Frequent compressor cycling when using a ASHP for shower water heating alone

[yonatanb]

Hello,
I am new to your forum, and as I am not a native speaker, I hope you would excuse my lame English.
A PEREG CRH-5 (input:5kW, output 18.8kW ) ASHP was recently installed in the building where i live. PEREG is a local Israeli brand, that uses imported compressors.
The heatpump's only use is heating shower water for 9 appartments lodging ~27 residents.

The entire system is made of the aforementioned heatpump, a 120L buffer vessel, 3 water tanks with internal spiral heat exchangers, each containing 300L of water, a water pump that circulates the buffer vessel water in closed loop through the heat exchangers, and a 2nd water pump that draws water from the same vessel and injects it directly to the heat-pump. Both pumps run without interruption.

The ambient temperature in Tel-Aviv, Israel, is 23°C-30°C (73°F-86°F), so CoP is at its peak value of ~ 4.
The residents consume ~700L of hot water per 24h.
17kWh of electricity is consumed during the same 24h period. The consumption figure includes both the heat-pump and the water pumps consumption.
The operating temperature delta is 8°C (ranging from 47°C [116°F] tp 55°C[131°F])
I use a basic recording device for monitoring the compressor cycling.
I currently experience as many as 5 compressor start-ups per hour (during peak time, 22:00-23:00h). i counted an overall of 27 compressor start-ups during 20h of monitoring. During peak hours, the compressor cycle last 120 seconds at most, thus heats ~70L (60% of the 120L) of the buffer vessel water.

I curiously read Afternine's thread regarding a similar problem with an Altherma unit. Though the malfunction symptom is similar, Afternine's use of the heat-pump and ambient operating temperature is quite different from mine.

This is why i pose the following questions:
A. Is over 30 compressor start-ups a day an acceptable/high figure?
B. Will this number of cycles affect the reliablity of the compressor?
C. What is the expected life span of similar scroll compressors?


Thank you, Yonatan.




== Raw Data ==
Legend:
hh:mm-hh:mm - current # of cycles after adding a 2nd water pump [# of cycles after temp delta modification 6°C->8°C] ( # of cycles in the beginning)

17:00-18:00 -1 * [ 2 ] (3)
18:00-19:00 -0 [ 1 ] (4)
19:00-20:00 -0 [ 4 ] (9)
20:00-21:00 -1 [ 6 ] (8)
21:00-22:00 -4 [ 8 ] (7)
22:00-23:00 -5 [ 6 ] (8)
23:00-00:00 -4 [ 1 ] (8)
00:00-01:00 -0 [ 1 ] (5)
01:00-02:00 -1 [ 2 ] (8)
02:00-03:00 -2 [ 1 ] (4)
03:00-04:00 -0 [ 2 ] (3)
04:00-05:00 - no datum [no datum] (3)
05:00-06:00 -no datum [no datum] (3)
06:00-07:00 -no datum [ 2 ] (3)
07:00-08:00 -no datum [ 2 ] (9)
08:00-09:00 -1 [ 4 ] (8)
09:00-10:00 -1 [ 6 ] (5)
10:00-11:00 -1 [ 8 ] (3)
11:00-12:00 -0 [ 7 ] (3)
12:00-13:00 -1 [ 2 ] (no datum)
13:00-14:00 - 2 [ 6 ] (no datum)
14:00-15:00 - 0 [ 1 ] (no datum)
15:00-16:00 - 1 [ 2 ] (no datum)
16:00-17:00 - 2 [ 2 ] (no datum)

[chillerman2006]

First Yonatanb

welcome to the forum

your English is very good, better than mine, but then I'm a Londoner

the least amount of start ups the better & I am expecting a heat pump wizard here (Mike) to say you need a bigger buffer tank fitted, as then your unit will run less often and you will have more hot water reserve's

R's chillerman

[al]

From a simplistic look i'd guess the buffer tank is undersized, running the heat pump for 120seconds is short cycling, i again am guessing that the anti recycle timer is set to 300 seconds, giving the 5 starts an hour.

al

[yonatanb]

Chillerman2006 & Al, thank you for you quick replies.

@Al: I learned from an expert (and from my bad experience) that the timer in this heatpump is set to 6min between start-ups. indeed, before a 2nd water pump was added to the system, the heatpump reached a record of 9 start-ups during a single hour. :-)

Following your joint advice, i read a previous thread regarding buffer vessel sizing.
Though the thread was exhaustive, only 2 forum members provided the forum with coarse quantitative rules for picking up the "correct" buffer vessel sizing.
Being just an end-user rather than a pro, could you provide me with your rule-of-thumb for the range of volumes recommended for the buffer vessel (assuming the output heating power is 18.8kW and the mean minimum temperature in the winter here is ~10°C, the absolute minimum exceeding 5°C ) ?
Thank you again for sharing your experience and wisdom !

