Carrier Heat Pump

[acnerd]

Seeing as it's coming up to winter again, I thought I would ask the question. One of my customers rely solely on underfloor heating in a converted barn. Their heat source is a Carrier 30RA-033 heat pump air cooled unit. See link below: (The model number may be slightly different!)

http://www.carrieraircon.co.uk/index...quid_chillers/

In the dead of winter, the climate in which this unit operates can regularly fall below zero degrees Celsius. This unit needs to maintain 50'C water. Unfortunately it spends a lot of its time defrosting (quite a moist air) as much as every 20 minutes if below zero. January this year it was standing in 3 inches of ice!!

I know the refrigerant charge (R407c) is correct as I have had to repair a leak recently. I also understand that efficiency in heating can drop rapidly the colder it gets outside. So the question is: Can I do or add anything to the system to prevent it icing up as much and allow it to run in heating longer? I have checked the frost build-up on the condenser coil and it DOES need to defrost often, so it wouldn't be a straight case of modifying defrost times.

Any suggestions gratefully appreciated.

Craig

[goshen]

hi there if unit operates at temps u stated there realy is not much u could do with out modifing the entire machine.
however i would suggest to add a heating element that would heat the water when the temp goes well below 0 deg c or add a buffer tank that would store the heat ,should make life of machine a little easier.as u said when your energy is going twardes defrost cycle it is wasted energy that is why a backup heting element is probably the best idea .
we have had very good experiance with a heatpump inverter controled unit by mcquay even at below o deg the unit kept going with minimal defrost time, but! this unit was designed for work till -10 deg
good luck

[acnerd]

hi there if unit operates at temps u stated there realy is not much u could do with out modifing the entire machine.
however i would suggest to add a heating element that would heat the water when the temp goes well below 0 deg c or add a buffer tank that would store the heat ,should make life of machine a little easier.as u said when your energy is going twardes defrost cycle it is wasted energy that is why a backup heting element is probably the best idea .
we have had very good experiance with a heatpump inverter controled unit by mcquay even at below o deg the unit kept going with minimal defrost time, but! this unit was designed for work till -10 deg
good luck

Thank you Goshen, that's a good idea which I never thought of!

[desA]

If you really want to make a simple fix, place a heater box over the AWHP inlet, which uses finned electrical heating elements. This will provide warm air onto the evap coil.

Of course, the air heater could also have steam coils as the heat source - anything to raise the air temp slightly. You could also pass returning water from the system back through the coil, if this is still warm - in other words, cool it a bit further through the air-warmer... This should cut down on the coil defrosting.

[goshen]

If you really want to make a simple fix, place a heater box over the AWHP inlet, which uses finned electrical heating elements. This will provide warm air onto the evap coil.

Of course, the air heater could also have steam coils as the heat source - anything to raise the air temp slightly. You could also pass returning water from the system back through the coil, if this is still warm - in other words, cool it a bit further through the air-warmer... This should cut down on the coil defrosting.

BUT WOULD AGAIN WASTE ENERGY!!

[Gary]

The bottom line is that anything you do to avoid defrosting is going to be less efficient than defrosting. I would leave it as is.

[mad fridgie]

If you really want to make a simple fix, place a heater box over the AWHP inlet, which uses finned electrical heating elements. This will provide warm air onto the evap coil.

Of course, the air heater could also have steam coils as the heat source - anything to raise the air temp slightly. You could also pass returning water from the system back through the coil, if this is still warm - in other words, cool it a bit further through the air-warmer... This should cut down on the coil defrosting.

NO. losses out way benifits
If you decide to use an element directly into the water.
Storage tank, unless it is massive. other wise no.
Do you have plenty of water, could look at water for defrost (from cold tap), during cold periods (a bit of smart think is required)

[mad fridgie]

Or use an EPR valve to keep the coil temp just above freezing.

Coil with EPR above freezing, ambient below freezing. Where is the heat exchange?
Will the energy from the coil go into the air if the coil is warmer.
I may a have misread, but I sure he wants to heat his house not cool it.
Second law of thermodynamics, energy travels from positive to negative!

[Gary]

Coil with EPR above freezing, ambient below freezing. Where is the heat exchange?
Will the energy from the coil go into the air if the coil is warmer.
I may a have misread, but I sure he wants to heat his house not cool it.
Second law of thermodynamics, energy travels from positive to negative!

Yeah... I thought about that after I posted it... couldn't delete it fast enough... lol

[goshen]

The bottom line is that anything you do to avoid defrosting is going to be less efficient than defrosting. I would leave it as is.

well said
however i did see a co2 unit that is capabale of heating water till -20 deg c did anybody else hear of these units made by sanyo?

[desA]

NO. losses out way benifits
If you decide to use an element directly into the water.
Storage tank, unless it is massive. other wise no.
Do you have plenty of water, could look at water for defrost (from cold tap), during cold periods (a bit of smart think is required)

Why bother with an AWHP, when its obviously either in the wrong location, or wrong application - at that time of year.

