Posted by: @sunandair

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Posted by: @derek-m

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. I think that you should be multiplying 17kW by 0.46, which would give you an answer of 7.82kW at an IAT of 20C

Thanks Derek. That clarifies a lot. It has been very useful to work out the DT ratings of radiators when operating at the lower flow temperatures of a heat pump.

EG the 17kw rating of @riponowl s installation only actually has a capacity of 7.82kwh when operating at a Flow temperature (LWT) of 50c. 
And at the lower modulation when operating such as at a flow temperature of 35c the output capacity of these radiators drops to 2.8kwh.
To my mind this would almost certainly be below the minimum operating output of a larger heat pump.

Derek, Apologies if this is a nonsense question but-Can you confirm :- does this mean the lower modulating heat range of a heat pump would not be reached due to the radiators not having the heat dumping capacity? Is this what might cause rapid cycling? Or does something else happen to allow a heat pump compressor to operate at a lower flow temperature? 

other output examples 

LWT40 = DT22.5 : 17kw X 0.255 = 4.3KWh

LWT30 = DT7.5 : 17kw X 0.085 = 1.4KWh

Posted by: @sunandair

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LWT40 = DT22.5 : 17kw X 0.255 = 4.3KWh

Correction : LWT40 should read DT17.5

eg: LWT40 = DT17.5 : 17kw X 0.255 = 4.3KWh

Posted by: @riponowl

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@derek-m 

I did adjust the ABV, firstly to 0.5 then 0.6. I noticed the flow rate drop, not sure if this was coincidental but the Samsung controller went as low 11.5 lpm. I don't if the ABV change caused this or whether it was coincidental. I thought it would error below 12 lpm, so this surprised me. Is it possible to bypass the shut down in settings? Maybe my installer did that, I wouldn't put it past them.

My system was supposed to be configured for 45c but again, I have no faith in them at all.

 

the flow rate dropping when you upped the ABV pressure indicates that the ABV was letting flow through, proving an alternate route. I don't know of a way to bypass the "flow rate too low" shutdown , but I'm not a samsung engineer. Its also plausible is that the rate below which it will error, has some error margin itself.

The system needs repiping:

•  1 pump not 2 (whether either of the 2 that you have is adequate to re-use, I don't know - but a competent heating engineer can work that out)
•  no ABV
•  the right flow meter in the right place.
• A higher flow rate mag filter
• possibly higher flow rate zone valves (or a high flow rate 3 way valve)
• possibly upgrading the first leg of the distribution to the rads (as we're not sure if they combine into a 22/28 or come in as 15)

I think you've got a decision to make: whether to put up with the system as-is for the winter, or take the hit for a drain down and repipe now?

given the physical difficulty of access, and the number of things that need changing to "get it properly right", it is not a quick simple job.

I think you said the system as-is does reliably heat your house, although perhaps not particularly efficiently? Given the time of year, I'd be tempted to put up with it for the winter, and use the winter months to find someone to do the work for you in the spring. 

Posted by: @sunandair

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Thanks Derek. That clarifies a lot. It has been very useful to work out the DT ratings of radiators when operating at the lower flow temperatures of a heat pump.

yes you have to do this. there are plenty of online tables to show how the output of the same radiator drops given different  DT.  most mfr's don't quoute really low temps but there are generic conversion tables such as this 

Posted by: @sunandair

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And at the lower modulation when operating such as at a flow temperature of 35c the output capacity of these radiators drops to 2.8kwh.
To my mind this would almost certainly be below the minimum operating output of a larger heat pump.

Can you confirm :- does this mean the lower modulating heat range of a heat pump would not be reached due to the radiators not having the heat dumping capacity? Is this what might cause rapid cycling? Or does something else happen to allow a heat pump compressor to operate at a lower flow temperature? 

there's a lot of discussion about this on other threads - have a good read around. the heat pump will modulate down,  to what level depends (I am deliberately summarising a huge discussion) on a lot of factors - how big it is , the install , the make and model etc... But yes, in mild weather, with a low LWT, the output of the rads can be less than the min output of the HP, so it will cycle.  in your case 3kw for a 12kw is likely to be a feasible low point.

A lot of us monitor the HP's performance in real time . That will tell you whether its cycling or not and over what period. You already have a device that does this, your heat meter, just a question of getting the data out. Where does the data go now?

