Posted by: @sunandair

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I guess one advantage of a PHE over a LLH is that there won’t be any mixing between Flow and Return. However is there a quantifiable energy loss through the plates of the PHE? 

Even a lossless PHE will reduce the system efficiency because the flow on the CH side is necessarily colder than the flow on the CH side and thus the HP has to increase its flow temp to achieve the same flow temp at the radiators.  In the @cathoderay case this difference was measured at 5C, corresponding (crudely) to a 10% reduction in HP efficiency.

Posted by: @sunandair

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is it fair to say that the industry stated these were required particularly for the retrofit business as some sort of guarantee that the system would work EVEN WITH bad or unknown existing pipework? 

Not really sure how it guarantees that.  @cathoderay reports that he does not know how the pump speed (or anything else) on the CH side is controlled.  This being the case, what guarantees that the two sides are properly matched to avoid unnecessary cycling on the HP side or other potential issues.  If the existing pipework is not sufficiently large etc for the flow required to get sufficient heat transfer at the chosen deltaT, adding in a PHE doesn't fix this (it might of course mask it - but that's a bad thing).   

What a PHE does do of course is reduce the amount of glycol needed in the system (if you are using glycol), which seems to be a significant reason for fitting them.  I guess its open to debate whether the use of glycol is to be preferred over antifreeze valves and whether the saving in glycol if you fit a PHE is worthwhile.  

@cathoderay You stated in an earlier post on your other thread, quote "I think the heat loss as the house currently is (I am slowly improving insulation/draft-proofing where I can) is something just over 12kW. The rads are matched plus ~10% to the room heat losses at a flow of 55 degrees. "

So just like your radiators need to be hotter than the room you are heating ( to create a heat differential), when incorporating a heat exchanger the primary primary circuit needs to be at least 5 deg C hotter than the secondary circuit (radiator circuit) in order to transfer the desired heat quantity across (on a cold night you need as much of that 11 to 12 kW as possible). So when you need 55degC supplying the radiators you are going to need 60degC from the heat pump primary circuit.
Your weather compensation curve has to take this into account, which it doesn't at the moment.
Remember that most who are commentating on this forum about their weather compensation implementations don't have a PHE and therefore their selection criteria will be always be slightly different to yours.
A better matched weather compensation curve will improve you experience during cold spells as well as reducing cycling, all be it at a higher running cost. The choice is yours of course.

Posted by: @derek-m

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There is no bog standard control philosophy that meets all requirements throughout the whole year.

I can certainly agree with that.  I reckon that there are a fair few PhDs in 'optimised' control of heat pump central heating systems under various sets of conditions.  Cross disciplinary ones at that, as it is both engineering and human psychology!

Perhaps central heating control systems should have a dial, in addition to 'ideal temperature' allowing you to choose the relationship between tolerance to deviation (from temperature) and efficiency, much like investments ask about your tolerance to risk vs potential reward.  In my own case Im also convinced that humidity is another factor which affects the 'perceived' temperature.

Posted by: @iaack

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So when you need 55degC supplying the radiators you are going to need 60degC from the heat pump primary circuit.
Your weather compensation curve has to take this into account, which it doesn't at the moment.
Remember that most who are commentating on this forum about their weather compensation implementations don't have a PHE and therefore their selection criteria will be always be slightly different to yours.
A better matched weather compensation curve will improve you experience during cold spells as well as reducing cycling, all be it at a higher running cost. The choice is yours of course.

Surely its more likely that @cathoderay adjusted his wc curve with the PHE in place, so the settings already take account of its effect.  Of course, assuming that this is the case, he will be able to rack it down if he removes the PHE, improving system efficiency.

Posted by: @jamespa

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You are mixing up cause and effect.  The heat delivered by the heat pump to the system absolutely is equal to (mass) flow rate * specific heat capacity of the medium x deltaT.  Thats just basic physics and its not dependent on steady state. 

However this is a result of heat lost at the emitters which is dependent on the temperature difference between the emitter and the room (in the case of the radiator, raised to the power 1.3).  The HP adjusts the amount of heat it delivers to the water to match the heat lost by the radiators and maintain the delta T it targets.

I don't doubt the physics, I've even applied the equation to two conditions, but am still left confused, and I'd bet the house on the fact that I am not the only one confused! 

Sometimes analogies work for me. Maybe we can see the transfer/delivery of energy as happening like a goods train. The amount of goods - lets say coal - a train can deliver is determined by its speed (which is the flow rate), the capacity of the wagons (the specific heat) and how much coal actually gets off loaded at the destination (the delta t). Increase any one of those parameters, and more coal gets delivered.

