Hi Everyone - Newbie with ASHP Questions

[Madam1980]

Hi community, 

 

Over the last 2 weeks I've been trying to take in lots of info regarding ashp since having one installed in a new build, thanks so much to all the people that contribute with their esteemed knowledge, much of it on here is well above my basic understanding and it seems there are a myriad of things to consider when trying to optimise the systems. 

 

Bit of background, 186m2 timber frame kit from a company called scotframe, as a non builder this seemed like a means of eradicating some of the problems with builders as it progressed.  Product has been good aside the nordan windows and doors imo, the erectors not so good, wish I'd hired a chippy with lots of new build experience to keep and eye on them but I am where I am now.. if anyone has questions about scotframe always happy to share my thoughts and experiences. 

 

Installed I have an ecodan 8.5kw system with ufh downstairs dfor heating and DHW. no buffer tank just the ecodan cylinder with 6 room stats downstairs. Vent axia sentinel mvhr yet to be commissioned / balanced. 75mm screed on top of 140mm celotex. pipe spacings at 125mm (mostly anyway). Been running now for 2 weeks, the house is at a comfortable temp downstairs, 17 deg, and that what's the stats are set at, not lived in yet but in this cold weather seems heating will be needed upstairs with points for electric heaters of some sort ready for the future. 

 

After lots of reading on here I've decide to run the ufh at a low flow temp, presently 29 degrees and run it continuously aside from peak pricing times with octopus agile.. . The house seems to hold heat reasonably well but until I move in to a semi building site in around 6 weeks I wont know exactly how it performs. 

 

Whilst the heat pump installer / plumber was a lovely chap and had a  love for making perfect copper piping I'm not entirely sure how technical he is when it comes to these systems. 

 

I have lots of questions but the thing that's initially baffling me is on the control panel for the heat pump when the ufh is running its shows the flow and return temps are very similar circa only 1 or 2 degree loss between the two. From reading on here that seems far too narrow so the lovely heat the pump is generating heat but returning the majority of it . If someone could help me understand the impact this will be having on the heat pump / efficiency and then how I would go about increasing this difference that would be appreciated. The installer left all flow rates at 2.5 which was nothing like the ufh design I've referred back to, so a few days back I reduced these to what the design recommended, but this hasnt affected these flow and return temps which I assumed it would have. 

 

As said for many on here with knowledge in this area it may well be like pulling teeth trying to get me to understand what's going on but I'm game for the challenge

 

Thanks. 

 

 

[joe90]

I don’t know what my return temps are but logically if the return temp is close to the flow temp then the slab is up to temp!!! I remember many years ago when I started reading about ASHP and UFH that someone cited if you set the flow temp 1 or 2 degrees above required temp it would be self controlling!.

[Madam1980]

Thanks for the reply. I probably should have added more info here to help. Yesterday I used my little infrared heat sensor on the flow and return pipe work from each ufh circuit. unsure how accurate it is but was showing roughly 29 deg on the flow and about 20 deg return for most loops. Assume this indicates the slab isnt quite yet up to temp but getting close? also to note when the installer was commissioning the system he said the variance between the flo and return was low but didn't add anything further or do anything, I didnt think much more about it at the time as I was like a rabbit in the headlights, excitement of finally having heat and hot water after a few months of my fingers wanting to fall off whilst working on the build etc. at that stage the slab would have been cold of course, tbh though I cant recall whether he was looking at the hot water production or ufh then but either way the tank would have been chilly as would have the slab etc. Confused  thanks again,

[ProDave]

I would not get hung up on flow and return temperatures.  The difference will depend on how many floor zones are on and how long it has been on.  Turning on from cold in a morning when all floor loops are likely to be calling for heat you will get a big difference between flow and return, and then later in the day when it is all warm and perhaps just 1 or 2 rooms are calling for heat, the difference between flow and return will be small.

