UPDATED: Air Source Heat Pumps/Air Con - Full Info & Guide, is it cheaper to run than mains gas?
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You can't use a heat pump to keep it at 75 degrees - they'll only manage about 55 degrees. An immersion heater then takes it up to 75 degrees - that's what probably explains some of the poor system performances and higher running costs especially if you have to do it all the time rather than once a week or even once a month.Never under estimate the power of stupid people in large numbers0
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In 2005 I was impressed with the concept of thermal stores and decided to get a system based on an electric water heater linked to a thermal store. I ran the idea past various people including my local plumber all of whom told me it would not work. I phoned Macdonald Engineers and explained my proposal, they suggested it seemed like a reasonable idea and suggested a 250 litre TS fitted with an extra 15 metre coil. This was connected direct to the rads and to a 12 kw Heatrae Sadia (Amptec) in-line heater. The heater was set to heat the thermal store only in the off-peak periods of an economy 10 regime. DHW was from mains cold water going through the heat exchanger coil giving mains pressure hot water.
This worked reasonably well but the thermal store was not big enough to supply enough heat during peak electricity periods without switching on the electric heater.
When the RHI was first announced the figures that were bandied about suggested to me that an ASHP would be a no-brainer. I got quotes from various companies and eventually settled for one that assured me that all would be well. I got a Danfoss AX 12 installed, the Amptec is still in the system but is not used at all. The house has never been warmer. Electricity consumption has dropped from 16-17,000kwh per annum to 11,000kwh per annum. This is in a set-up that has just about everything wrong - solid walled house built pre-1835, conventional radiators, micro-bore piping.
So Cardew, maybe you will see one somewhere.
Would I do it again - no - I'd put in a wood pellet boiler.0 -
I've got a vented Thermal store for my ASHP setup. its only a small one and is only for DHW, the system is zoned so the heatpump feeds directly to the unvented heating system on rads. Works well so far.I heat the TS up to 55 degc with the heatpump and get the immersion to heat it up another 3 degrees to 58 once a day as recommended by Mitsu.
Re the worries about defrost cycles, i think one of the most important considerations is to make sure that your heat pump isn't oversized, if there isnt enough heat in the system to defrost the pump then you will get huge and expensive defrost cycles, again no problems with my system (if anything we slightly undersized it) .0 -
HiMikeeuropa wrote: »... And just to say that #1315 doesn't really stand with the Gledhill TS mentioned as there isn't a slug of mains water sitting inside the TS growing bugs. What does dismiss it, in my eyes, is the fact you have to keep the TS contents at 75C in order to get sufficient heat transfer into the mains feed. Why use a HP to keep the TS at 75C if you can use it to keep a "tank" at 50C and just occasionally sterilise it?
There really seems to be an issue of understanding thermal stores here ... I'll try to simplify as much as possible.
Forget about the heat-store or how the heat arrives for the moment, just think about a measured length of pipe which is constantly heated from the outside and where mains water enters at ground temperature (say 10C ?), is heated as it flows through the pipe and exits at a raised DHW temperature .
(i) - Initially, the standing contents of the pipe will all be at DHW temperature.
(ii) - As a HW demand occurs, flow starts and cold water enters. This will create a linear temperature gradient along the length (all/part of) of the pipe with the exit temperature (depending on flow rate) being the required DHW temperature.
(iii) - When the HW demand (flow) is removed the water at the input end of the pipe will warm to the DHW temperature, taking the system back to condition (i).
As can be seen, the temperature of the inside wall of the pipe has been ...
(At the input) - reduced to mains groundwater temperature for a short period, before being reheated to DHW temperature.
(In the middle) - reduced to ~50% of the temperature difference for a short period, before being reheated to DHW temperature.
(At the output) - stays at DHW temperature throughout the cycle.