Now comes the bit where I get stuck: how can we extend this analogy (assuming it can be done) to cover the same delta t (amount of coal off loaded) at different flow temps. Logically the analogous situation to a higher LWT is a train carrying more coal (say 100 tons instead of 50) but the delta t (coal off loaded remains the same (say 5 tons). But it is still delivering the same amount of coal (energy).      

For a moment, I thought your second paragraph above made everything clear: the heat pump has to put more heat in when the rad/room delta t is larger (the yard has to load more coal on the train) - but then the train hits the buffers again, whatever the LWT/preload of coal, the basic physics means that at a constant LWT/RWT delta t, the heat pump always delivers the same amount of energy (because, at least in a Midea heat pump, the flow rate is constant, and the specific heat is also constant).

To further complicate things (for me), the Midea data charts I have posted (eg 3rd April 2023) appear to show the heat pump adjust both the LWT and the delta t (indeed, delta t is the only thing in the equation that it varies): on the left hand side, with ambients just above zero, the LWT is mostly around 49 degrees (when not fluctuating), and the delta t is around 5, while on the right hand side, with ambients over ten, the LWT averages out at about 38 degrees, and the delta t is around 3 to 4. because the delta t is just a simple multiplier in the equation, a 1-2 degree drop from 5 degrees reduces the energy transferred by 20 and 40% respectively.

Maybe this is what happens: the ambient goes up, and the LWT goes down, and this is what lowers the LWT/RWT delta t: because the room temp remains fairly constant, the lower LWT means less heat/energy is transferred to the room, 20 to 40% less in the 3rd of April left hand/right hand side comparison, and as a consequence the LWT/RWT delta t drops. I am mindful of the fact that the only variable in the equation a Midea unit changes is the LWT/RWT delta t, but it seems to me it doesn't do that directly, instead, it alters the LWT, and as a consequence the LWT/RWT delta T changes.

Sorry to be so thick about this, and persistent in trying to be less thick, but I have always been one who has to understand the basics. It used to drive some (but thankfully not all) of my teachers mad, just as I fear it will be driving some of you mad.  

Posted by: @iaack

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So just like your radiators need to be hotter than the room you are heating ( to create a heat differential), when incorporating a heat exchanger the primary primary circuit needs to be at least 5 deg C hotter than the secondary circuit (radiator circuit) in order to transfer the desired heat quantity across (on a cold night you need as much of that 11 to 12 kW as possible). So when you need 55degC supplying the radiators you are going to need 60degC from the heat pump primary circuit. Your weather compensation curve has to take this into account, which it doesn't at the moment.

At the design stage, when my installer completed the Freedom Heat Pump calculator, there was no mention of the PHE, it appeared at installation time. Prior to the installation, I understood the system would need a LWT of 55 degrees at -2 ambient, but even that is an oversimplification, because a LWT of 55 at the heat pump does not mean 55 degrees arriving at the rads, which is what on paper they needed to heat the rooms to design temp at - 2 outside. I suspect this is 'accommodated' (on paper) by the fact the rads are ~10% over-size.

I absolutely get that if there is a 5 degree drop across the PHE, then to get the necessary temp at the rads, then the LWT at the heat pump needs to be increased by whatever the drop is, in this case 5 degrees, giving a LWT (at the heat pump) of 60 degrees. Had I known this at the time, I might have asked whether this is a good idea, given heat pump efficiency drops at high LWTs.    

My weather curve does in fact partly take this into account, I changed the low ambient end from 55 @ -2 to 58 @ -4. At the time, I did it mainly to extend the range over which there was a gradient, because the Midea curve flat lines either side of the two set points, but it also took the LWT closer to 60 degrees at the low ambient end. I was loathe to take it any higher for fear of what it might do to running costs.

Posted by: @jamespa

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Surely its more likely that @cathoderay adjusted his wc curve with the PHE in place, so the settings already take account of its effect.  Of course, assuming that this is the case, he will be able to rack it down if he removes the PHE, improving system efficiency.

See the above, that was me adjusting the low ambient end of the curve with the PHE in place, so it did take the PHE into account, insofar as it was in place. I have to say after the adjustment, done on 24 Jan 2023, the curve worked pretty well, most of the time the house (or rather kitchen, which is what I monitored) was within a degree or so of design temp, and perhaps a bit over as it got milder:

I do agree all this amounts to a compelling reason to remove the PHE. As I said earlier, I am going to talk to my installer about this. Once I have been persuaded AF valves are fail-safe in practice (or if they are not, how you manage that), then I will have the glycol removed, which will both make the system easier to manage (and cheaper to fill), but will also of course increase the specific heat of the circulating fluid, with consequent benefits.    