 

The other thing often not explained about heat pumps is usually they heat the space heating OR the DHW, never both at the same time.  So when the ASHP is heating the DHW it stops heating the floor for that time.  In most cases that will allow the ASHP to run at a high temperature to heat the DHW and then run at a lower temperature when it is only heating the floor.

 

Make sure the installer has shown you how to adjust the floor heating temperature.  You might need to adjust that to get it to meet your heating needs. Too cold and the house won't get warm in a very cold spell, too hot and it is wasteful and could damage some floor materials.  29 degrees is probably a good starting point but you really need to know how to adjust that should you need to.

 

 

[Madam1980]

Hi Dave, thanks for the info and that I shouldn't be so fixated on the flow and return temps that's helpful advice as thats where my little mind has been stuck for the past few days wondering if the heat pump has been running for little purpose if its working but only delivered 1 deg of heat. I've worked out how to adjust the the flow temp I believe on the ecodan control panel hence dropped it from 40 to 29 to see what happens as from reading on here the lower the flow temp the better the cop. Again starting from little knowledge on all this but consider myself frugal I suppose so working on the basis I start cold and adjust upwards rather than other way around. the control panel is telling me I've consumed 166kwh thus far and delivered kwh is 268kwh in the time its been running which doesnt seem great so felt dropping flow tempo for ufh could help. Down in west sussex, obviously been extra cold since it was commisioned. I understand its either one or the other re hot water or space heating as there is a diverter valve depending on whats calling for heat. Not sure which takes preference but trial and error of running both at same time should answer that question. cheers 

[ProDave]

Chances are it is just circulating the water and the compressor is not actually running.  When you delve into the settings there are a lot of things you can alter.  For instance as well as setting the set temperature on mine you can set the hysteresis so it won't start the compressor until (in my case) the flow temperature has dropped 3 degrees below the set point.  This reduces short cycling (starting the compressor for a very short period)

 

Over what time period is your measured consumption above and is that all consumption, heating, hot water or both?

 

It has been particularly cold in the last week.  My heating consumption went up by half in the last week, that is understandable as it was extremely cold here, but my hot water consumption also went up, not because we were using more hot water but because it will have been working leff eficciently and defrosting more in the extreme cold.  P.S I am talking of night temperature regularly -10, coldest -16 one night, and daytime high -2.  It's positively tropical here now at +6

[Madam1980]

Thank you again for more helpful advice, I shall attempt delving into the settings further and try to unpick the ecodan control manual, its not the most intuitive bit of technology I've ever come across. Controlling the set point for the compressor to run makes lots of sense thank you.  

 

The pump was fired up for the first time 25th of jan, running since but i've been fiddling with things, the stats, one of which seems to trigger more than the others which I find odd as its mostly open plan downstairs, ive also reduced the flow temp to ufh as mentioned about a week ago so hopefully that could help. I think I may try and remove this questionable stat then wire the actuator to another one in the same space that is more stable. The reading i gave cover both dhw and heating, the system doesnt seem to break down the split between consumption for dhw and heating, which would be helpful for analysis. The DHW has barely been used as at present there is one tap in the house and its unoccupied, the dhw is its set to come on 1pm each day to take temp up to 55 with a one week legionella cycle to 65 via the immersion. Appreciate the dhw will not be friendly on the heat pump especially when it's so chilly outside but have no benchmark to compare against so its hard to know whether its good, bad or just plain ugly on the usage front. I've noticed that the ecodan cylinder doesn't seem to do a very good job of keeping the heat in the tank even when not being drained, assumed these fancy new things would leak very little, again I will find out in the months to come how this performs. Roll on the sunshine, shorts and lower elec consumption  

[SteamyTea]

Welcome.

Not sure where to start with explaining this.

Heat is not temperature. Heat is the old word for energy.

So don't worry, at this point about temperature differences too much.

As your whole house is new, there will be a lot of mass that will absorb energy, without changing temperature significantly. There will also be a lot of moisture being evaporated, and this, by the very nature of physics, takes more energy than just heating up liquid water (air pressure and humidity levels make a difference as well).

