Buffer Tank Assisted Defrost

[pilko]

Hi everyone,
I am new to this forum and as I was just lurking,I discovered buffer tanks.
I run a 2 1/2 ton air to air unit.
I would like to use a buffer tank to assist the defrost process and eliminate the
cold air into my living area by stopping the indoor fan during defrost.
If this mod.is feasable would the buffer tank/heat exchanger be installed in the
position shown in the upper diagram or lower diagram?

Regards,

pilko.

[Bigfreeze]

Not really feasible as yours is an air to air unit and therefore the unit is designed to heat air and not water.
In diagram one, when heating, the majority of heat will be lost to the buffer until the buffer reaches temp which means cooler air at your duct for a long period of time and my guess is it could become very problematic when the system defrosts as the hot water will raise the temp of the suction gas to such a level that the cooling that the gas provides to the compressor would be virtually nil, so your compressor life would be shortened.

In diagram 2, it adds no benefit to the system at all as in heating it will act as it does now except it will also have 200L of water to heat and in defrost you will still get cold air at your duct, although your defrost may be a little quick due to the extra energy provided by the tank.

If it was me I'd leave well enough alone. They're not very compatible in this application.

[pilko]

Thanks Bigfreeze,
My way of thinking (although it may be flawed) is as follows:-
In scenario #2 in heating mode the vapour will be cooled as it heats the buffer water on is way to the compressor.
In defrost mode, the vapour heats up as it passes through the buffer on its way to the indoor coil. The indoor fan would not be running so the heated vapour will continue out to be compressed and then heat the frosted outdoor coil.

regards

pilko

[nevgee]

Have you considered the amount of superheat and what effect that would have on the compressor?

[mad fridgie]

The fan should stop on defrost any way, and not restart until the coil has warmed back up.

[pilko]

@ vevgee
"Have you considered the amount of superheat and what effect that would have on the compressor?"
I would ensure that the amount of heat recouped is no greater than that in a normal defrost.

@ mad fridgie
"The fan should stop on defrost any way, and not restart until the coil has warmed back up."
The indoor fan does not stop during a normal defrost as it is required, to pull heat from inside the home in order to heat up the cold vapour and hence the outdoor coil.

The heat I am suggesting using from the buffer tank would be used instead of that heat which is pulled from the house which causes cold draught and room cooling.

[mad fridgie]

You have enough thermal mass, natural convection and heat of compression to defrost the outdoor coild without the fan running.

[pilko]

@ mad fridgie
"You have enough thermal mass, natural convection and heat of compression to defrost the outdoor coild without the fan running."
The only thermal mass that I have (without the buffer) is in the 20 feet or so of liquid line.

regards

pilko

[mad fridgie]

What about the mass of the coil, the hot refrigerant within the coil and the mass of the housing
This nothing new, this how it is done in most parts of the world, I have had these in my home for 20 years, no problems. Just try it!

[pilko]

Thanks so much mad fridgie.
I am certainly going to try it as soon as the OD temp dips again. We are having a heat wave right now (+6C)
This has blown me away, every one I know has their indoor fan running during defrost.
I will report back as soon as I have tried it.

regards

pilko

[MikeHolm]

Pilko, where are you?

[pilko]

Hi Mike,
I live in New Germany, Nova Scotia. My son lives in Toronto so I go there quite often.

pilko

[MikeHolm]

Cool, I'll be in Mahone Bay for Xmas

[pilko]

Great,--- Mahone Bay is a beautifull spot.
I just checked your site. I see you're an Electronics Tech. Electronics is my background too.
My Heat Pump is my hobby and I have made several mods to the control circuitry. I also measure and track the parameters on my computer and display them on my TV ( the missis thinks I'm crazy)

pilko

[MikeHolm]

My electronics was farther in the past than I want to admit and for some reason I am much better with relays and old style controls than figuring out pre-programmed controllers. I've been doing way more boilers and solar than pure electronics anyway. BTW, my misses KNOWS I'm crazy

[tectron]

low pressure problem in defrost mode on air water heat pump with EEV
ideas?

[Bigfreeze]

low pressure problem in defrost mode on air water heat pump with EEV
ideas?

Lack of water flow.
Water drops in temp, which causes gas temp and pressure to drop. Its possible the safety for the HX is controlled by low pressure rather than a min flow temp, hence the LP fault.