Posted by: @cathoderay

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Maybe we can see the transfer/delivery of energy as happening like a goods train. The amount of goods - lets say coal - a train can deliver is determined by its speed (which is the flow rate), the capacity of the wagons (the specific heat) and how much coal actually gets off loaded at the destination (the delta t). Increase any one of those parameters, and more coal gets delivered.

Its not a bad analogy

Posted by: @cathoderay

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Now comes the bit where I get stuck: how can we extend this analogy (assuming it can be done) to cover the same delta t (amount of coal off loaded) at different flow temps. Logically the analogous situation to a higher LWT is a train carrying more coal (say 100 tons instead of 50) but the delta t (coal off loaded remains the same (say 5 tons). But it is still delivering the same amount of coal (energy).     

However the coal is offloaded at the radiators through the bottom of the truck, so the amount offloaded (deltaT) depends on  how full the truck is in the first place (temperature difference between the radiator and the room).

The driving force for the delta T is the radiators not the heat pump.  The heat pump can maintain a given delta T by varying either the flow rate or the flow temp, but all its doing is 'keeping up' with the heat lost from the radiators.

To extend the analogy further, as well as the hole in the bottom of the waggon, each radiator has an aperture at the top which fits snugly against the hole.  You can adjust the size of the aperture thus adjusting the rate at which coal is emptied for any given level of fill.  That's balancing the radiators.

Posted by: @jamespa

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However the coal is offloaded at the radiators through the bottom of the truck, so the amount offloaded (deltaT) depends on  how full the truck is in the first place (temperature difference between the radiator and the room).

Brilliant extension of the analogy, thank you very much.

Picture of a bottom discharge hopper wagon courtesy of Peco

Posted by: @cathoderay

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See earlier reply (secondary pump is very basic, internally controlled). So far as I can see, the primary and secondary flow rates are totally independent of each other, neither has the slightest clue what the other is doing. Furthermore, it looks as though the primary pump, although it can be externally controlled isn't, or if it is, it is controlled to have a constant flow rate (as seen on the charts).

 

IF the above statement still holds currency then this is more and more pointing to your  PHE as the cause of both Low cop and increased cycling. One event may be compounding another.

Going back to basics the fundamental efficiency of a PHE is when the flow rates on either side of the plates are equalised. If your secondary pump even slightly slows it will immediately narrow the DT And so lowering your cop. If it slows further then it will be closely followed by stopping the HP.

so from what theory I have learned, if your secondary pump does operate independently of flow rate of your HP pump speed as you say, then this wiil be mounting up several stages of energy losses.

@cathoderay

Using your analogy.

The train has 10 coal trucks, each carrying 10 tonnes, so 100 tonnes total. At the loading end (heat pump) the trucks can be loaded at a variable rate up to a maximum of 150 tonnes per hour.

At the unloading end the trucks are emptied into a 20 tonne hopper, but the coal can be removed from the bottom of the hopper at a variable rate between 0 and 100 tonnes per hour.

The train is initially loaded with 100 tonnes of coal which it takes to the unloading hopper, which as the train arrives holds 10 tonnes of coal. Coal is being removed from the bottom of the unloading hopper at a rate of 50 tonnes per hour. The first truck is unloaded into the hopper, which brings the quantity up to the maximum 20 tonnes, so no further trucks can be unloaded for the time being. After 12 minutes, 10 tonnes have been removed from the hopper so a further truck can be emptied.

This continues for 1 hour, so 5 trucks have been emptied, but now the train driver is told to return to the loading hopper. So the train goes back with 5 empty trucks and 5 that are still full. At the loading end it is therefore only necessary to fill the 5 empty trucks before the train goes back to the unloading hopper.

The rate at which the coal is supplied is therefore dependent upon the rate at which it can be unloaded from the train, not the rate at which the train can be loaded.

Posted by: @derek-m

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This continues for 1 hour, so 5 trucks have been emptied, but now the train driver is told to return to the loading hopper. So the train goes back with 5 empty trucks and 5 that are still full. At the loading end it is therefore only necessary to fill the 5 empty trucks before the train goes back to the unloading hopper.

 

....But because you have a plate heat exchanger the coal on the train from MeashamLWTCB has to change trains mid journey to a different train running on a different track in order to get the coal to its final Destiny - GWR Emitterville. 

not all the coal can be offloaded at Station PHE being controlled by a notoriously intransigent workforce who will only come in to offload the train when they know there is a partially empty train returning from GWR Emitterville. And even then they will only offload enough coal to fill the empty wagons on the train.  If there are no empty wagons the workforce will have no option but to park the full train from MeashamLWTCB. Until the Emitterville train returns with some empty wagons.

Now, is this the right point to start discussing the calorific value of the coal? 😉 Regards, Toodles.