If you can, check that the ASHP is not short cycling. This may be a bit hard to establish from  indoors if the circulation pumps are constantly running.

 

As a general rule, energy transmission is most efficient at the median temperature.

So if the flow is at 29⁰C, the return at 20⁰C, the slab should be, on average, at 24.5⁰C. This does not translate into the room air being at the same temperature as there are losses through the building fabric.

Inside the heat pump there is a heat exchanger that takes the hot side of the heat pump and 'passes' the energy to the colder side, the flow to the slab.

Generally, you do not have much control over how hot, or cold, the hot side of the heat pump is, that is a function of the refrigerant gas and the fraction of the Carnot cycle used.

But as a general rule, you want the flow and return median temperature to be significantly higher than the highest flow temperature going to your space heating and hot water.

 

Edited February 15, 2021 by SteamyTea

[ProDave]

Reduce the DHW temperature to 48 degrees. Most of us find that plenty hot enough.

[Madam1980]

Thank you Dave, shall duly reduce the target temp for the hot water and see what transpires when I eventually get to enjoy the first soak. Coming from a house with a combi boiler that worked without thought or well I didn't give it much as you can tell. 

 

Steamy, thanks for the physics 101. I like the general rules especially as my understanding is very basic / non existence right now ? At times like this I wish I'd been more interested in education rather than the girl sat next to me or a football. Once the weather warms seems like I'll be camping next to the pump to understand exactly what's going on. As you say from inside it's hard to tell what's really happening with the circulation pumps whurring away. I'll also take note to remove random buckets of water dotted around the place or building site carnage so to not waste my precious heat on them before they make their way to the tip. 

 

You last point baffles me (suprise suprise), 'But as a general rule, you want the flow and return median temperature to be significantly higher than the highest flow temperature going to your space heating and hot water' how can I make the flow and return median significantly higher than the highest flow temp going to space heating or hot water? This probably isn't relevant to your statement but the system I have the ecodan has what I've learnt has a dumb manifold for the space heating, i assumed that the heat pump is delivering the coolant at a set temp (29 degrees currently as before) and the return temp is whats left after the energy has dissipated after running through the ufh pipes and ending up back at the heat pump. Feel free to rate my statements on a stupidity level each time for others amusement ? 

 

 

[Madam1980]

Sorry steamy, when I said heat I meant energy! 

 

As another point of no doubt wrong assumption, i figured if the heat pump produced low temperatures for ufh it would not only run more efficiently but be less likely to short cycle as the warm up time in rooms would be slower thus the heat pump running for longer rather than stop start with a higher flow temp that could cause it to kick more often from the stats. Oh now saying that I suppose this is down to the heat loss rates  of the house and the stats calling for heat  rather than flow temp.  Or maybe not ??

[SteamyTea]

1 hour ago, Madam1980 said:

'But as a general rule, you want the flow and return median temperature to be significantly higher than the highest flow temperature going to your space heating and hot water

Yes, I can see it is a bit confusing.

The external air temperature is constantly varying, what we know as weather.

But inside the heat pump is a controlled environment, known gas volume and pressures, this leads to a constant temperature being produced (based on the ideal gas laws PV/T).

So almost regardless of what the external air temperature is doing, once the cold liquid refrigerant that is pumped into the outside heat exchanger, the bit with the air fan, warmed up a bit and changed into a gas, recompressing it back to a liquid will produce the same set temperature, time and time again.  It is this temperature that has to be significantly higher, or it will take a very long time to heat the water that is used for space heating or DHW.

Heating is not a linear process, as temperatures get closer to each other, the time taken to transfer energy increases.  This is why, if you leave the lid off an electric kettle, it will carry on boiling till it is dry.  It is trying to heat the whole kitchen to 100°C.