[pilko]

@ mad fridgie
I did some more research and testing re. your recommendation to stop the internal fan during defrost.I realize that is what you are doing but I am nervous.
Firstly my supplier says he doesn't recommend it. Then I found the attached comment on the HVAC Talk forum.
Because of my nervousness I did two defrost tests,one at 960 CFM and the other with a lower fan speed producing 530 CFM (the graphs are attached).
I would greatly appreciate your comments --- or anyone elses for that matter.

Regards

pilko

[mad fridgie]

Just a question first, you do turn off the outdoor fan during defrost!

[pilko]

"Just a question first, you do turn off the outdoor fan during defrost! "

Yes

[mad fridgie]

It would seem that in your neck of the woods, that old principles rule.
With the fan off will the indoor coil become very cold, "yes" who cares!, your discharge pressures will be very low, so compression ratios will be low so no issues.
Do your system have an accumulator.
Your outdoor coil 'in defrost' should be temp terminated, not just time.
You indoor fan should restart on temp (when the coil gets above 40C, to stop cold draft start).

[pilko]

@mad fridgie,
I really appreciate your help and patience.
I have just received another conflicting answer in the last few minutes from the HVAC Talk forum.

regards

pilko

[pilko]

Hard to read ---attaching link

http://hvac-talk.com/vbb/showthread.php?t=978262

[mad fridgie]

Of course a big heater would cure the cold draft problem. No arguments.

It is clear that your type of system is common in North America, I suspect they are just AC units, with a reversing valve installed, basic controls and not real thought but into the heating side.

This issue has been resolved for at least 20 years, check out all your well known Japaneses brands.

[MikeHolm]

There are a lot of Chinese brands starting to be installed around, Haier and the like. A 4kw HP is going in for about $1800-2000 CDN from what I hear. I suspect there is not much knowledge on service by the installers, knowing most of our trades.

Most of the Japanese ones are Fujitsu and Mits. Daiken has only just started selling in North America.

[pilko]

@ mad fridgie,
Are you by any chance refering to the split type with a wall unit whereas the others and myself are refering to the split, ducted air system.--- I just wondered if that would explain the differences in opinion.

pilko

[MikeHolm]

The rest of the world, to my knowledge, does not heat homes the way we do, forced air, A coils etc. so all the stories i hear are using the split wall hung systems, fujitsu, mitsubishi etc.

Sorry to answer for you Mad. Deny me beer if I am wrong.LOL

[mad fridgie]

"No" a fully "16Kw nominal" ducted system (heating 220M2) 7 rooms (11 outlets) with hall acting as a mixing chamber (so indirectly heated by all the return air) outdoor bit is outdoors, indoor bit is in the roof cavity. 12 meters pipe between. This one is toshiba, the last one had 2 outdoor units to one indoor unit (daikin). Similar set up (more rooms) We get down to -6C outside, design is based upon 2C averaged.
Regardless if ducted or high wall the same problems occur. The refrigerant does not know if it is in a duct or on the wall.

[pilko]

Hi mad fridgie I do not doubt what you are doing at all, I am envious but nervous.
Please check the link in post #23, there have been some additional comments on the other site.
Would it be possible for you to post your comments on that site as they are adamant about not stopping the fan.
It is hard to make a decision when experts have opposing opinions.

regards

pilko

[mad fridgie]

I do not, have any aux heaters in my duct!
Your ducting, are the outlets at high level or low level?

[mad fridgie]

Tried getting onto the other site, can not remember password.

[pilko]

@ mad fridgie,
My ducting, indoor fan and "A" coil are all in my basement. My outlets/registers are on the main floor (bungalow)
Did you read the comments on the other site?

pilko

[pilko]

This shows performance for last 2 hours.
Defrosting seperation 72 minutes.

Green = Discharge air temp
Dark blue = Return air temp
Green = Outdoor temp
Light blue = Frost level on outdoor coil

pilko

[mad fridgie]

Right time to get technical (copy onto the other site)

Rough examples

In normal heating mode ; (R22 I guess)
1;ambient = -2C
2: return air = 20C
3;Estimated SST -8C
4;Estimated SCT 45C
Basic COP = 2.85 (heat)

When defrost with fan on!
SST 0C
SCT 27C (min limit for comp, but would expect lower in real world)
COP 5.51 (heat) Remember that the energy for defrost comes from inside your house, and has to replaced.The cooling COP is much higher

So what does this mean, you require twice as much energy to replace the heat you loose when on defrost when the fan is running.