Posted by: @iancalderbank

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yes you have to do this. there are plenty of online tables to show how the output of the same radiator drops given different  DT.  most mfr's don't quoute really low temps but there are generic conversion tables

Hi Ian,

I think you’re mistaking me for the OP...😉

But you’re right the tables produced seem to suit boiler installations rather than the lower flow temp sensor of Heat Pumps. So I produced my own table which also accounts for the mean temperature of the radiators which operate with a 5degC drop rather than conventional boiler systems. So with a 5deg drop across the radiators the table needs to have 2.5deg scale to account for the mean point temp. of the rad.

The figures Ive worked out go down to 25degC less 2.5C which is off the scale of manufacturers tables. 

👍

Posted by: @sunandair

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Posted by: @derek-m

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. I think that you should be multiplying 17kW by 0.46, which would give you an answer of 7.82kW at an IAT of 20C

Thanks Derek. That clarifies a lot. It has been very useful to work out the DT ratings of radiators when operating at the lower flow temperatures of a heat pump.

EG the 17kw rating of @riponowl s installation only actually has a capacity of 7.82kwh when operating at a Flow temperature (LWT) of 50c. 
And at the lower modulation when operating such as at a flow temperature of 35c the output capacity of these radiators drops to 2.8kwh.
To my mind this would almost certainly be below the minimum operating output of a larger heat pump.

Derek, Apologies if this is a nonsense question but-Can you confirm :- does this mean the lower modulating heat range of a heat pump would not be reached due to the radiators not having the heat dumping capacity? Is this what might cause rapid cycling? Or does something else happen to allow a heat pump compressor to operate at a lower flow temperature? 

other output examples 

LWT40 = DT22.5 : 17kw X 0.255 = 4.3KWh

LWT30 = DT7.5 : 17kw X 0.085 = 1.4KWh

Probably a good analogy for a home heating system is a juggler balancing a number of spinning plates on poles. There are various factors continually being balanced.

If we consider the hypothetical home, with a fixed LWT of 40C, at an OAT of 5C, giving an IAT of 20C, with the system in balance. So the heat loss, and hence heating demand, will be set by the DT between IAT and OAT, and the thermal energy supply will be dictated by the DT between LWT and IAT, along with the DT between LWT and RWT. So many DT's, it is enough to make your head spin. Oh, and don't forget about the flow rate.

If the OAT were now to increase to 10C, the system is no longer in balance. The heating demand has reduced, but the LWT is still at 40C, so supplying approximately the same amount of thermal energy to the heat emitters. This of course would cause the IAT to increase, which in turn increases the heat loss and hence heating demand, until the system may once again balance. The IAT may now be much higher than the desired value and the system is wasting energy.

So how can the IAT be kept at a reasonably constant level? Probably the simplest solution is to use a thermostat to switch the heat pump on and off, but this may not be the most efficient or desirable. The preferred method is to reduce the thermal energy output of the heat pump to match the heating demand, but how can this be achieved?

The primary method is to reduce the LWT setting, to a value where the thermal energy output matches the heating demand. This is the purpose of the WC curve, which tries to balance thermal energy supply to heating demand. An alternative method that could be employed is to vary the water flow rate, since a lower flow rate could reduce the quantity of thermal energy being transported from the heat pump to the heat emitters. Some system have this capability along with the use of WC.

So to keep a fairly constant IAT, the thermal energy output from the heat emitters needs to balance the heat loss from the home, and the thermal energy supply from the heat pump is required to match the heating demand presented by the heat emitters. The problem of course is that heat pumps have a lower operating limit, below which they cannot run continuously.

So to try to answer your very valid question.

The required LWT is dependent upon the heating demand presented by the heat emitters, so the lower the heating demand the lower the required LWT. But the quantity of thermal energy being absorbed and distributed by the heat emitters is also dependent upon the flow rate and the DT between LWT and RWT, which are interrelated. If the flow rate is reduced, then to supply the same quantity of thermal energy the DT will need to increase. Obviously, if the DT remains reasonably constant, but the flow rate is reduced, then the quantity of thermal energy is also reduced.

What happens at the heat emitter end is therefore fairly easy to understand and quantify, what is more difficult to understand and quantify is what happens at the heat pump end. If the heating demand has reduced, then less thermal energy is being absorbed by the heat emitters, so the RWT will start to increase. This has the knock-on effect of causing the LWT to increase above the calculated required LWT, which is sensed by the controller, which in turn lowers the speed of the compressor, to reduce the pressure, temperature and flow rate of the refrigerant gas going into the condenser. The quantity of thermal energy being produced by the heat pump is therefore reduced to match the heating demand. This arrangement works fine until the compressor reaches its lower operating limit, but this lower limit itself is not fixed, but varies with both the required LWT and the OAT and the capacity of the heat pump. Looking at the data tables for a 14kW Ecodan and a 14kW Midea gives the following minimum output values.