[chillerman2006]

Hi yonatanb

I dont know how to size a buffer vessel, have looked through the thread you mentioned and have a good idea of the two you are meaning, one I dont know but the other I do and will drop him a private message if he does not spot this thread when on later or unless Al has the calcs to do this

Regards Chillerman

[Gary]

I'm wondering how the buffer tank is piped in relation to the AWHP?

[chillerman2006]

Evening Gary

whats the do & donts with positioning a buffer tank on these systems mate ???

R's chillerman

[Gary]

... 2nd water pump that draws water from the same vessel and injects it directly to the heat-pump.

This is the circuit I am wondering about?

[Gary]

Evening Gary

whats the do & donts with positioning a buffer tank on these systems mate ???

R's chillerman

I have no particular expertise on these systems... other than having built my own AWHP.

On a commercially built system, I would think the water in and water out would be labelled on the AWHP in order to get counter flow. Something to check.

At the vessel end, the coldest water (bottom of vessel?) should be flowing towards the AWHP.

[chillerman2006]

Thanks Gary

I have very limited knowledge as you know on design but that makes good sense, cheers

R's chillerman

[yonatanb]

@Chillerman, thank you for pulling your strings accross the Atlantic :-). I will add you to my list of much favored Englishmen right next to Gen. Allenby :-).

@Gary, thank you for guiding me through this debugging process.
I have hand drawn a naive sketch of the system. No pipes are directly interconnected.
It seems the cold water, from the bottom of the vessel, is indeed flowing towards the input of the heatpump.Looking at the drawing, can you identify another possible flaw?
i also added pictures of pump #1, which pumps water into the vessel, pump #2 (hiding on the bottom left corner), which pumps into the heatpump, and a closeup (well, maybe too close) of pump #1 specification plate.
Both pumps are identical 2683[revs. /min], 256[Watt] water pumps.
On Sunday (worry not - it is not his rest day ;-) the expert that installed the system is going to replace both water pumps, by a single powerful pump. In case that does not do the job, he will consider replacing the 120L buffer vessel by one of the three 300L water tanks.
I will keep you posted regarding the developments.

[Gary]

As I said, I am not an expert on these systems.

That said, I am not sure I understand the piping strategy.

I would have piped the ASHP in series with the outlet of pump #1, i.e. pump #1>ASHP/in>ASHP/out>bottom of buffer vessel.

This would encourage temperature stratification in the buffer vessel and would ensure delivery of the hottest water to the three water tanks.

Having the ASHP/out dumping into the top of the buffer vessel mixes the vessel water, eliminating stratification and could deliver partially heated water to the tanks.

Possibly there is a reason for the two pump circuit strategy that I am not aware of?

[chillerman2006]

Gen. Allenby
.

What a strange coincidence you mention 'Allenby' - my ancestors are from this part of the world where this took place - respect to him for entering on foot & guarding what was of importance to others

R's chillerman

[yonatanb]

@Chillerman: I cherish Gen. Allenby, who by thorough planning, spared the lives of his ANZAC troops, without jeopardizing his mission (which was chilling his enemy's ambition). He was a thorough planner (a virtue i admire in the chilling business as well :-).

@Gary: I will recheck my drawing. I am not sure as for the internal piping of the buffer intself, but other than that, your suggestion is both simple to implement and logical. Would you make house calls outside FL? say, in TLV?
I will convey your design to the local expert. THANK YOU!

[Gary]

Airports are not smoker friendly... and it's a long swim to TLV.

[mad fridgie]

You are starting at wrong end.
Your 3 * 300L water tanks, how are they piped, water in, water out parallel I presume.
The internal coils how they configured inside they tanks.
Control scenario is incorrect,.
The heat pump is doing what it is designed to do, anti cycle timer is correct.
Without totally re-designing the system.
This what you do.
Thermostat connected to storage cylinders, sensor placed 1/2 way up the cylinder (approx at the top of the internal heat exchangers).
This stat controls the water pump that circulates between the buffer and the cylinder heat ex-changers.
Let the heat pump heat the buffer ( i am not a lover of buffers for this type of application, but it is in) as is, only difference is turn off this water pump when temp is reached. ( I gain i must presume that temp sensor for heat pump is in the buffer tank)

[yonatanb]

@ mad fridgie:
Thank you for your comment!
I rechecked my drawing and it seems to correctly portray the currently installed system at the building where i live.
Do you mean both spiral heat exchnagers and actual shower hot water from the tanks should have parallel in/out piping?
Other than saving the energy for running the water pumps indefinitely (which is far from negligible in my system), your thermostat controlled water pump configuration suggests you hold heat loss outside the tanks to be responsible for the buffer vessel quick temperature drop. Is this a correct description of your motive when suggesting such configuration?
Each 300L water tank is installed with a backup electric heater. Is it possible to use the thermostats of these heaters for the job?

