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Hi,

Just had one of those crazy ideas you get once in a while! You know the COP of an ASHP is poorer in winter and even more so when you ask it to heat water to 55/60 degrees, this reduces the COP rather badly.

So....what if you put the water through the ASHP twice (heat it up in stages) or bought two and placed them in series, would that be more efficient? Use the first to heat the cold water up from say 10 degrees to 30 and then the second from 30 to 55/60? If you get a COP of say 3.0 in those cases vs a COP of say 1.0 for a single ASHP? Would that hold true? 

I'm sure it's not that simple but was just curious? I doubt you get anything for free but if you stay within the best performance levels / COP would it reduce the electricity demand? 

If anyone can enlighten me?! 

Cheers

Mike

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What you are describing is a 2 stage heat pump. I believe the additional problem is needing a different refrigerant for the high temperarture stage, so two ordinary ones cascaded may not work well.

And the second stage one would be a water to water heat pump, not an air source.

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Thanks Dave!

I was sure someone had thought of it. Getting the underfloor heating temperature seems easy enough but going up to 60 degree appears to be much harder with an ASHP. I was at the Bicester Self Build live event and had a good chat with Andy from Sunamp - so was looking at alternate heat sources for underfloor and hot water. 

Cheers,

Mike

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1 hour ago, mike2016 said:

Hi,

Just had one of those crazy ideas you get once in a while! You know the COP of an ASHP is poorer in winter and even more so when you ask it to heat water to 55/60 degrees, this reduces the COP rather badly.

So....what if you put the water through the ASHP twice (heat it up in stages) or bought two and placed them in series, would that be more efficient? Use the first to heat the cold water up from say 10 degrees to 30 and then the second from 30 to 55/60? If you get a COP of say 3.0 in those cases vs a COP of say 1.0 for a single ASHP? Would that hold true? 

I'm sure it's not that simple but was just curious? I doubt you get anything for free but if you stay within the best performance levels / COP would it reduce the electricity demand? 

If anyone can enlighten me?! 

Cheers

Mike

COP doesn't really change much with air temperature, the really big impact is when it's cool (not cold) and wet.  In cold and dry conditions ASHPs perform very well, which is one reason they are used a fair bit in countries where the winters are cold and dry.

You can get 2 stage heat pumps, as Dave mentions, but they are expensive and there are challenges in getting the refrigerant in the second stage to work well, because of the high condensing temperature.

The best practical solution is to pre-heat hot water with an ASHP and then just boost to get hot water.  An ASHP run at no higher than 40 deg C will have a pretty good COP no matter what the outside temperature or humidity, as it may well never, or only rarely, run a defrost cycle (and it's defrost cycles when it's cool and wet and the heat pump is being asked to work hard that kill the COP).  If you heated a UVC to 40 deg C with an ASHP, then just put some form of boost heater inline then the boost heater doesn't need to do much, as you only want the DHW to be at around 45 to 50 deg C, anyway.

This is pretty much what the hybrid heat pumps do, and they are probably one of the best solutions around for hot water.

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Using an ASHP to get water to 40' and boosting for "hot " water is exactly what I plan to do. I did wonder if it was possible to get the ASHP to recognise when it was just about to run a defrost cycle and get it to stop itself, that way you could get the ASHP to get the max temp out of it without ruining the COP.

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You could likely get it to shutoff, but could be harder to get it to turn on again.  Unless you maybe put a timer on it but may be still in defrost area.

Could also be controller dependent.

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On 13 June 2016 at 14:27, joe90 said:

Using an ASHP to get water to 40' and boosting for "hot " water is exactly what I plan to do. I did wonder if it was possible to get the ASHP to recognise when it was just about to run a defrost cycle and get it to stop itself, that way you could get the ASHP to get the max temp out of it without ruining the COP.

I have been wondering that. Or if not a sensor to detect that it has just started a defrost cycle.  My thinking is if the heating and hot water load is low, and you have plenty of time for it to heat the water so are not in a hurry, it would be better to shut it off for half an hour or more and let it defrost naturally.  The thinking being most icing occurs when the climate is cool and wet, i.e still above freezing so it will defrost naturally.

If it needs defrosting when it's below zero, you have little option but let it do its thing.

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1 hour ago, joe90 said:

 I did wonder if it was possible to get the ASHP to recognise when it was just about to run a defrost cycle and get it to stop itself, that way you could get the ASHP to get the max temp out of it without ruining the COP.

