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Low Loss Header is losing 1.5c from flow temperature

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(@sunandair)
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We have worked out that our flow temperature is cooler on the central heating side of our installed low loss header by approximately 1 to 1.5c. Is it worth considering removing it and running the system as a single pump 3 port setup for DHW and central heating?

Here are two schematics of what we have.

694A4E54 102E 4880 920F F734496D2B12

we were thinking that we could bypass the header somehow. Which may make the second pump redundant. 

617347E1 AE43 47BD B2EA C4CE32D04825
E34D71AA 6026 4716 90D2 3D733B4F69CE

not sure if we need to consider close couples tees etc? 
any considerations or thoughts?

Is it worth saving 1.5c of a 40c flow?


   
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(@derek-m)
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Posted by: @sunandair

We have worked out that our flow temperature is cooler on the central heating side of our installed low loss header by approximately 1 to 1.5c. Is it worth considering removing it and running the system as a single pump 3 port setup for DHW and central heating?

Here are two schematics of what we have.

694A4E54 102E 4880 920F F734496D2B12

we were thinking that we could bypass the header somehow. Which may make the second pump redundant. 

617347E1 AE43 47BD B2EA C4CE32D04825
E34D71AA 6026 4716 90D2 3D733B4F69CE

not sure if we need to consider close couples tees etc? 
any considerations or thoughts?

Is it worth saving 1.5c of a 40c flow?

Is your primary pump being speed controlled by the FTC controller? If it has two cables connected that may be the case.

You could try reducing the speed of the secondary pump or increasing the speed of the primary pump. Ideally you require the water flowing into the LLH to be the same flow rate, or slightly higher, than the flow rate coming out, which should reduce or eliminate any mixing taking place.

Your proposal would probably be the best overall solution in the long term, but you would need to ascertain that the primary pump is adequate for the job, and that your overall system has sufficient water volume to meet the minimum requirements.

 


   
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(@sunandair)
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Posted by: @derek-m

You could try reducing the speed of the secondary pump or increasing the speed of the primary pump. Ideally you require the water flowing into the LLH to be the same flow rate, or slightly higher, than the flow rate coming out, which should reduce or eliminate any mixing taking place.

Your proposal would probably be the best overall solution in the long term, but you would need to ascertain that the primary pump is adequate for the job, and that your overall system has sufficient water volume to meet the minimum requirements.

Thanks @derek-m I will check the pump speeds. I recall the primary pump was adjusted to flow at 12 LPM but the secondary pump was left operating slightly faster. As this is the opposite of what you have suggested I will get the speeds and flow rates adjusted and see what that does to the flow temps.

I’ll let you know how it goes...👍


   
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(@batalto)
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@sunandair removed my LLH - only positive benefits from what I can see.

12kW Midea ASHP - 8.4kw solar - 29kWh batteries
262m2 house in Hampshire
Current weather compensation: 47@-2 and 31@17
My current performance can be found - HERE
Heat pump calculator spreadsheet - HERE


   
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(@allyfish)
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@sunandair Derek is bang on. More flow on the secondary side than the primary means cooler return water from your radiators mixes with primary heated flow from your ASHP and recirculates round. OK to have LLH primary flow slightly higher or equal than secondary, but secondary flow must never be more than primary. LLHs can be advantageous, especially where the flow characteristic of the ASHP, (usually high flow, low delta T) does not match that of the secondary heat emitter circuit, or the secondary flow can vary due to heating controls.

Houses with microbore or smaller pipework might not be able to achieve the higher optimal flow rate an ASHP requires, even with a secondary pump. They will have been designed for lower flow & higher delta T fired boilers. That's where a LLH can help, and can make retrofits possible in less than ideal applications. (Installers fit them far too often as a get-out-of-jail-free card however, rather than properly design & size the circuits) LLHs do rob some efficiency as there is always some heat exchange inside the header between flow and return, and conduction through the header cylinder itself, even if the header is hydraulically balanced with equal primary and secondary flows. Ideally for ASHPs; one circuit, one pump and no LLH.