[mad fridgie]

Are the coils inside the cylinders at the top of each cylinder.?
Looks like a right real **** up to me.
You have loops fighting loops.
Let me think for an easy solution, if there is one! ( first thought rip it out start again)

[mad fridgie]

Hi, thought the first post made was was lost! must of been hiding somewhere in cyberspace. My first comments were based upon what I thought your system was, and had presumed that at least someone had a clue, but maybe not the all the tricks. How ever who ever installed this has NO clue on heat pump installations. (did not realize you had given drawings)
Your unit is cycling because of the setup of the water cylinders (not external losses, even though should be considered)
It does come down to the position of the cylinder internal spiral heat ex-changers. These need to be at the bottom of cylinders. I do not like coils in cylinders for heat pumps! (OK for boilers)
Your present system just looks a system for a boiler or the like (was this designed by a plumber?)
How many water ports do you have at the bottom of the water cylinders (not buffer tank) not ports that go to the top of the cylinders. I have tried to post a pic, but RE for me is really slow at the moment

[Gary]

I rechecked my drawing and it seems to correctly portray the currently installed system at the building where i live.

Are you saying that the spiral coils are actually in the top of the tanks?... not the bottom of the tanks?

[yonatanb]

Thank you for keeping up with me.
By "correct", i meant the piping outside the tanks is the same as in reality.
The drawing of the spiral heat exchangers inside the tanks is due to pure imagination, since as an amateur I can only draw what I see, and the pipes there are hidden. what I can say is that the tanks are not custom made.
I took a photo of the bottom of a tank (4 ports), and the side of a tank (2 ports. I may have missed other ports on the opposite side, which I cannot reach).
The system is meant just for boiling shower water.
The firm that installed this system has installed many systems in private houses, small (80 room) hotels, large hotels, and kibutzim.

[mad fridgie]

The fact is that you last tank ( the one that feeds the showers) heats the second tank which then heats the first tank. which causes the heat pump to cycle. If the heat pump was a once past system then maybe sort of acceptable, but from what you have described, it sounds as if it a standard circulation system. Your saving grace is that you do have a high ambient temp (masking the poor design and performance)
I have sent the drawing to chillerman, and asked him to post.
Just because a company have installed many systems, does not mean they know what they are doing, what you have is a boiler system not a heat pump system. "Not the same, and is where most mistakes are made around the world.
In your weather conditions i would hope for a COP of close to 7. You use 700 litres and have 900L storage i would expect that your heat pump should only run 2 or 3 times a day.

[desA]

Perhaps I missed this in the previous threads. (Saw MF alluded to this earlier).

Where is the heatpump temperature sensor (thermostat) positioned?

[yonatanb]

Dear all, i really did not expect experts coming from 4 continents assisting an end-user in distress located in the 5th !
The sun never sets on the heatpump business.


@ mad fridgie: I am looking forward to your design. thank you again !

@ desA: There are no electrical connections between the water tanks and the heatpump. I deduce that the only thermostat is located inside the heatpump. The internal compressor is started exactly when 50°C is diplayed on the heatpump LED dispaly. Thus, I speculate it is the circulated water temperature, which drives the only thermostat on and off.

[mad fridgie]

RE heat pump.pdf
try this

[desA]

Herein lies your problem. The sensor is cooling off too rapidly. Pump probably stops when heat-pump drops off. Water local to sensor then cools off rapidly. Your second circulating pump may have helped a tad.

This is an age-old chestnut...

[yonatanb]

@ mad fridgie : Thank you for taking the time and effort! I will convey your design (and ) to the local professional, and keep the forum posted.

@desA: Actually both water pumps run indefinitely. You are right again regarding the rapid temperature drop. When only a single water pump was installed, and after a 2 minutes of compressor startup and shutdown, it didn't take more than a couple of minutes for the heatpump thermostat to drop to a few tenths of a centigrade above the restart temperature threshold. As the raw startup data in the opening post shows, you are also right about the improvement the system experienced when a 2nd water pump had been introduced. I think mad fridgie's design tackles all the issues you raised. Thank you for pointing this out!

