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Author Topic: air to water ashp  (Read 6950 times)
dhaslam
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« Reply #15 on: July 20, 2017, 05:17:17 PM »

The compressor can overheat in warm weather so there is normally a sensor to protect it.

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DHW 250 litre cylinder  60 X 47mm tubes
Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
Fionn
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« Reply #16 on: July 20, 2017, 05:39:52 PM »

It sounds to me like your cylinder coil is undersized and the HP is running at the upper limit of it's output temperature range.

The delta T between the water in the coil and that in the cylinder is much reduced (vs an oil boiler) due to the low flow temperature from a heat pump.
If you had a larger coil the water would have more of a chance to cool before returning to the HP.
As it stands, the HP is tripping out on over temp as the set point is too high for it to run in high ambient temps without the benefit of cool return water.

This would still occur once the tank fully heated up though, even if you had a bigger coil, so it sounds to me that the HP is at the upper limit of it's ouput temperature range and you'll need to dial back this setpoint.
One would have to assume that the refrigerant level would be set correctly in a new unit with fixed volume, there's no variation here due to refrigerant pipe runs etc.
« Last Edit: July 20, 2017, 05:42:00 PM by Fionn » Logged

PV - 2.75kW East, 1.5kW South, 2.5kW West. 3 x Flat Plate Solar Thermal with side arm FPHE on 268L cylinder
JonG
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« Reply #17 on: July 21, 2017, 07:31:40 AM »

Flow rates are critical on heat pumps and usually with a design delta of 7 degrees across the plate, pipework ends up being 28mm or above. It looks as if you have plastic at 22mm? Which with its thicker wall and liners may be causing flow rate restrictions.

Is the circulating pump on full speed, is there a 3 way valve passing to heating perhaps and as per Fionn is the cylinder a dedicated heat pump variant and tested with the unit?
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eabadger
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« Reply #18 on: July 21, 2017, 09:06:46 AM »

thanks for suggestions, just to confirm it isnt the motor that is tripping out, it is the gas pressure sensor.

since fitting i have considered pipe size having read the samsung manual, but outlet of heat pump is 3/4" and inlet of cylinder is 3/4" which is 19mm, even my speedfit pipe 22mm has a bore of that, isnt flow restricted to smallest piece or would over size pipe make a difference? pump is a modulating one but set to full speed, the motorised valve is a 28mm shared with rest of system.

cylinder is not a dedicated HP one but is a oso solarcyl, and i have seriesed up the two coils, feeding hp in at top of cylinder and out of lower coil at bottom.

what i have noticed is the high pressure max on these pumps is 4.2mpa my split air air heat pump is 4.5mpa, when my air water pump is at rest at 17* temp the pressure seems to be .2mpa greater than manual states as new fill.

look it isnt an issue and the temps we have had recently are not normal, but i was expecting great results with the extra heat not lower results.

i am so far really happy with the system, make unsure of but am now convinced this is the way for off us, am setting up monitoring on it both heat and power so will know for sure, am going to set my morningstar rd1 to trigger the immersion whenever the batteries reach float or on timer if that not reached once per week.

steve
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JonG
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« Reply #19 on: July 21, 2017, 04:34:04 PM »

What is the heat pump rated capacity and I can calculate flow rate from that for you? I would pipe the coils in parallel, in series there is likely to be too much resistance, can you use temp clamps to assess flow and return temps to the cyl.
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eabadger
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« Reply #20 on: July 21, 2017, 07:31:56 PM »

heat pump is 8kw, the cylinder coils are 11 and 18 i think.
not in to dynamics of flow, but isnt flow reduced by smallest point?
i had just one coild connected, and worked, but error 001 when warm out, two coils does it less.
gauge on side of unit shows error occur spot on 4.2
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JonG
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« Reply #21 on: July 22, 2017, 10:19:32 AM »

Ok 8kw at 7 degree delta is 16 LPM, very roughly 22mm plastic will only carry around 6.5kw at that velocity, if you had a bigger pump or higher pump speed it could do it, but might create harmonics on the coil.

This is the maths though, real world can be different.

If it happened on 1 but was better on 2, I would say it is a combination of factors. Not enough surface area for heat transfer, when 1 was plumbed in and too much resistance when 2 are in series.

Options therefore include, bigger pipework, coils in parallel, heat pump cylinder with a suitable coil size, larger pump etc.
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eabadger
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« Reply #22 on: July 22, 2017, 10:36:59 AM »

thanks for info, the heat pumps says flow of 1.2m per hour, the pump is set at and showing near 4m per hour.
happy to fit 28mm pipe if that improves things, but no real problems just wondering.
but if the pump is 3/4" will this not be the limit to it all?
again can paralel up the coils, but would this not have adverse effect as at different heights in the water? so different temperatures.

reading the manual for same pump but sold in dubai they have different settings on the eev and superheat temperature.

steve
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1600w PV main array at 24v, excide 2v 1000a forklift cells now x 2, 320w PV secondary array at 12v. Enfield 1944 ex RAF 5.6kw diesel genset (now in pieces, big ends gone), Petter AC1 28v diesel charging set at 2.8kw.
1kw wind turbine.
26kw wood stove back boiler to underfloor heating and dhw
JonG
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« Reply #23 on: July 22, 2017, 11:05:19 AM »

1.2 m3/h is around 20 litres per minute which would mean a design delta of around 6. 4m3 is around 66 litres per minute which is pretty immense, you would probably hear that in the coil and would be a pretty big pump at that duty.