I'm considering ditching my LLH, but it has the advantage of a 3kW immersion heater in very cold weather to aid defrost, supplement output, etc. My 10kW ASHP did struggle a little in the really cold weather weeks due to the number of defrost cycles. I'm going to give it some while longer and see where my SCOP ends up. The LLH is balanced on a 22rad retrofit, and it's all working well now. Wasn't at first. My COP is not really where it could be, but I've found that Grant has exaggerated their COP and SCOP values by about 20% in their literature compared to Chofu's literature. Since Chofu make the ASHPs, I think I know who's telling porkies...


   
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(@sunandair)
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Posted by: @derek-m

Ideally you require the water flowing into the LLH to be the same flow rate, or slightly higher, than the flow rate coming out, which should reduce or eliminate any mixing taking place.

Early test results showing opposite is happening in our system. 

I’ve opened up the primary pump valve until flow rate went up to 15 LPM (was 13) and closed the CH pump then opened it back up to give us a total circulation 13LPM flow. This was a bit tricky since I lost circulation in the CH loop at one point so opened the check valve up a bit more. 
(This must be a fine adjustment since a quarter turn gave over 1LPM change)

Results after half an hour settled operation were not as suggested. In that the Tdrop on the CHFlow thermistor was 4c compared to 1.5c when set with a 13L flow on the primary pump and open valve on CH pump.

I’ve currently reverted to the earlier setting -opened the ch valve half a turn and closed the primary pump valve to give 13 LPM flow. we are now only getting 2c Tdrop. Still not as good as original 1.5c drop. 

(Both pumps are on speed setting one and can only be adjusted manually using the isolation valve otherwise the speed will be 15 LPM)

Posted by: @allyfish

OK to have LLH primary flow slightly higher or equal than secondary, but secondary flow must never be more than primary.

We have been running with a slightly higher flow in the secondary and, as above, it’s been giving us a better result at the moment. Of just a 1.5c drop. It’s a bit puzzling to say the least.

It’s quite hard to finely regulate the flow of the CH pump since if it is over adjusted it simply isolates the flow in the two loops without much of a sound indication. Whereas the primary is directly linked to the flow sensor so is a direct reading.

 

 


   
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(@derek-m)
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Posted by: @allyfish

I'm considering ditching my LLH, but it has the advantage of a 3kW immersion heater in very cold weather to aid defrost, supplement output, etc. My 10kW ASHP did struggle a little in the really cold weather weeks due to the number of defrost cycles. I'm going to give it some while longer and see where my SCOP ends up. The LLH is balanced on a 22rad retrofit, and it's all working well now. Wasn't at first. My COP is not really where it could be, but I've found that Grant has exaggerated their COP and SCOP values by about 20% in their literature compared to Chofu's literature. Since Chofu make the ASHPs, I think I know who's telling porkies...

If you have a LLH with reasonable volume or a buffer tank, it may be preferable, and easier, just to bypass the flow side and in effect create a volume tank in the return side. This would help create a volume of warm water, at return temperature, which can provide heat energy for defrosting, rather than taking energy from the heat emitters.

Any immersion heater that may be installed in the above mentioned vessel, should only be used in extreme circumstances, since the energy supplied would be at a COP of 1, whereas the stored energy will have been provided by the heat pump at a COP of at least 2.

 


   
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(@derek-m)
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Posted by: @sunandair

Posted by: @derek-m

Ideally you require the water flowing into the LLH to be the same flow rate, or slightly higher, than the flow rate coming out, which should reduce or eliminate any mixing taking place.

Early test results showing opposite is happening in our system. 

I’ve opened up the primary pump valve until flow rate went up to 15 LPM (was 13) and closed the CH pump then opened it back up to give us a total circulation 13LPM flow. This was a bit tricky since I lost circulation in the CH loop at one point so opened the check valve up a bit more. 
(This must be a fine adjustment since a quarter turn gave over 1LPM change)

Results after half an hour settled operation were not as suggested. In that the Tdrop on the CHFlow thermistor was 4c compared to 1.5c when set with a 13L flow on the primary pump and open valve on CH pump.

I’ve currently reverted to the earlier setting -opened the ch valve half a turn and closed the primary pump valve to give 13 LPM flow. we are now only getting 2c Tdrop. Still not as good as original 1.5c drop. 