Now with out fan running (no cold draft and minimum energy drawn from the house)
SST -20C
SCT 27C
COP 3.39 (heat)

So as you can see, the lower COP in this defrost cycle means less energy is removed form your house.

Also remember that on this nominal 10Kw the comp we still draw 2.3Kw, which is defrost the evap coil (which becomes a condenser during defrost) What does this mean in defrost terms, the comp power draw alone will defrost 54.5lb of water an hour. 1/2 the weight for 1/2hour and so on. You are still getting some energy from the mass of the indoor coil and surroundings.

Defrost will be slower, as the total amount of heat rejected to the defrosting coil will be less when the fan is not running.

Well I hope that makes sense

[mad fridgie]

Yes did read what they said.
The only thing I do not know is how your machines are built or designed, I suspect same design as Noah used on his ark!
You need a control scenario
The fan is OFF during defrost
The fan ONLY comes on when the indoor coil is up above 40C (this reduces the cold draft) However you still have a bit of cold air in the duct, I presume your basement is colder than your living areas!
Defrost is terminated when the outdoor coil is above 4C (then time as back up)

[pilko]

@ mad fridgie,
"Right time to get technical (copy onto the other site) --- do you mean you have copied to the other site or are you asking me to?

pilko

[mad fridgie]

Yes...............................you

[pilko]

---------------Done

[mad fridgie]

Just seen the experts response. What expert!!!!!!!!!!!!!!!!!!!!!! (on another site)

Of course is takes the same amount of energy to defrost 10lb of ice. Who said it did not!

The argument is that there is not enough energy to defrost coil without the fan running.
I proved that there is, that it is more energy efficient and most of all more comfortable.

I get the impression you do not even have temp defrost termination!

[mad fridgie]

Mad Fridgie is barbar

[pilko]

@ mad fridge,
"I get the impression you do not even have temp defrost termination!"
Are you refering yo me or the expert on the other site.(this is all getting very confusing)

pilko

[mad fridgie]

So Pilko, to both really, tried to post of hvac-talk, sort got kicked off,
I question and gave answer to your questions, but they have been deleted.
So I have tried another approach, lets see what happens.
No kicked of that way as well?
So sorry can not help on the other forum.

[pilko]

In an attempt to settle the argument I have recorded a defrost at 1000 CFM and a second defrost at 500 CFM.

Red = Discharge air temp From "A" coil
Dark blue = Return air temp to "A" coil
Green = Outdoor temp
Light blue = Frost level on outdoor coil

[mad fridgie]

Do this by hand
Turn off the fan, at start defrost! Do not turn the indoor fan back on until the coil reaches 40C! (back in heating mode)
Turn defrost off when outdoor coil is free of Ice (not on time)
Oil is not an issue!
The only reliability issue is the chance of liquid flood back, do you have an accumulator ( a big thing before the comp inlet "big pipe") if not then only "maybe" would you need the fan on. (a bit more complicated than I just stated)

[mad fridgie]

You are measuring all the wrong data!
But i am also interested in how you are measuring your ice build up?

[pilko]

Do this by hand
Turn off the fan, at start defrost! Do not turn the indoor fan back on until the coil reaches 40C! (back in heating mode)
Turn defrost off when outdoor coil is free of Ice (not on time)
Oil is not an issue!
The only reliability issue is the chance of liquid flood back, do you have an accumulator ( a big thing before the comp inlet "big pipe") if not then only "maybe" would you need the fan on. (a bit more complicated than I just stated)

The test I just did at 500 CFM was as follows:-
1 As soon a HP went on DF I reduced fan speed to give 50% CFM
2 My defrost ends as soon as the frost is removed (detected by my frost sensor)

pilko

[pilko]

There is no accumulator.

[mad fridgie]

Good about the defrost termination.
Turn of the fan.
Do not measure the coil outlet temp. It will give you false readings. Less airflow lower temps, no air flow very low temps. Less mass so reduced energy removed from your house.
Turn fan back on only when coil is hot.

[750 Valve]

This is quite a funny read, I have long subscribed to the theory that the North America is about 10 years behind the rest of the world when it comes to refrig and air and this proves it!

I can't believe it's that hard a concept to grasp! The rest of the world caught on and designed R/C systems the same way for the last 15 to 20 years but no, good old US of A reckons it can't be done!

[mad fridgie]

Not to good then!
We measure pressures and temp around the comp (as the temps you are reading are the process variables (after the fact) not the system variables (driving force)