[desA]

My concern is as follows:
It sounds like the heat-pump temp sensor is attached to a component (typically condenser) which is cooling off even if the water is pumped around e.g. discharge, or vapour line.

Under this scenario, & with the circulating pumps running, it would be best to re-locate the sensor onto a water pipe where it can pick up the temperature of the circulating flow, or better still, onto one of the hot water vessels.

Grab the manufacturer's technical representative & ask him why they build their units in this way.

[Gary]

It sounds to me like the heat pump sensor is attached to the outgoing water pipe, instead of the incoming water pipe.

And I still don't see the point in having two pump circuits... or piping the tanks in parallel.

[Gary]

I would leave the tanks piped as is, pipe the heat pump in series with the water returning from tank #3 as outlined previously, resulting in a one pump system. Then I would place a thermostat in the center of tank #3, which would cycle both heat pump and water pump.

To answer an earlier question... no, you can't use the electric backup thermostat in tank #3 unless you are willing to sacrifice the electric backup in that tank, but you can install a second thermostat right next to it.

[MikeHolm]

OK, I can't read the Mads pdf but from the original drawing I would say that the tanks should be in parallel as they will have different temps, as is, and the pumping power is un necessarily high. With this arrangement there will be a high dT across the buffer which I don't think is desired and I don't see any other advantage to having them in series as they are hydraulically separated already from the HP. It is hard to tell from the picture but it looks like the HX is in the middle of the tank which is a bit of a waste. Is it possible to get a spec sheet for the tanks showing tube diameter and length?

[yonatanb]

Gary,
A. wouldn't placing the the termostat in the 3rd and last tank cause frequent cycling, since each time an end-user takes a bath (although this is discouraged by the government here), this tank is injected with cold water? wouldn't the 2nd tank be a better choice for positioning a thermostat?
B. it seems there are 3 temperature sensors (which are the on the upper right corner of the unit's rotated connection circuitry scheme). one senseor is labeled "water input sensor", the next is labeled "water output sensor" and the third is labeled "battery sensor" (the gibrish is Hebrew). I feel I am misleading you here, since I do not know which of these the LED display actually displays.

[yonatanb]

Mad Fridgie's pdf again.

[Gary]

Considering the average shower uses 7 liters, I'm thinking it would take quite a few showers to substantially drop the mid level temperature in a 300 liter tank... and I would want the heating to start long before half of the hot water is used up.

In addition, I would not want the electric backup heater in tank#3 to be activated before the heat pump is activated.

[yonatanb]

Gary, Got it now. Is your argument valid even in my case where the heatpump is so powerful (too powerful) it can heat 30L within ~4 min? My calculation shows that the average in this building is 25L of hot water per shower (its so humid here, that nobody misses a shower, at least once a day). statistics of course is a higher form of a lie - there are users here that consume 40L-50L of hot water per shower (based on an annual survey). By choice, the electric backup here is manually switched on, since the conditions here are ideal for heatpump operation year long (theoretically speaking), and in order to have full control of the system, I do not want the direct electric heating to step in automatically.

[Gary]

You stated earlier that the operating range is 45C-55C. You also stated that that compressor starts when the display reads 50C. This would lead me to believe that the display is showing (and starting from) water output temperature, since the input ranges down to 45C.

[Gary]

Using the 50 liter figure: This would be 1/6th of the tank volume. If the tank is at 50C and 50 liters of water at 20C is injected, the average temperature would be 45C... IF the water were thoroughly mixed, which it is not... it is stratified.

[MikeHolm]

Mad Fridgie's pdf again.

I'm with Mad on this (sorry Gary). I have installed hundreds of systems with tanks like this and it is a traditional tank layout that works well. Gary is right though, that it would be best to start the heating sooner provided there is enough tank volume to prevent a high HP start rate. Your control issues will be much simplified.

By the way, I thought is was code in Israel to have a Solar water heater before the back up system.

[Gary]

The 7 liter figure is from memory... maybe it was 27... or maybe something entirely different... I'm old, you know... they say the second thing to go is the memory... I forget what the first thing was.

[MikeHolm]

The feeling in your toes, i think

[MikeHolm]

The typical low flow showerhead is 7-10L/min

[Gary]

The only advantage I see to parallel flow is more reliable water pressure at the showerheads. Series flow would give more reliable water temperature at the showerheads.