So imagine that, on the cold side of the heat pump, the gas is initially expanded, cooling as it does so, to say -30°C.  As it passes though the external heat exchanger, it warms up to say 10°C on a hot day, or only to -5°C on a cold day.  This 15° difference makes only the slightest temperature difference to the recompressed gas which may have a temperature of say around 70°C.  But it does vary the efficiency.

It is never possible to get your DHW (realistically the highest temperature you need) higher than that 70°C, and the last 10°, from 60°C upwards could take hours, so this is why heating it to a lower temperature, say 50°C is preferable.

There will be a sweet spot, for any given external temperature, where you can heat the water to.  This is a bit more complicated than purely a ratio between temperatures.  This is because (with modern 'inverter' ASHPs) the mass of air that is allowed to flow though the external heat exchanger is constantly varied.  Slow flow and more energy is extracted from the air, high flow, less energy is extracted, this is exactly the same as what happens with the internal flow rates around the plumbing system.

There are two reasons to vary the air flow rate, one is keep the temperature difference between the cold and hot sides of the refrigerant gas at a maximum, and the other is to stop icing up.  This is a bit of a balancing act, but usually works fine, though in the recent cold snap, some ASHPs have totally cut out, and other have frozen up, but it was extreme weather for the UK and we probably don't have the same set ups as they do in Canada, Sweden, or Japan.

Possibly the second image may explain it a bit better.

Worth noting that, as I said earlier, temperature and energy are not the same thing, though they are related.

 

 

 

 

[Madam1980]

brilliant ,a perfectly written statement thank you that you make sound simple but at the same time complicated enough to make my little mind shudder last night, thought I'd sleep on it. I hope you copy and pasted that as must be sick of saying the same thing again again to the uninitiated average person that ends up with a heat pump as they live in a place without gas and don't wish to burn oil! 

 

So stupid questions on the back of it to help demonstrate I still don't get it..

 

If the output variance between the gas temps and the external air temp are minimal in the grand scheme of things, what drives the efficiency, ie what part of the system is having to consume more electricity to achieve the same set return from the controlled environment or is it that it consumes the same amount of elec just the output is less when the air temp is lower as less energy is forced across the heat exchanger by the fan?

 

Also this seemed to dictate to my small man mind that the flow rate and the temp of the flow to the unit is more important than the outside air temp. How do I know what the correct flow rate to it should be or what the best temp to send to it is in any particular condition, or do I not get to decide the flow rate and the heatpump is taking care of controlling its the capacity to heat said coolant / water at whatever rate it decides based on the temp that is being pumped in to it or is it controlling all of this by itself.

 

More stupid questions then.. is short cycling when there is demand for heat but the unit cuts in and out because it hasn't found the sweet spot of balance between gas temps, air temps and flow rates to run efficiently so things starts freezing up and has to stop to thaw out before restarting again.

 

Expect I'll get my head around it all by 2022!  

 

 

 

 

[SteamyTea]

27 minutes ago, Madam1980 said:

what drives the efficiency

It is the Carnot Cycle Heat Engine principles.

The electrical energy that you put into a heat pump just drives a compressor, fan and a water pump, the rest of the energy comes from cooling the air though the external heat exchanger.

If you cool a mass of anything, it releases energy, how fast you do it is the power.  Energy is measured in joules [J], power is in watts [W].  A watts (not Watt, as that was the man) is 1 J.s^-1.

If the ambient air is very cold, i.e. close to the temperature of the external heat exchanger, very little energy can be extracted, and what can be extracted takes along time, which is more electricity to drive the compressor, fan and pump

When the compressor control gear senses these conditions, it turns on a very basic electrical resistance heater, this drops the efficiency to just below 1 i.e. more energy is going into the ASHP tan is coming out.

Another way to think of it is as a lever with a moveable pivot point.  When conditions are idea, a lot of mass can be moved with one stroke, when they are suboptimal, many strokes, of a small mass have to be used.

So basically the compressor is spinning without compressing the refrigerant gas much, the air fan is at full speed, but little energy can be extracted from the cold air as it is almost at equilibrium with the expanding refrigerant gas in the evaporator.