They seem clever enough to try and do this, or at least some are.  I've noticed ours modulate the output down, then back up again, for no apparent reason, and it seems to do it when it's wet and cool.  It has an external temperature and humidity sensor fitted to control defrost cycling, so I suspect it may well just reduce output initially as a way of seeing if that's enough to get the primary circuit temperature differentials looking better and only if that doesn't work does it turn off and run in reverse to defrost.

It would be nice to know what these things are really doing, rather than have to guess, because it seems clear that the inverter controlled units are pretty clever at managing their own performance, within the limits set by the user.

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Some use a 4 way valve to defrost using the heat in the refrigerant - no idea how it works but it means it doesn't draw warm water back to use as a defrost cycle.

What is interesting is whether you would want to use an inline heater pre or post UVC - many of the ASHPs now have an "external" heater controller that could easily heat water from 40-50c with a willis style immersion given the flow rates.

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On 13 June 2016 at 15:43, PeterW said:

SWhat is interesting is whether you would want to use an inline heater pre or post UVC - many of the ASHPs now have an "external" heater controller that could easily heat water from 40-50c with a willis style immersion given the flow rates.

I think the concensus there is store the hot water at a low temperature to minimise standing losses and only heat on demand with an in line water heater as you draw water off.

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Very interesting comments & advice in particular that cool & wet is more of an issue than the outside temperature, and the need to avoid the defrost cycle, both of which affect the COP. 

Just two things I'm not clear on:

  • What does "UVC" stand for in this context? I'm sure it's not "Ultra Velcro Chamber"...(!)
  • Does staging water pre heated to 40-50 degrees not give you a legionella risk? 

 

 

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It's maybe not a fair comparison, but when we had our EAHP,  it would freeze up after 45 minutes of operating.  It would then require a full 15 minutes of warm household air (circa 20C) flowing over the evaporator to defrost it.  I'm not convinced that an ASHP positioned outside would defrost that quickly.

8 minutes ago, mike2016 said:
  • What does "UVC" stand for in this context? I'm sure it's not "Ultra Velcro Chamber"...(!)

UVC - unvented cylinder

1 hour ago, ProDave said:

I think the concensus there is store the hot water at a low temperature to minimise standing losses and only heat on demand with an in lime water heater as you draw water off.

Interestingly, when I last costed this out, I came to the conclusion that the overall cost of providing DHW by this method (so assuming normal tariff electricity for ASHP and inline boost) was the same as using off peak electricity to heat up a cylinder of DHW.   

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Our ASHP, like the majority (probably all) inverter controlled units goes into reverse to defrost, so goes into cooling mode, which heats up the external heat exchanger to 40 to 45 deg C or so, very quickly.  When it does this, it draws heat from the buffer tank or UFH and pumps back cold water at around 8 deg C  to either/both.  A single defrost cycle can drop the temperature of our 70 litre buffer from around 35 to 40 deg C to about 15 deg C...................

It does defrost fairly quickly, but the cycle still takes around ten minutes, as it has to switch to cooling mode, pause while the 4 way valve operates, start off slowly and then ramp up to full power, presumably so that the thermal stresses of pumping refrigerant at around 45 deg C into a heat exchanger that's probably at around -5 to -10 deg C, and covered in ice, is minimised.  It then has to reverse back again when it's defrosted and repeat the process of switching the 4 way valve back and ramping the power back up to maximum in order to try and re-heat the buffer tank/UFH that it's been actively cooling whilst defrosting.

 

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1 hour ago, Stones said:

 

Interestingly, when I last costed this out, I came to the conclusion that the overall cost of providing DHW by this method (so assuming normal tariff electricity for ASHP and inline boost) was the same as using off peak electricity to heat up a cylinder of DHW.   

I propose to heat a tank of warm water and UFH using off peak electricity and maybe top the tank to hot using same off peak just before off peak stops ( 5 or 6 am?) 

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1 hour ago, mike2016 said:

 

  • Does staging water pre heated to 40-50 degrees not give you a legionella risk? 

 

 

Yes it increases the risk so most have a Legionella cycle every week / fortnight to use the tank immersion to heat the whole tank overnight to 65c

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On 12/06/2016 at 18:06, mike2016 said:

If anyone can enlighten me?!

I shall try.

What I think would happen is that the HP would run for longer, using more energy.

I tend to think of a HP working with 'packets' of energy.  So imagine you have a large box of cool air, you squeeze it down, thus making it smaller, but hotter.  We have a simple formula for this PV/T = C.