The size of the pump inlet/outlet is not hugely relevant, the pressure drop across it will affect flow inevitably but short restrictions provided there are not too many won't cause too much issue.

As an aside if you increase the pressure available from a pump the resulting increase in resistance is something like a factor of 4 from memory.

Using the coils in parallel isn't a perfect solution, but it does increase surface area and decrease resistance, both of which may be causing the undesirable alarm. Whilst the alarm is intermittent and only annoying at this stage, the high temperatures that the compressor experiences and the cycling that occurs will rapidly result in wear and potentially premature failure.

Flow rate and long run times (or adequate run times) are high priorities when we design heat pump installations, hence the use of buffers, big bore pipework, minimal zoning and tank in tank or thermal store type hot water delivery systems.

I think the gauge that you referenced will be pressure of the refrigerant, if you check the pressure on tables pertaining to the refrigerant, it will tell you what temp is equivalent to that pressure, you can then check this against the temp at which the HP stat is tripping to see if the stat is out of calibration, which does happen. Alternatively the stat should have its tripping pressure on it or it should be in the MI.

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eabadger
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« Reply #24 on: July 22, 2017, 01:08:20 PM »

so instead of replacing the 22mm speedfit with 28mm copper, what if i double up the 22mm flow and return?
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1600w PV main array at 24v, excide 2v 1000a forklift cells now x 2, 320w PV secondary array at 12v. Enfield 1944 ex RAF 5.6kw diesel genset (now in pieces, big ends gone), Petter AC1 28v diesel charging set at 2.8kw.
1kw wind turbine.
26kw wood stove back boiler to underfloor heating and dhw
Countrypaul
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« Reply #25 on: July 22, 2017, 01:31:47 PM »

You could look at 28mm plastic, I have just got a length of 28mm polyplumb as a coil - it was recommended as being much more flexible than speedfit which appear to only be available as straight lengths like copper and so much easier to run the polyplumb. This to be used between our thermal store and ASHP when we get one later this year. The cross sectional area of 28mm vs 22mm is asbout 50% more but using a larger diameter pipe should give less resistance though how significant that it I have no idea. Using a flexible pipe with bends rather than a more rigid pipe with elbows should also reduce resistance.
« Last Edit: July 22, 2017, 01:33:44 PM by Countrypaul » Logged
JonG
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« Reply #26 on: July 22, 2017, 01:44:50 PM »

28mm plastic should be fine, 28mm copper better and yes with as few 90 degree bends as possible, 22mm copper will actually carry 8.5kw at 7 degree delta, but the heat pump power is likely to be much higher than this in warmer temps, again the MI should provide a range of power outputs at different ambients to allow you to size correctly and not cut it fine.

You can double up the pipework providing that the length and resistances are kept equal, so same number of fittings etc. It would be good practice to use a flow setter on each leg though to ensure they are balanced, don't take the readings as gospel though (around 10% variance usually unless you spend heavy).

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eabadger
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« Reply #27 on: July 22, 2017, 02:33:09 PM »

if i do redo in 28mm i think i will do in copper due to price, i still cant get my head round why the 3/4" 19mm outlets and inlets dont reduce the flow negating the bigger pipe/tube?
i have a straight run after arriving in house, would it make any positive difference to do just the straight run in 28mm? about 6 meters. total run about 8 eachway.

steve
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1kw wind turbine.
26kw wood stove back boiler to underfloor heating and dhw
djh
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« Reply #28 on: July 22, 2017, 05:48:56 PM »

You could look at 28mm plastic, I have just got a length of 28mm polyplumb as a coil - it was recommended as being much more flexible than speedfit which appear to only be available as straight lengths like copper and so much easier to run the polyplumb.

My entire house is plumbed using speedfit coils, so it definitely exists!

http://www.johnguest.com/speedfit/product/pipe/jg-layflat-polybutylene-pipe-in-coils-2/
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Cheers, Dave
Countrypaul
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« Reply #29 on: July 22, 2017, 06:02:10 PM »

You could look at 28mm plastic, I have just got a length of 28mm polyplumb as a coil - it was recommended as being much more flexible than speedfit which appear to only be available as straight lengths like copper and so much easier to run the polyplumb.

My entire house is plumbed using speedfit coils, so it definitely exists!

http://www.johnguest.com/speedfit/product/pipe/jg-layflat-polybutylene-pipe-in-coils-2/

The web site says available in June, so maybe it had not yet got through to my supplier when I ordered the polyplumb - plus I needed slightly less than 25m, so a 50m coil might have been serious overkill, pity as all the rest of the house exept the WCs are done in JG speedfit.
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