Author Topic: Heat Pump Hot Water System vs Gas Hot Water  (Read 29380 times)

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Offline EEVblogTopic starter

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Heat Pump Hot Water System vs Gas Hot Water
« on: November 03, 2022, 04:44:12 am »
Comparing gas hot system to a heat pump hot water system before I install a Reclaim 415L heat pump hot water system at home.

 
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Offline SeanB

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #1 on: November 03, 2022, 05:08:13 am »
Quick tip before you cut over, measure temperatures of hot water outlet and cold water input. Likely gas heater keeps that tank at 90C plus, while the heat pump works best as a heater to around 60C Most use you blend hot and cold, so for a shower you probably blend around a third of the hot water with two thirds of cold water, likely at around 18-20C, to give you the 40C or so shower water. Same for the sink, hotter than 40C will burn.

So take into your calculations for water use that dropping the temperature ( over 70C heat pumps drop drastically in COP, a hidden figure, just like Peltier coolers there is a very steep curve) to around 60C will result in you using probably equal amounts of water from both, so the heat pump tank is now about equivalent to the gas heater.  Same total volume of water used, just different settings now on the shower mixer if single lever type, and same for sink.

Best added thing to do is go and install insulation on all the hot water pipes, both inside the roof and outside, so that you do not lose that expensive heat, as you can easily have a 10C or more loss along a long run, especially at low flow. You pay money for that heat, why throw it away heating up the attic, it generally is hot enough already, and you are not going to get more heat into it, so insulate. Insulation is cheap, and you get zip up types as well from the industrial AC suppliers, or just the regular slip on black neoprene and slit it, then use the tape to close it up. $2 per 2m length, a tiny cost. Also on the first 1m of the inlet pipe, as you will see the greatest heat loss is those inlet and outlet pipes, along with the mandatory pressure and temperature relief valve. Water best to install pressure control for the whole house if you do not already have it, balanced hot water system is a great thing, no fighting if somebody flushed the toilet and the shower flips cold boiling cold a few times.
 
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Offline ssander

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #2 on: November 03, 2022, 06:13:24 am »
Just checked my councils noise restriction times. Times for heat pump water heaters are more restrictive than AC units.

Fortunately you're going to run it during the day. Although if you have showers at night, it'll probably want to kick in at night and reheat the water in the tank.
 

Offline SeanB

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #3 on: November 03, 2022, 06:28:57 am »
Not really needed, simple enough to just leave the tank to heat up the next day when power is available, and the sun can provide it. I did find that insulation works wonders, with adding extra insulation helping a lot in holding the water hot. I did accidentally find out, by messing up the timer setting, so it was left in the off mode (tiny little switch that moved to wrong position when closing the cover), and I only noticed 3 days later on that the hot water was now coming out merely warm, and tending to cold.

This on a 100l standard electric hot water heater. biggest gain on any stored heat system is improving insulation, and Dave should, before hauling the old one out, take one of his thermal cameras and look at all the heat leaks in the gas system, especially with just a trickle of hot water running at the furtherest tap in the system, and use this as a guide to insulate the system a lot better. My one is a big ball of green fluff, recycled PET bottle insulating batting, normally used in drywall as insulation, but I had a part roll around from repairing a drywall section at work, and as the add on is the same stuff with a fancy quilted cover, I just finished the roll. Also insulated the hot and cold pipes as well, as far as possible till they hit brickwork, and only drawback is now the cold tap has a slug of warm water when first opened. The copper pipes condust heat that 2m length well.
 

Offline Dread

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #4 on: November 03, 2022, 06:34:22 am »
This might not be what your expecting.  I do not know the model your getting but if it's anything like the Rheem models they sell in the USA the noise from the compressor is going to drive you crazy!
The Humming sound is so loud (70db) that you can hear it in almost any room in the house and it's super annoying.  They claim it's 41db but it's not!

Second issue is that it they are easily burned out by power outages and surges.

