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Gas Combi to ASHP - Making sense of survey results and proposal.

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(@irmartini)
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@harriup

If the radiators need to be twice the size then this sounds like a step too far.

If I understand your reply re: volumisers and buffers they are actually two different things!

With volumisers not really affecting efficiency but buffers can.

 

Which leads nicely to

@ajn9000

Reading your reply I understand a buffer may be needed, depending on the heating setup.

Which I hope will mean I do not need a buffers as I have what I Believe to be a rather simple setup.

  • I only have one heating circuit which will always be open and only contain radiators.
  • I’m having the main circuit re plumbed in 28mm and all microbore radiators will be replumbed in 15mm so I’m hoping flow rate should not be an issue.
  • Not sure I understand this one?
  • I will be asking for a 50ltr volumiser in the heating circuit (so its not used when only in DHW mode) again so I hope the heating volume will be sufficient for defrost cycles.

Also it sounds like even though your rads were spec’d at 45deg flow rate you are able to run cooler so there is hope with the current proposal I may be able to achieve a slightly lower flow temp also.

 

@mike-h 

No need to apologise I appreciate everyone’s input whether it is positive or negative understanding the Cons is as important if not more so than the Pros!

Your first point I’m hoping will not be a problem especially after reading your interesting article I would like to think my scenario will not require a buffer tank due it being a single permanently open heating circuit in 28mm.

 

Your second point from what I’ve read ( I could be wrong) about the Samsung Gen7 HTQ which was originally quoted they do not have the pump in the outdoor unit however I believe the HTQ models are all R32 and as I’ve now specified a R290 unit there are I understand only two R290 Samsung units (ignoring capacity options) one with the pump in the outdoor unit and the other does not but at the moment I don’t know which unit they plan on using I’ve asked the question.

That said I’m not following with regard to the anti-freeze valve and no glycol is that with an outdoor pump or an indoor pump?

If you have an antifreeze value do you not need glycol?

 

Your third point is interesting and not something I had considered or full understand, if I read it right when you don’t need ‘full power’ from the ASHP it can throttle its output down to run continuously at the required demand. In my head I’m comparing this to my combi boiler once the heating circuit is heating up and the returning water is not stone cold I you can hear the gas throttling back again to meet the current demand.

However, the ASHP cannot throttle back past a certain point and so ends up turning off, again using my combi boiler analogy the heating circuit is up to temp and the returning water is the same temp and it shuts off completely.

But in the gas boiler scenario when heat is needed again it just fires up and we are back in business but the ASHP turning back on is not as quick or efficient as the gas boiler and needs time and power to get going again.

Have I understood this correctly?

Which would make sense then if you have ASHP which is to powerful and cannot go low enough for the demand will keep turning off and back on again wasting energy where as if you have the right size ASHP or at least one that can work at the level required it remains on and running efficiently.

Is this what people mean when they say ‘Cycling’ I’ve read it a few times but not really understood what it meant and when it happens?

“Anything worth doing, is worth doing right.”


   
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 JEH
(@jeh)
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@irmartini Hi, Why not lower your gas boiler flow temp to 35° and leave the heating on permanently? Then you'll know if your radiators are big enough.


   
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(@mike-h)
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Posted by: @irmartini

Which would make sense then if you have ASHP which is to powerful and cannot go low enough for the demand will keep turning off and back on again wasting energy where as if you have the right size ASHP or at least one that can work at the level required it remains on and running efficiently.

Is this what people mean when they say ‘Cycling’ I’ve read it a few times but not really understood what it meant and when it happens?

Yes, when the ASHP cannot produce less heat than the radiators are delivering, the return water temperature and leaving water temperature (LWT) tend to rise above the set levels and the compressor switches off (cycles off). Depending on your settings, the water pump continues to run for 1 or 3 minutes or continuously. Once the LWT has fallen to around 2degC below its set point, the compressor starts again (cycles on) and the process then goes on repeat. My Samsung and several other Samsungs that I have seen data for cycle on and off around 8 times per hour in this scenario. It is generally accepted that this frequency is bad for wear and tear and the COP isn't as good.

There are ways to get round this.

1. Bigger radiators will deliver more heat. But this may make the home too warm.

2. Run higher flow temperatures, so that the radiators deliver more heat. But this may make the home too warm and is less efficient.

3. Run higher flow temperatures (or fit bigger radiators) and switch the compressor off with a thermostat set to desired room temperature. The thermostat that is part of the wired remote controller has a hysteresis of 1.1degC. If set to 18C it switches the ASHP on at 17.9C and off at 19C. Although this is less efficient than the 'low and slow' way of running a ASHP, it seems to be quite cost effective as the heat pump remains off for several hours at a time. Some other manufacturers have more sophisticated controllers which keep room temperature within closer margins.

4. Get a smaller ASHP!


   
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(@irmartini)
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Posted by: @jeh

@irmartini Hi, Why not lower your gas boiler flow temp to 35° and leave the heating on permanently? Then you'll know if your radiators are big enough.

I've missed the opportunity to try things like that, already this year the boiler turns on in the morning and generally doesn't turn on again all day except when we have a few cold days bacxk to back generally its just not cold enough now to do any meaning full experiments. If only i start the ASHP journey a few months ago 😣 

 

“Anything worth doing, is worth doing right.”