 

Luckily these conditions never last long in the UK (I expect not many people will complain about their ASHPs now that air temperatures have risen 15°C).

 

Edited February 16, 2021 by SteamyTea

[pdf27]

2 hours ago, Madam1980 said:

If the output variance between the gas temps and the external air temp are minimal in the grand scheme of things, what drives the efficiency, ie what part of the system is having to consume more electricity to achieve the same set return from the controlled environment or is it that it consumes the same amount of elec just the output is less when the air temp is lower as less energy is forced across the heat exchanger by the fan?

 

Also this seemed to dictate to my small man mind that the flow rate and the temp of the flow to the unit is more important than the outside air temp. How do I know what the correct flow rate to it should be or what the best temp to send to it is in any particular condition, or do I not get to decide the flow rate and the heatpump is taking care of controlling its the capacity to heat said coolant / water at whatever rate it decides based on the temp that is being pumped in to it or is it controlling all of this by itself.

I think of it like pushing a weight up-hill. Heat naturally wants to go from hot to cold ("downhill") - but the bigger the hill (the bigger the temperature difference between the outside air and the water flowing around the heating system), the more work you need to do to push it up the hill.

Flow rates make a small difference, basically there's a sweet spot for efficiency where the work you do pumping the water or air is more than the improved heat transfer. Best just to leave the heat pump to do it's own thing there - that sort of thing is programmed in at the design stage.

[Madam1980]

Excellent thank you on both on all counts and replies, appreciate the time others are prepare to give. analogies also always help the simpletons. Bottom line being, to really understand what's going on is I need to learn more about up physics / thermo dynamics and mechanical engineering for the full picture, something over time with helpful input like this and reading could potentially happen. Or not.

 

Consolidating these thoughts or alternatively more likely veering off piste from them despite feeling a connection of sorts..

 

In short, Forget the flow rates for the unit itself, this is something the heat pump will establish and continually adjust. Trying to force a bit of kit that has been optimised to run differently is likely to do the opposite. However...Does the heat pump control the speed of which the indoor circulating pump spins or are these not that clever and provide a fixed flow or does another part of the system take on this job? Also does the heat pump have any capacity to store heat, set numbers of litres or such like, ie if it runs then stops will there be any further heat to be extracted or is it once the ‘carnot energy cycle has completed?’ that’s that and if I want more heat from it it needs to start up again or is it a delayed process, ie compressing and expanding of gases takes say 2 mins and the warming of a set amount of water / coolant another 2.

 

Secondly, am I causing the heat pump to potentially short cycle by setting a flow temp for the ufh? am I better off leaving it on the compensation curve setting so it decides what is most suited target temp to the present outdoor environment?

 

Thirdly, not much i can do about the resistant element from coming on. seems to be 2 boosters on this ecodan pump or well settings on control panel indicated there is, one at 2kw and one at 4kw, does that sound about right? when it’s too cold outside for the pump to run efficiently it is forced to use either depending what’s being called for in heat demand? 1 for 1 or sub that not being great but if that's the weakest part of the system / process it will run at in the infrequent times when it remains below freezing or another set low temp outside then so be it. Anything beyond this is the bonus and the point of the system making it a viable heating method 

 

And finally, for now anyway, back to short cycling as its the wasteful part of the process it sounds like aside the needed resistance heaters, what’s the most efficient way to set up the hot water aside from reducing the target temp of the tank to 48. Realise how I use hot water will affect this but assuming the biggest calls come from an evening and morning shower like most homes. If this takes precedent over space heating when its switching from one to the other will there be a lag causing the heat pump to short cycle..shutting down and start up again rather than switching eloquently between the each other… does how I configure the heating alongside this make much odds?  Am I better it constantly monitoring the tank temp for say a 10 degree drop and letting the heatpump do its thing or say running the DHW timer twice per day when elec prices are cheaper. More maths or equations no doubt ?  

 

Thanks again