If this is a free demo unit then great but if your paying the full price I highly suggest that you get some sort of guarantee on the Sound level or you will be defaulting to using only the elements just to keep your sanity.
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Online Halcyon

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #5 on: November 03, 2022, 09:34:54 am »
Just out of curiosity Dave, did you also do the maths on continuous hot water systems? Those that only heat what you use, on-demand, as opposed to keeping a tank of water hot 24/7?
 

Offline mjwurtz

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #6 on: November 03, 2022, 01:12:39 pm »
If it's for a new house, the installation cost is lower : only the difference between a classic (gas or electric) and a heat pump system...
But you must take in account the external temperature : if it drops too low, you will consume a lot more to have really hot water.

For the noise, I'm surprised by the very loud 70 dB ! The external 8kW heat pump for heating my house is much less noisy... (low temperature central heating with floor heating at 20°C and some radiators for first floor at 24°C).  I have a classic wood stove for very cold days in winter  ;)

In my house, I use a water heater with solar tubular captors using fluid(water+additives) and a electric resistor if no sun for a few days. Even in winter, by -10°C (I live in Alsace, France), if the sun is present, the captor temperature is high enough for keeping the tank hot enough. Generally don't use electricity (apart form the captor circulation pump, which consume nothing) between April and September

Just another hint : to avoid too hot water, place a thermostatic mixer just after the hot water tank, so you never deliver water above 45-50°C at the taps...
 

Offline EEVblogTopic starter

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #7 on: November 04, 2022, 12:14:23 am »
Just checked my councils noise restriction times. Times for heat pump water heaters are more restrictive than AC units.

It's 36dB.

Quote
Fortunately you're going to run it during the day. Although if you have showers at night, it'll probably want to kick in at night and reheat the water in the tank.

It won't, it has a timer that only comes on during the day.
 

Offline EEVblogTopic starter

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #8 on: November 04, 2022, 12:23:54 am »
Just out of curiosity Dave, did you also do the maths on continuous hot water systems? Those that only heat what you use, on-demand, as opposed to keeping a tank of water hot 24/7?

Wasn't a consideration, they use massive amounts of instantaneous power, around 7W/30A for a single phase unit. Use hot water and charge our EV at the same time and we are at our house 60A current limit already, not including anything else like stove or dryer etc.
And we'd need to use it mostly at night that would have to come from the grid as it would likely exceed even battery storage peak power delivery specs. Totally changes the type of battery storage you get if you want to cater for that. We like the idea of having a thermal battery using excess solar.
Also, with a big tank of hot water, if the power fails you still have a decent amount of hot water for a few days, and it takes very little power (<1kW vs 7kW) to keep it topped up.
Instanteneous seems to be only an advantage if you lack space and don't care about power consumption or power failures. It's not something you would choose if you have solar or battery storage system.
« Last Edit: November 04, 2022, 12:30:48 am by EEVblog »
 
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Offline EEVblogTopic starter

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #9 on: November 04, 2022, 12:25:53 am »
This might not be what your expecting.  I do not know the model your getting but if it's anything like the Rheem models they sell in the USA the noise from the compressor is going to drive you crazy!
The Humming sound is so loud (70db) that you can hear it in almost any room in the house and it's super annoying.  They claim it's 41db but it's not!
Second issue is that it they are easily burned out by power outages and surges.
If this is a free demo unit then great but if your paying the full price I highly suggest that you get some sort of guarantee on the Sound level or you will be defaulting to using only the elements just to keep your sanity.

36dB average is the claim. Will onle come on for a few hours per day during the day, never at night. We have 4 external aircon units that we can't hear at all, and I think they are higher rated.
 
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Offline Dread

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #10 on: November 04, 2022, 01:03:53 am »
36dB average is the claim. Will only come on for a few hours per day during the day, never at night. We have 4 external aircon units that we can't hear at all, and I think they are higher rated.