OAT      LWT       Ecodan       Midea

  7         30         4350W       6034W                   L

  7         40         3700W       6644W

 15        30         6650W       5966W

 15        40         5900W       7331W

As can be seen from the above, even the point at which cycling is likely to occur probably cannot be predicted with any certainty.

Heat pump cycling will occur during milder weather conditions, and it may be necessary to try different ways to try to keep the frequency of cycling to an acceptable minimum.

One way could be to set a higher required LWT, so that the heat pump warms up the heat emitters and then stops running whilst the heat emitters cool down again.

Another method is to use a room thermostat to stop and start the heat pump and again use the variation in IAT to limit how often the heat pump needs to run.

It will be down to the individual to find the method that best suits their particular system.

Thanks Derek. Lots of variables to consider. And another reason to try to get the right size heat pump and right size emitter set. Is it also time to banish TRVs and zone thermostats? These all seem to introduce avoidable variable conditions. 🤓🤓😜

Posted by: @sunandair

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Thanks Derek. Lots of variables to consider. And another reason to try to get the right size heat pump and right size emitter set. Is it also time to banish TRVs and zone thermostats? These all seem to introduce avoidable variable conditions. 🤓🤓😜

You could probably keep TRV's in bedrooms where a lower temperature may be preferred.

A thermostat may be useful to help reduce cycling frequency during milder weather conditions, but should not be used for normal room temperature control during the heating season.

Posted by: @iancalderbank

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the flow rate dropping when you upped the ABV pressure indicates that the ABV was letting flow through, proving an alternate route. I don't know of a way to bypass the "flow rate too low" shutdown , but I'm not a samsung engineer. Its also plausible is that the rate below which it will error, has some error margin itself.

The system needs repiping:

•  1 pump not 2 (whether either of the 2 that you have is adequate to re-use, I don't know - but a competent heating engineer can work that out)
•  no ABV
•  the right flow meter in the right place.
• A higher flow rate mag filter
• possibly higher flow rate zone valves (or a high flow rate 3 way valve)
• possibly upgrading the first leg of the distribution to the rads (as we're not sure if they combine into a 22/28 or come in as 15)

I think you've got a decision to make: whether to put up with the system as-is for the winter, or take the hit for a drain down and repipe now?

given the physical difficulty of access, and the number of things that need changing to "get it properly right", it is not a quick simple job.

I think you said the system as-is does reliably heat your house, although perhaps not particularly efficiently? Given the time of year, I'd be tempted to put up with it for the winter, and use the winter months to find someone to do the work for you in the spring. 

The system is heating my house satisfactorily with efficiency of around 3.5 (if I can believe Passivliving). Of course I am always looking for ways to improve that. Cycling has been a concern though. It is a little noisy (inside), I'm not sure if that is normal. Are two pumps noisier than one? The only reason I can think of for the two pumps is because there is an 8m run to heat pump which is 28mm pipe (not 35mm as I first suggested).

I've looked up the details of the Fernox Omega TF1 and according to this website the max flow rate is 50 lpm so I assume that is ok.

https://www.cityplumbing.co.uk/p/fernox-tf1-omega-22mm-filter-with-slip-socket-connections-62248/p/125256  

I have failed to find any max flow rates for the Honeywell motorized valves.

The heating pipes that go up from the heating system into the ceiling are 22mm, directly above is the WC which is a tiled floor. Just over 1m to the left is the first radiator which is the bathroom, also a tiled floor. I don't see an easy way to figure out what the hidden pipes are. That might have to remain a mystery.

Having had the cost of the system being drained down recently I can't currently stomach that cost again for a while. I've been contemplating UFH in the conservatory so there might be an opportunity to have work done then. The most important thing is that I now know that the flow rate isn't as bad as I first thought. Also if I ever do get low flow rate error I will know exactly what it is, rather than spending weeks with my system down trying to figure it out. 

I have certainly learned a lot. Creating the flow chart really helped me to understand what is going on inside that cupboard so I'm really glad people requested that.

Thank you for all your help.