So now imagine that you are about to squeeze the box even smaller, you will need more power to do that (but you don't have a bigger motor), and it will take longer, bit like sitting on a packed suitcase to close it.  C stays the same (think of this constant as your CoP, so Cn, where n=0 to infinity).

So yes, you can make your own 2 stage pump, but you will gain nothing (you will actually loose to general thermal losses).

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Thanks!

Getting to grips with the reality of ASHP's a bit better now! 

Just on the issue of defrost cycles - I was speaking with a supplier at the weekend about a particular MHVR system for which they can provide a geothermal heat exchanger for it, basically a 30-40 meter silver lined underground pipe to heat the incoming air. It's not cheap at a few thousand pounds but I was wondering if there are similar solutions for ASHP or are they overkill? It would be great to have this to supply air to both the MHVR and the ASHP to prevent any defrosting cycles entirely? 

Anyone any comments on this or a better way to tackle the defrosting issue?

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There are ASHPs that come with a group plate heater, which is an awful lot cheaper than a few thousand.

I would be dubious about the payback of such a system.  

I was looking at a pre-heat system for MHRV from PV in-roof panels and it only adds about £70-80 the the price of the in-roof system but the issue balancing the input and output air was raised so i'm unsure if its a workable solution as yet.

 

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I believe there was a thread on the Geothermal MVHR on the old site and a house that had major issues with Mold and Fungus spores causing health issues. It was probably a case of a bad install or the system not being maintained correctly by the users being heavily publicised but it stopped me from looking at potentially someting that could have been a good idea.

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It could be a workable solution but i'm wondering would it be good value.  For me anything that costs thousands really has to be offering big savings.

If for example you are putting something like this on a mortgage, you need to double the cost and see if it still pays back over 15-20 years.

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I looked at earth pipes, but the cost (and a few horror stories, including the infamous one that made the house uninhabitable) put me off.  The payback time was going to to be far longer than my lifespan, too.

Knowing what I've learned now, I'd keep things as simple as you possibly can, and accept that you might have to use small amounts of direct electricity to provide boost heat for hot water.  The cost of this is small, and so not really worth worrying too much about.

For example, if you have an ASHP that works at an average year-round, COP of 3, delivering water at 40 deg C (quite probable, if set up and sized correctly), you want DHW at 45 deg C and your incoming mains water is at 7 deg C, then you get about 85% of your hot water at 1/3rd of the electricity unit price and 15% at full price. 

Another way of looking at it is to pretend that the boost heater lowers the average COP.  For the above conditions, the average COP would be lowered from 3 to about 2.55 just for the hot water delivery, the COP for heating/cooling would still be around 3.  If you compare this method of providing hot water with, say, an electric shower, then a 10 kW electric shower for 10 minutes probably costs around 25p, the same shower using an ASHP and boost heater would cost about 9.8p. 

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That certainly puts it all in perspective! You must link me to the horror story you mentioned, sounds like a fun read! 

 

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1 hour ago, mike2016 said:

I was speaking with a supplier at the weekend about a particular MHVR system for which they can provide a geothermal heat exchanger for it, basically a 30-40 meter silver lined underground pipe to heat the incoming air. It's not cheap at a few thousand pounds but I was wondering if there are similar solutions for ASHP or are they overkill? It would be great to have this to supply air to both the MHVR and the ASHP to prevent any defrosting cycles entirely? 

Supplying air to the MHVR via underground pipes is the scary scenario as pointed out above.

Supplying geothermally preheated air to the ASHP would be less controversial - but sounds like more hassle than its worth. I think the key advantage of an ASHP over a GSHP is that an ASHP avoids the whole issue of ground works, pipes etc. If you were going to do that anyway just go the whole way and do a GSHP with liquid filled pipes (using a low freezing point fluid).

I have a GSHP - and whilst it does avoid the issues of defrosting etc - it was quite expensive. If doing again I might well go with ASHP instead.

- reddal

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I was reading about pumping brine filled pipes to heat the ASHP but sitting back it appears to be a lot of work for a small benefit. I'm meeting with a company tomorrow about their products, once of which is ASHP's and will be interested in their experience using their models in the Irish Climate. Maybe with all this global warming we'll not get below 6 degree much anymore anyhow?!! That or I'll position a dyson hair dryer in front of the ASHP fans......! 

Just curious - what is the cost multiplier of GSHP over ASHP? Is it more than 2x?

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