[mad fridgie]

How do you achieve high efficiency with a heat pump. "keeping the discharge pressure low for as long as possible"
How do you achieve this in a thermal storage system, By heating a store of cold water.
Start from the beginning cold tanks), tanks and H/E in parallel. The heat is introduced into the tanks, natural stratification will cause the heat to rise to the top. Discharge pressures will remain low, slowly increasing over time.
When the tanks reach set point half way up, most of the mass of water in the tank will be close to the set point temperature. Units turns off. So good so far?
Right, somebody a has a small shower, will effect the control thermostat or the temp at the top of the tanks 'NO".
The use age increases, until about the half the tanks are now full of cold water. (the top is still hot) The heat pumps thinks it is heating cold water, so discharge pressures are low rising after time.
OK I do not like buffers or internal heat ex changers for this application.
I suspect that the reason for the the 2 pumps is the unknown pressure drops through the cylinder heat ex changers.

[yonatanb]

@ MikeHolm: regarding the specification of the 300L tank, i have no clue, and finding out will be practically impossible. Nevertheless, the design of this firm (named "Ideal" :-) should probably mimic the design of Israel's most valued off-the-shelf water tanks - Chromagen - located in Kibutz not far from Armageddon. For a specification of their tanks, take a look in their single spiral 300L tank in this document on pp. 32. The inlets & outlets are of course differently positioned, but the tube specification should be similar. The supplier did tell me, howerver, that the spiral heat exchanger is 12[meter] long. I am impressed by your knowledge of the local codex. In case i am not mistaken, for ~30y now, it is obligatory to install, in buildings up to 9 stories high, a public solar system backed up by electric heaters. each appartment has a small tank with a spiral heat exchanger, which is fed by solar heated water. each tank also has electric heating backup. Generally speaking, basic passive solar water heating is THE most efficient and cost effective way to boil water in Israel. Unfortunately, this building was built 31 year ago, and the contractor chose another heating option - a central diesel water heating system. Since the penthouses in this building occupy most of the roof area, we are only left with 15[m^2] of public roof, and after seriously considering a pure solar system, it was dropped due to bureaucratic reasons way-way-off this topic. Actually, i made an exhaustive economical reserch regarding the available heating alternatives: I calculated the net present value of each option, taking into account even the rate of accumulation of scale, and the resulting degradation this inflicts on the electric and solar systems. @Gary: I think that having to do with alternative energy resources is good for your health. Take, Zvi Taborr, for example (a native English speaker as well). This kid is only 94 year old, and his mind is crystal clear.

[yonatanb]

@ Mad Fridge:

when only a single pump was installed the compressor cycled more than 100[times]/24[hr].

increasing deltaT from 6 to 8°C dropped this to ~70[times]/24[hr].

installing a seconf water pump resulted in a fruthe decrease to ~30[times]/24[hr].

[mad fridgie]

Had a look at your cylinders, as normal short of surface area for heat pump applications (the coils are not big enough) But they are at the bottom which is good.
Measure the temperature coming out of the cylinders (feed to the showers)
around the world designers are using boiler or solar systems for heat pump applications, and these methods are incorrect.
If you want 50C in the storage cylinders, then really you need at 60C feeding the internal heat exchangers.
The efficiency of a heat pump is determined by the temperature/flow of the water entering the heat pump heating side, colder the water/higher the flow the more efficient.
The short cycling is an effect, not a cause. I could just tell you to reduce your cycling, but that would just be another mask, not fixing the problem.
Even though I have given you a drawing, this for your application and cheap to fix, this is not what i would design from new.

[chillerman2006]

I am following this closely here, learning plenty, am definately going to make sure I order a 'mad fridgie' from my suppliers this week and stick a mini you in my toolbox, to make all I dont quite understand fall into place.... and am really looking forward to our pint and seeing your new 'Heat Pump' release for the uk....Top Man MF & thanks for looking at this for Yonatanb

R's chillerman

[Gary]

The efficiency of a heat pump is determined by the temperature/flow of the water entering the heat pump heating side, colder the water/higher the flow the more efficient.

I would assume that series tanks with stratification would be more effective at separation of water temperatures than parallel tanks with stratification, and therefore provide colder water to the heat pump for a longer period of time... on the other hand, I assume you have tested both strategies and found the opposite to be true?

[chillerman2006]

Apologies Gary, if that read odd, I was not glossing over your expertise in just about everything....I just thanking 'Mad' for taking the time to look at this for yonatanb as per the message I sent him the other day

R's chillerman

[Gary]

[Jedi hand gesture]
These are not the droids you are looking for. Gary is an expert at just about everything. Move along.
[/Jedi hand gesture]