... the issue is that bacterial cultures tend to grow on surfaces, so in this case that would be on the inside of the pipe. Operating the system temperature at levels which the bacteria would consider as being either 'nice & cosy' all of the time to 'nice & cosy' most of the time with a temporary chilly period, simply means that the bacteria have conditions where they would survive and thrive, with the likelihood of breaking away from the surface & entering the main HW flow, unless action is taken to prevent it (ie 60C+ cycle).
Regarding the heat-store temperature. The manufacturer will design & build the thermal store to heat mains water to a specific DHW temperature at a specific flow rate. The recommended store temperature will be the one which will achieve the designed exit temperature under 'standard' conditions, so if you reduce the store temperature significantly the result would be an exit temperature which is significantly below the store temperature, unless the flow rate was significantly reduced to compensate (ie, a slow flowing bath tap etc.), or the manufacturer increased the surface area of the heat exchanger to compensate.
To me, it's all pretty academic as heat-pumps operate most efficiently at temperatures which are below MrsZ's shower temperature threshold I'd just go for a more suitable solution, which certainly wouldn't include using a HP to heat more mass than necessary to DHW temperatures .... ever considered solar or multi-fuel as a heat-source with the HP's electric immersion as a backup?
HTH
Z"We are what we repeatedly do, excellence then is not an act, but a habit. " ...... Aristotle
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Hi
There really seems to be an issue of understanding thermal stores here ...
HTH
Z
IMO there is more of an issue with the terminology on 'Thermal Stores.'
My understanding has always been that there is a hot water tank for storing Domestic Hot Water(DHW). That description applies regardless of the water in that tank being heated by a gas/oil/LPG/solid fuel CH boiler, an immersion heater or an ASHP/GSHP. The usual way to divert the hot water from a boiler to radiators/UFH or this HW tank is by a 3 way valve.
The DHW water in this Hot Water tank is heated by a 'normal' heat exchanger(usually a coil)
This Hot water tank will require raising on occasion to 60C for sterilisation - regardless of the heat source.
That Hot water tank is NOT normally referred to as a 'Thermal Store'. Indeed until the introduction of heat pumps I had never heard of the term 'Thermal Store' in a CH system.
Again my understanding of a 'Thermal Store', when used with a heat pump, is that it is used as a reservoir of hot water for supply to the radiators/UFH; it is often referred to as buffer store.
As the output of a heat pump is relatively low, in those installations where a heat pump would be unable to cope with the space heating demand under some conditions, a buffer store can be specified. This is heated by the heat pump and if necessary an immersion heater.
This buffer store water would be heated to the same temperature as the water temperature specified for the radiators UFH(say 30C to 40C) and in the context of this conversation, will not require sterilisation.0 -
Hi CardewIMO there is more of an issue with the terminology on 'Thermal Stores.' ....
What you're first describing is really a standard DHW cylinder. We have a large DHW cylinder (vented system - I like 'simple') which fills with water from a header tank and is heated by hot fluid flowing through 2 separate heat exchanger coils, one for the GCH, the other for the solar thermal ... there's also the option for immersion heaters.
The recent discussion seems to concern a DITS type 'thermal store' which normally relates to a similar cylinder, where the contents form part of the heating system, not as a storage for DHW. The fluid contained can be shared by various heat sources ... eg GCH boiler + log-burner ... and can be delivered to the radiators or underfloor circuit by a separate attached circuit (which could be either directly or not). If you have a solar thermal system, the store would normally have a separate solar heat exchanger with a glycol mix heat transfer/antifreeze fluid. DHW will have a coil running through the TS which transfers stored heat to the DHW circuit. As you say, as well as the contents acting as a 'shared' thermal store, they also allow the stored heat to be immediately available on-request ... when there is a demand for wet space heating it's immediately available for delivery at full operating temperature, with the heat source (GCH/HP ?) acting to replace (/top-up) what has been used ... this is what I would consider to be a buffer. This becomes particularly useful to improve the reaction capability of a relatively low heating capacity heat-pump ... I agree that in this situation the 'buffer' would not be used to heat the DHW at all, this should be in a separate cylinder.