It's going to be interesting to see if the EU or Aus versions of these things are better made than the USA versions.  On paper they sound great but between the noise and the lack of hot water in the winter and the less than advertised power saving we returned ours for a loss.   I think the idea is very sound but they just have not gotten the implementation worked out properly in the West.  Looking forward to your Video.
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Offline nctnico

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #11 on: November 04, 2022, 01:25:42 am »
I'd look carefully at possible legionella infections in the hot water pipes; you really need at least 60 deg. C from kettle to tap in order to make sure legionella can't survive. IOW: there is a reason gas heaters are set to rather high temperatures like 90 degrees. Recently my youngest son couldn't go to school because they found legionella and had to flush all the water pipes in the building (which is less than a decade old).
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline james_s

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #12 on: November 04, 2022, 03:02:53 am »
This might not be what your expecting.  I do not know the model your getting but if it's anything like the Rheem models they sell in the USA the noise from the compressor is going to drive you crazy!
The Humming sound is so loud (70db) that you can hear it in almost any room in the house and it's super annoying.  They claim it's 41db but it's not!

That doesn't seem right at all. If it's actually 30db louder than specified that suggests there is something wrong with it.
 
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Offline Poe

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #13 on: November 04, 2022, 04:03:40 am »
Heat pump hot water tanks cool/dehumidify the house nicely.  The one I installed for my Dad has an exterior vent so that you aren't cooling the house in the winter.  Bit cumbersome though.

A traditional electric heating element hot water tank might have been a better pick for Dave though.  Would have been paid off within one year.  It would only save a couple hundred dollars (vs the $722), since you would be missing out on some solar revenue.  But I'd rather make $300 a year for seven years, than invest $5k in a heat-pump so that I can start saving +$500 more, each year, after eight years.  I've had two hot water heat pumps die within five years.  This heat pump appears to have a 1yr warranty on valves, 2yr warranty on electronics, 6yr warranty on pump. 

Now I have two traditional  60 gallon hot water tanks with a "desuperheater" connected to my air conditioner.  Instead of the air conditioner just dumping heat into the outside atmosphere, it first dumps it into my hot water tanks.  Free hot water in the summer and my AC takes less electricity to run.  In the rest of the year, my hot water cost is massively reduced since it's a large house heat pump doing most of the heating.  The desuperheater is just a heat exchanger, water pump and some water lines so it's cheap.

..legionella..

There's good reason most hot water tanks do not feel the need to recommend 60C.   Legionella is already in the cold supply water.  When the water is warm, the bacteria can multiply, but not significantly in sealed systems like water heaters.  Legionnaires' disease is an issue when the water is open to the atmosphere (stagnant condensate in cooling towers, not the pressurized water).  The reason they test for Legionella in the water is not because of the hot water system, but because the water supply (cold) itself can have excessively high counts due to poor treatment.

Regarding sound, my heatpumps were quiet.   Like entire home HVAC units, the 'hum' or vibration complaints typically are due to how and where they're mounted.  Small closets or floors which easily transmit or amplify sound.
 
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Offline nctnico

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #14 on: November 04, 2022, 02:33:41 pm »
..legionella..
There's good reason most hot water tanks do not feel the need to recommend 60C.   Legionella is already in the cold supply water.  When the water is warm, the bacteria can multiply, but not significantly in sealed systems like water heaters.  Legionnaires' disease is an issue when the water is open to the atmosphere (stagnant condensate in cooling towers, not the pressurized water).
The Dutch government thinks otherwise. Legionella can and will multiple rapidly in warm water. Therefore it is mandatory (=by law) that hot water systems reach at least 60 deg. C and the hot water coming from a tap reaches at least 55 degrees C. Over the years there have been quite a few (lethal) incidents caused by legionella in the Netherlands. IOW: it is not to be underestimated.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline Marco

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #15 on: November 04, 2022, 02:46:23 pm »
I'd look carefully at possible legionella infections in the hot water pipes; you really need at least 60 deg. C from kettle to tap in order to make sure legionella can't survive. IOW: there is a reason gas heaters are set to rather high temperatures like 90 degrees.