@riponowl COP of 3.5 is ok as a long term, worth keeping an eye on what it does day to day if your system has that capability (Sorry  I don't know what Passivliving is). The flow chart will be your reference now everytime!

Flow rates etc: Its not about the "maximum flow rate" quoted for a component. What matters is the amount of pressure  (a.k.a head) that is needed to push the water through the component at that flow rate. 

the data sheets for the TF1 are here . and the graph on page 3 

shows that to achieve 50lpm, 20kpa of pressure drop will occur through the TF1, which equates to 2.04 metres of pump head. (there are umpteen online conversion tables for this). A typical CH pump for a house will have somewhere between 4metres and 10metres (if a really big one ) of head capacity. So you'd burn a large %  of the head capacity of the pump, just to push through the filter, leaving not a lot to push through the rest of the circuit. What happens in practice is that the flow rate has to drop because there just isn't enough pump capacity. It is literally a case of pushing water fast through a narrow hole - you have to push harder the narrower the hole, the tireder you get, the less power you have left for everything else.

also (in case you thought it did) this head figure has nothing whatsoever to do with how high your house is, in a circulating design , because its a circle , every up has the same amount of down. 

there are similar pressure loss tables for honeywell zone valves, which I checked into before when building my system, I can't lay my hands on them right now, but trust me, they also have narrow apertures and the same issue. There are some ASHP installers on here who will make simple blanket "dont use honeywell zone valves for an ASHP" statements.

8m of 28mm pipe to your heat pump is not a big deal at all. at your flow rate of 24lpm , assuming its copper 28mm, it consumes approx 4.8kpa of head loss = 0.48m  head (maths here) .  Going back to the TF1 graph, the TF1 at 24lpm also consumes about 4kpa. So to think of it another way - pushing 24lpm through the 20cm or so of the TF1, takes the same effort as pushing through 16m of 28mm pipe (there and back from airing cupboard to heat pump). Quite a comparison isn't it!

It is quite likely that the reason you have two pumps is because your system needs an awful lot of pump head (i.e. "push") because of the aperture size constraints in the filter and zone valve components. Thus your pumps are running flat out, and there are two of them, so they are noisy. If you make the changes that I suggest, you will be able to run with 1 pump at lower power. Also with one pump (because you have a heat pump with this capability) you will be able to connect the circulating pump (providing it is the right type) to the PWM controlled outputs of the heat pump, so the heat pump will dynamically control the flow rate in response to how much heating power its trying to deliver. At lower power levels you won't hear it.

depending on where your isolators are you may not need a full drain down, a partial is often possible. If you don't have isolators that allow this, get them fitted the next time you have work done.

@iancalderbank Thank you so much for the detailed response. Do you have a suggestion for what to use instead of the Honeywell zone valves?

@riponowl heacol (an installer on this board) used to recommend Bellimo or ESBE when I asked him exactly this question. I went with ESBE. I think his company is now ultimaterenewables.com they may be able to help with a specific product. I managed find a honeywell data sheet here . kv value will be 6.9 if 22mm. kv values explanation. higher is better. esbe kv values, I went for 32mm (1 1/4") which has a kv of 16. you need to get the hang of the conversion tables as they all use possible units (kpa, mwc, bar) for pressure and (l/s, l/m, m3/h) for flow interchangeably. there are conversion tables online.

a thing to look for to visualise whats happening with the opening size is to find on youtube an internal shot of a honeywell zone valve . Versus a "full bore isolation ball valve" - the esbe works exactly the same as one of those.

same goes for the mag filter, you want something with a much lower loss for this flow rate. spirotrap AE125 is a good place to start - is what I used. This has a drop of 0.3 kpa at 24 l/min i.e. 0.03 metres of pump head which is almost negligible, vs 0.4m pump head taken by your TF1.

Posted by: @riponowl

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The system is heating my house satisfactorily with efficiency of around 3.5 (if I can believe Passivliving).

just did a quick google: is passiving something that does online monitoring of your system via data from the heat meter? I'd be interested to see any output from that system. Not just because of your system, but more because I am interested in heat pump monitoring systems.

Posted by: @riponowl

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The heating pipes that go up from the heating system into the ceiling are 22mm, directly above is the WC which is a tiled floor. Just over 1m to the left is the first radiator which is the bathroom, also a tiled floor. I don't see an easy way to figure out what the hidden pipes are. That might have to remain a mystery.

22mm at that point would seem reasonable for your number of radiators. does this go to all the radiators, or is there another 22mm branch going to another floor?