(Both pumps are on speed setting one and can only be adjusted manually using the isolation valve otherwise the speed will be 15 LPM)

Posted by: @allyfish

OK to have LLH primary flow slightly higher or equal than secondary, but secondary flow must never be more than primary.

We have been running with a slightly higher flow in the secondary and, as above, it’s been giving us a better result at the moment. Of just a 1.5c drop. It’s a bit puzzling to say the least.

It’s quite hard to finely regulate the flow of the CH pump since if it is over adjusted it simply isolates the flow in the two loops without much of a sound indication. Whereas the primary is directly linked to the flow sensor so is a direct reading.

 

 

I suspect that adjusting valves to vary the flow rate is far from ideal, since the flow rate should really be adjusted by varying pump speed. I am also not certain what the temperature difference is to which you refer, is this across the LLH between the flow in and flow out temperature, or between the flow and return temperatures at each side of the LLH. As simple sketch would be useful.

 


   
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(@sunandair)
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@derek-m hi sorry I’ve spent one hour writing a reply to you and when I sent it it was lost without a trace. 
I can’t face re writing it again


   
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(@derek-m)
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Posted by: @sunandair

@derek-m hi sorry I’ve spent one hour writing a reply to you and when I sent it it was lost without a trace. 
I can’t face re writing it again

No problem.

That has happened to me on a number of times, and it always seems to occur when you have written a really long post. I think that the system does save copies whilst you are typing, so it may be buried somewhere in the bowels.

 


   
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(@mike-h)
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Posted by: @batalto

@sunandair removed my LLH - only positive benefits from what I can see.

if the difference between the flow temperature from the heat pump and the flow temperature from the LLH to the radiators is only 1.5°, then I would expect this not to make a huge difference to the COP. 

My understanding is that the main reason that you get a lower COP with flow separation is the need for a higher flow temperature leaving the heat pump. Let us suppose that the expected COP of your heat pump at an ambient temperature of 7° is 4.8 with a LWT of 40° and 4.18 with a LWT of 35°. If you require a LWT of 40° to heat your building with flow separation and only 35° without flow separation, I would expect the COP with flow separation to be close to 4.18 and to be close to 4.8 without flow separation. 

in Brendon’s article, he tested these two scenarios and found the COP to be 4.71 without flow separation and 3.4 with flow separation. I really don’t understand why the latter figure wasn’t closer to 4.18, even allowing for the need for a secondary pump. 

Removing my buffer tank would cost several thousand pounds, so it would be nice to know whether I could expect a 30% improvement in efficiency or just 10-12%.

 


   
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(@derek-m)
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Posted by: @mike-h

Posted by: @batalto

@sunandair removed my LLH - only positive benefits from what I can see.

if the difference between the flow temperature from the heat pump and the flow temperature from the LLH to the radiators is only 1.5°, then I would expect this not to make a huge difference to the COP. 

My understanding is that the main reason that you get a lower COP with flow separation is the need for a higher flow temperature leaving the heat pump. Let us suppose that the expected COP of your heat pump at an ambient temperature of 7° is 4.8 with a LWT of 40° and 4.18 with a LWT of 35°. If you require a LWT of 40° to heat your building with flow separation and only 35° without flow separation, I would expect the COP with flow separation to be close to 4.18 and to be close to 4.8 without flow separation. 

in Brendon’s article, he tested these two scenarios and found the COP to be 4.71 without flow separation and 3.4 with flow separation. I really don’t understand why the latter figure wasn’t closer to 4.18, even allowing for the need for a secondary pump. 

Removing my buffer tank would cost several thousand pounds, so it would be nice to know whether I could expect a 30% improvement in efficiency or just 10-12%.

 

You should not actually need to remove your buffer tank, just change it into a volume tank. Disconnect both the flow pipe from the heat pump and the flow pipe to the heat emitters from the buffer tank. Connect these two flow pipes together using suitable copper pipe, bypassing the buffer tank.

Move the return pipe from the heat emitters to one of the upper connections on the buffer tank, then fit plugs to the two now open connections.

The water should then flow from the heat pump directly to the heat emitters, but via the 'volume tank', flowing  from top to bottom, on its way back to the heat pump. Having a volume tank provides a small heat store to assist in defrosting.

Any decent local plumber should be able to carry out the work at reasonable cost.

 


   
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