Depending on your use an anti legionella cycle might be sufficient. The only hot water user here is the shower really, close in boiler for the kitchen tap and don't really use hot water on any other tap. The shower runs often enough it will use the hot water from the cycle. Maybe if you have some rarely used taps which can't be relied upon to do that add a tankless heater to their hot water line?
« Last Edit: November 04, 2022, 02:55:01 pm by Marco »
 

Online Kleinstein

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #16 on: November 04, 2022, 04:10:04 pm »
Quite a bit of the heat is lost through the insulation, just to keep the tank hot. This was esepcially an issue with the older systems with thin insulation and little insulation.

The gas powered hot water systems usually have no problem reheating multiple times on a day. The 175 L is more like limiting on how much you can use in a short time, like 1/2 hour or so. The usually higher temperature in the gas fired buffer is also an issue. So the 415 L new buffer may not provide that much reserve for hot water.
It also depends on how well the buffer tank keep hot and cold water separate and perserve stratification - this can make about a factor of 2 on the capacity of the buffer, especially with not much headroom.  Ideally the heat pump does not have to provide all it's power at the peak temperature, but could use different temperature levels. So a good buffer with stratification could also improve the COP.  Not sure on how this is handled though. It may just be a point for possible future improvements still to come in newer models.

We have a solar heated buffer tank of some 800 L and this is normally good for serveral rainy days, without new heat. However it really depends on the usage. So I would expect the 415 L to be OK, but not extra large for just recharging once a day and not a very high temperature to start with.

At least in germany there are limits on how hot that water in the pipes should get. This especially applies to solar power systems that can get really hot in the buffer (like 90 C). This is somewhat conflicting with the minimum temperature wanted to keep legionella at bay.
 
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Offline thm_w

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #17 on: November 04, 2022, 09:40:35 pm »
I'd look carefully at possible legionella infections in the hot water pipes; you really need at least 60 deg. C from kettle to tap in order to make sure legionella can't survive. IOW: there is a reason gas heaters are set to rather high temperatures like 90 degrees. Recently my youngest son couldn't go to school because they found legionella and had to flush all the water pipes in the building (which is less than a decade old).

Manual says:
- Hot water temperature 63C
- Heat pump on / off temperature (based on the thermistor on the tank) °C 37/59
- Daily Self-Legionella control – 60°C at 45% level on tank

Quote
The unit fan comes on properly and the circulating pump starts to circulate that constant 63°C water through the tank.
Due to the smart top down heat return system your customer will have 50L of hot water within 20 minutes and a full tank in about 3 hours.
After 15 minutes touch the heat pump return connection to the tank and ensure it is hot to touch, if you feel good heat here you are done and can comfortably leave knowing you have completed the install successfully

https://reclaimenergy.com.au/wp-content/uploads/2020/07/Reclaim-HeatPumpController-Specifications-Issue01-July2020-web.pdf

Still, kind of sucks if you can't adjust the temperature up at all.
« Last Edit: November 04, 2022, 09:43:21 pm by thm_w »
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Online Kleinstein

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #18 on: November 04, 2022, 09:57:00 pm »
The specs are really strange, assuming 32.6° C ambient temperature and 21 C temperature for the "cold water".  Do you really need hot water in a climate that hot ?
So take the COP with a grain of salt.
 

Offline thm_w

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #19 on: November 04, 2022, 10:50:18 pm »
The specs are really strange, assuming 32.6° C ambient temperature and 21 C temperature for the "cold water".  Do you really need hot water in a climate that hot ?
So take the COP with a grain of salt.