Various forms of TS exist, but the following document was recently referenced (http://www.greenspec.co.uk/files/energy/storesperformance.pdf) ... have a look at the examples on page 6 ... you'll see how the contents (3) are used to heat the DHW from cold(5) to hot(4) in different configurations ...
HTH
Z"We are what we repeatedly do, excellence then is not an act, but a habit. " ...... Aristotle0 -
We really are at cross-purposes Z.
We went through the contents of the above post a few days ago. The idea of a ASHP supplied thermal store, with a DHW coil fed with mains water was ruled out.
Admittedly it can work if you have another heat source apart from the ASHP. In summer when your Solar Thermal panels have high output is obviously an elegant solution.0 -
Hi
There really seems to be an issue of understanding thermal stores here ... I'll try to simplify as much as possible.
I agree with and understand everything you describe in the rest of this post Z, however I suspect that you were saying that to disagree with my reference to #1315 not applying to the Gledhill TS mentioned? If that is the case, I suspect you may not realise that with that particular TS the DHW heat exchanger is not within the TS, therefore I suspect the "length of pipe which is constantly heated from the outside" basis on which your explanation is based is not true. And therefore while there still could be potential for legionella to grow at times of demand it's probably no greater than the chance of it growing on the insides of hot water piping everywhere which heats up when there is demand then cools down after.0 -
Hi MikeMikeeuropa wrote: »I agree with and understand everything you describe in the rest of this post Z, however I suspect that you were saying that to disagree with my reference to #1315 not applying to the Gledhill TS mentioned? If that is the case, I suspect you may not realise that with that particular TS the DHW heat exchanger is not within the TS, therefore I suspect the "length of pipe which is constantly heated from the outside" basis on which your explanation is based is not true. And therefore while there still could be potential for legionella to grow at times of demand it's probably no greater than the chance of it growing on the insides of hot water piping everywhere which heats up when there is demand then cools down after.
I've read this related to external plate heat exchangers before and I'm really quite unconvinced ...
The installation manual for the TS you raise contains the text "a primary thermal store it is not necessary to heat the store above 60°C for hygienic protection against growth of ‘legionella’. This is because the domestic hot water is heated instantaneously via a heat exchanger and so there are no legionella problems associated with stored water." (http://www.gledhill.net/pdf/Torrent%20GreenHeat.pdf) ... which for the thermal store itself is pretty self-evident ..... however, this doesn't really address the whole DHW circuit (any little used taps/showers ?) relative to the low store and heat exchanger temperature, or the materials which are used. I personally wouldn't directly compare the TS system with a combi boiler because of the relative HE DHW exit temperatures - a combi would heat to ~60C whilst the TS would need to be much lower due to HP efficiency issues ... a shower below ~40C starts to feel a little cool, so the TS minimum would be ~45C (?) and that's pretty close to the max efficient operating temperature of a HP, so none of the DHW circuit ever reaches 60C ... that provides for year, after year, after year of thriving/dormant cycles in pipework feeding a restricted flow showerhead ....
At the end of the day, I actually think that far too much is made of legionella in standard DHW systems, but personally wouldn't consider a solution which was practically incapable of raising the temperature to a level above which microorganisms thrive ... I'd consider it as being not only placing my health on the line, it's the personal 'duty of care' which I have to ensure that harm is not caused to others, again a potentially expensive option which needs to be accounted for .... what others choose to do differently isn't my concern, unless their system were to negligently cause me harm
....
HTH
Z"We are what we repeatedly do, excellence then is not an act, but a habit. " ...... Aristotle0 -
I hope someone can offer me some advise please?
I have an ecodan air source heat pump which was installed in December 2012. We keep getting a U1 error code on the panel. The radiators upstairs don't seem to be getting hot, the underfloor heating seems to be working fine and the water is always hot.
I've had someone out to look at it and he has replaced a sensor as he thought it may be faulty but the problem is still occuring, sometimes more than once a day.
Has anyone experienced this error before and if so what fixed it?
Many thanks0
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