overheat, I'm having to use TRVs upstairs to stop that even with the panel set 20C in day to 19C at night.  Could simply the effect of having upstairs lower than downstairs causes the increase to as the heat rises anyway?


   
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 Gary
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06/12/2024 9:40 am  

@crimson turn the lockshields down upstairs to reduce the flow to the rads so they output less heat rather than use the TRV's.


   
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 Crimson
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Topic starter 06/12/2024 9:51 am  

Thanks Gary, I'll do that with the Drayton balancing key.  I know the plumbers turned down the actual lock shield a bit as well so combined should really reduce it.  I think main issue is upstairs is so oversized now as was just under with K1s that are now K2s.

Mean to say, I did do this at one point, but the other half kept complaining it was still too hot upstairs

This post was modified 3 months ago by Crimson

   
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 Crimson
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Topic starter 09/12/2024 4:12 pm  

So with a flow temp maxing at 42C, managed to get living rooms to almost 21C at points in the day.  44C started to overheat one of the rooms so some good news.

It's been around 6-7C ambient.  At 7C – it runs at 36C so quite pleased with that.

That's by balancing down one living room further (that has the air locked rad), and 1 rad in the other living room slightly.  And now no TRVs used upstairs (all set to Max).  First floor rads are closed quite down heavily.  The main bedroom still getting a bit too warm at 19.5-20C.  I've now got that at 1/6 with Drayton Balancing key (not a TRV setting).  So I'm hoping installers will come and sort the airlocked rad so I can pull the flow temp down slightly.


   
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