21C for the cold is possible in AUS, although the average is ~15C: http://www.bom.gov.au/products/IDN65176.shtml
33C ambient is really optimistic though... average temps should be 12 to 25C.
Maybe they based it off of here: https://en.wikipedia.org/wiki/Marble_Bar,_Western_Australia

It is weird when coming from a colder to tropical country, that there is no "cold" tapwater, and yeah you can shower with the regular unheated water just fine.
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Offline Ed.Kloonk

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #20 on: November 05, 2022, 06:02:26 am »
..legionella..
There's good reason most hot water tanks do not feel the need to recommend 60C.   Legionella is already in the cold supply water.  When the water is warm, the bacteria can multiply, but not significantly in sealed systems like water heaters.  Legionnaires' disease is an issue when the water is open to the atmosphere (stagnant condensate in cooling towers, not the pressurized water).
The Dutch government thinks otherwise. Legionella can and will multiple rapidly in warm water. Therefore it is mandatory (=by law) that hot water systems reach at least 60 deg. C and the hot water coming from a tap reaches at least 55 degrees C. Over the years there have been quite a few (lethal) incidents caused by legionella in the Netherlands. IOW: it is not to be underestimated.

I know that instant hot water boilers for cafe's are a different class of water heater but for those of you that like to take things apart, don't. What's inside an old one will horrify you. I don't buy coffee made from water I didn't boil myself.

I would be interested to know if a instantaneous gas hot water heater would pay better in the long run. You know, offset the cost of free power verses a plain old Rinnai. If it were mine, I'd tidy up the gas plumbing so at least I could quickly connect a Rinnai if the house inverter chucks a fit. Tho, with that said, the 1kw water heater -could- run of a neighbors' powerpoint and extension lead.
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Online Kleinstein

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #21 on: November 05, 2022, 06:38:04 am »
Instant hot water from gas (that is a gas heater controlled by the wanted hot water flow) works reasonable well. With gas there is not problem to have some 30 kW of instant on power. The regulation may not be as good as electric, but for that there are regulators at the outputs. I had used such a system for some time - the main weak point was that it needs a certain flow to start. So it does not work well to get small amount of slightly warmed water for the tap.
The point for Dave is getting rid of the gas all together. Depending on the location there can be quite some fixed part to the bill.
 
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Offline james_s

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #22 on: November 05, 2022, 07:24:52 am »
Gas is the way to go for hot water IMO if you have access to it. My gas hot water tank uses no electricity at all and keeps right on working through a power outage. The tankless gas heaters take only a small amount of power and can be run from a pure sine inverter or a small generator. Not a big consideration everywhere but here in the heavily forested and mountainous area where I live the first big storm to roll through in the winter usually knocks down loads of huge trees and that knocks out the power. I know some will say just bury the lines, but the root balls of the same huge trees can tear those up too when they topple over.
 

Online Halcyon

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #23 on: November 06, 2022, 05:37:13 am »
Just out of curiosity Dave, did you also do the maths on continuous hot water systems? Those that only heat what you use, on-demand, as opposed to keeping a tank of water hot 24/7?

Wasn't a consideration, they use massive amounts of instantaneous power, around 7W/30A for a single phase unit. Use hot water and charge our EV at the same time and we are at our house 60A current limit already, not including anything else like stove or dryer etc.
And we'd need to use it mostly at night that would have to come from the grid as it would likely exceed even battery storage peak power delivery specs. Totally changes the type of battery storage you get if you want to cater for that. We like the idea of having a thermal battery using excess solar.
Also, with a big tank of hot water, if the power fails you still have a decent amount of hot water for a few days, and it takes very little power (<1kW vs 7kW) to keep it topped up.
Instanteneous seems to be only an advantage if you lack space and don't care about power consumption or power failures. It's not something you would choose if you have solar or battery storage system.

I meant the gas-powered instantaneous hot water. They basically use no electricity at all apart from the igniter (you can even get ones that don't require power at all and use water pressure to drive the spark). I understand the point about using excess energy and storing it in another form (like your hot water battery).
 

Offline DrGeoff

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Re: Heat Pump Hot Water System vs Gas Hot Water
« Reply #24 on: November 06, 2022, 06:17:42 am »
Interesting. I looked at heat-pump HWS when I designed the house I am currently living in about 20 years ago.
Figured that the most efficient conversion was to go direct from sun to heat and used three solar heating panels instead which works fine, but requires the traditional off-peak feed for those days of cloud and rain.
Was it really supposed to do that?
 


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