   
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(@irmartini)
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Posted by: @mike-h

Posted by: @irmartini

Which would make sense then if you have ASHP which is to powerful and cannot go low enough for the demand will keep turning off and back on again wasting energy where as if you have the right size ASHP or at least one that can work at the level required it remains on and running efficiently.

Is this what people mean when they say ‘Cycling’ I’ve read it a few times but not really understood what it meant and when it happens?

Yes, when the ASHP cannot produce less heat than the radiators are delivering, the return water temperature and leaving water temperature (LWT) tend to rise above the set levels and the compressor switches off (cycles off). Depending on your settings, the water pump continues to run for 1 or 3 minutes or continuously. Once the LWT has fallen to around 2degC below its set point, the compressor starts again (cycles on) and the process then goes on repeat. My Samsung and several other Samsungs that I have seen data for cycle on and off around 8 times per hour in this scenario. It is generally accepted that this frequency is bad for wear and tear and the COP isn't as good.

This has hammered home my understanding of cycling thank you.

I understand your points about ways to avoid cycling if its happening but your last point 'Get a smaller ASHP!' or rather as i read that get the correct size for my heat loss is my main takeaway!

Which leads me back to the heat loss survey and how accurate it is compared to my actual heat loss as the decision driver for what size to get.

I wish i wasn't such a control freak and was just able to let the installer make the decisions and do what they think is right and stop worry about it.

 

“Anything worth doing, is worth doing right.”


   
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Toodles
(@toodles)
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  1. @irmartini 😄

Toodles, 76 years young and hoping to see 100 and make some ROI on my renewable energy investment!


   
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(@ajn9000)
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This series is very good - to see a very poor ASHP install transform into a good one ...  

(325) Is a Heat Pump Cheaper to Run Than a Gas Boiler? - YouTube


   
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(@ajn9000)
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Also, to satisfy the inner control freak you could ensure that Open Energy Monitor is installed so you have lots of accurate data!  Having that discussion with your installer might send a powerful message - they probably wouldn't want to be too low down on the leader board!

https://heatpumpmonitor.org/

I don't really do regret, but I would have installed Open Energy Monitor had I known about it at the time - but luckily for me the Stiebel Eltron controller and web portal is very good at providing lots of data.  I'm just struggling to get margin of error figures out of them on their energy figures. 

 

 


   
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(@irmartini)
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Topic starter  

@ajn9000 

that is a great idea that dashboard and all the data is just what I’m after. I will ask my installer to add it to the quote 👍

“Anything worth doing, is worth doing right.”


   
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(@harriup)
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Posted by: @irmartini

Which leads me back to the heat loss survey and how accurate it is compared to my actual heat loss as the decision driver for what size to get.

An annual gas use figure of 13000kWh, as long as you are close to keeping the entire property at 20° for most of the time, would suggest a 5kW ASHP would supply enough heat. But that is a rule of thumb, and the MCS guidance which is supposed to be followed by installers in order to qualify for a BUS grant requires a heat loss survey to be done and the unit sized to the design temperature using that. And the track record is entirely that they overstate heat loss - and have a particular achilles heel around air change. 

The quote detail says your needs can be met with a flow temp of 45°. A high temp unit is capable of flows above 55° so your needs could be met with a smaller unit specified at a higher temp. Then when it turns out you only need 4.5kW when its -3°, not the 7kW of the heat loss calc you probably will be running at 45°. It could be presented to you as a choice you can make.

Perhaps talk to the installers about the heat loss calc, see how much store they want to set by it.

 

Mitsubishi EcoDan 8.5 kW ASHP - radiators on a single loop
210l Mitsubishi solar tank
Solar thermal
3.94kW of PV


   
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(@irmartini)
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Topic starter  

@harriup 

That 13000kWh of gas is with a thermostat set most of the time at 18deg and as the whole family are out all day at work and school the heating only come on for an hour in the morning for when we get up and leave then it is off all day. its then on from 4pm till 9pm and usually at some point after we eat we up it to 19deg and if we are in all weekend it will be on 19 for most of the day!

 

“Anything worth doing, is worth doing right.”


   
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(@irmartini)
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Posted by: @harriup

I would ask why a volumiser is required, and especially ask if a buffer tank is proposed. It used to be that a lot of installers put them in automatically as a backstop, even some manufacturers insisting that one be present. Buffers in particular can just cause a drop in working efficiency. But many systems don't need either at all, so I would ask the question – even if you can't follow the answer you should at least get a coherent relevant explanation.

 

Here is the answer I got to this question:

With the HTQ it will be a buffer as this will prevent excessive cycling within the unit as well as give the heat pump a hydraulic interface and therefore will not restrict flow within the heat pump especially if some of the radiators have closed down due to the rooms reaching temperature.

 

I understand that if a number Radiators start shutting down via TRVs then a buffer would be helpful, but I don't plan to restrict the Radiators with TRVs rather I would be looking to lower flow temp.

I'm not convinced I need one in my situation?

Is it really worth having just in case?

 

 

“Anything worth doing, is worth doing right.”


   
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