It is inappropriate technology, as it stands, therefore any 'fixes' will be far from optimal, but they will get the guy's system working.

Rip it out & put it in the attic. The cows belching & passing wind will do the rest...

[mad fridgie]

The cows belching & passing wind will do the rest...

" Luvin it"

[mad fridgie]

well said
however i did see a co2 unit that is capabale of heating water till -20 deg c did anybody else hear of these units made by sanyo?

Still needs to defrost, industry name for these products Ecocute

[acnerd]

Thank you for all the replies. I agree that it is doing the best it can in the conditions it operates in .... but tell this to a freezing customer!

My opinion is that the rural location of the barn had no gas mains to it, as a boiler would have been the best application for it; and an overzealous consultant decided a heatpump was the bees knees.

The barn is actually an office with hardly any employees in it (about 5) with high vaulted ceilings, so the heat goes up anyway. Problem being that if the room temp goes below 16'C, they have a legal right to go home.

So we agree on two things: a) Not the right unit for the job, (b) any task of adding heat around the evaporator coils to reduce frost build-up results in increased energy usage.

I still like the idea of the buffer tank though. There is a main header loop of about 30m in total between AWHP and plant room, after which a 3-way valve and circulating pump push the water around the underfloor manifolds.

[acnerd]

Although it just occurred to me that the buffer tank may not work. The defrost function is reverse hot gas, so water that is at 50'C becomes 30'C due to the obvious cooling cycle (plate heat exchanger becomes evaporator).

So then I would sit with a buffer tank that is trying to heat up water that the heat-pump is cooling down in defrost mode!!

[goshen]

hi the idea is that when you have a buffer tank u will indeed cool it a little when defrosting, but the enertia of the tank will hold, u will loose probably 2 -5 deg c
another option is having an outside thermostat that will activate a heating element in the tank and stop the heat pump using its power and raising water temp to resonable values and vice versa .the hardest time for a heat pump is the first climb when the water is cold once the water is hot 30-35 the heatpump should handle no problem unless -5 deg outside .
dont forget a defrost cycle should be short if u have a buffer because head pressure is higher !!

[mad fridgie]

Acnerd,
Right we can sort this out. Now we know your application.
Questions
Is Heat pump only on when when the office in use?
What is your norminal Kw output of your heat pump.
Have you calculated the load of your building? what is it.
What is the floor area of the building.?
How much piping is in the foor?
What are your pipe centres.?
How many Circuits?
What is the total water flow through the floor.?
What is the temperature difference between water enter the floor and watr leaving the floor

[acnerd]

Acnerd,
Right we can sort this out. Now we know your application.
Questions
Is Heat pump only on when when the office in use?
What is your norminal Kw output of your heat pump.
Have you calculated the load of your building? what is it.
What is the floor area of the building.?
How much piping is in the foor?
What are your pipe centres.?
How many Circuits?
What is the total water flow through the floor.?
What is the temperature difference between water enter the floor and watr leaving the floor

This information I will have to find out next time I'm on that site. I am not sure if I will be able to tell you pipe lengths and spacing under the floor, other than it is fed from 5 manifolds and there is 100mm screed on top of the pipes.

To what end are you heading based on this information?

[mad fridgie]

The biggest mistakes with underfloor heating and heat pumps is that they are designed around boiler systems.
So with the limited information, here we go. (some of this may not may sense, I will try to explain) do not comment on a comment until you have read all.
100mm scree, very slow reaction time.
Needs to be on 24hrs per day. this will act as a thermal store.
Do NOT use air thermostats (wire out) can be affected by solar gain!
You need to dramactically increase the water flow through the underfloor.
Approx 10L/M per circuit (not manifold) This is done to reduce the TD of the inlet and outlet water, and gives a much more even heat distribution and thermal storage.
You need to control system on return water temperature. (Low to mid 30s)
Supply temp is not important, should be 3-5C higher than return.
This may seem condridictory to what you think. If you have high inlet and low flow heat is concentrated in a small area.
Make sure you always have a high flow of water through the heat pump.
This will increase you nett heating and reduce power (much better COPs)
Defrost nature of the beast, these changes should reduce the effect seen by the defrost.
If the unit is to small then, you have to add a second.
there will be times when it is slightly under, (extreme cold normal design practice), have an element in series with the underfloor supply.
Is there some sort of tank.
make sure it is stratified and not a mixing tank (big mistake).

[acnerd]

They are good points, thanks. We do use an air thermostat, but as you mentioned they aren't the best.... it sits on one side of the main area and controls the entire building. Yes 100mm screed is very slow to react, a point I have made countless times.

From memory we supply the main header loop with 50'C, and the circulating pump will supply water to the floor at around 35'C. There is about 3 to 5 TD. The system doesn't run 24 hours per day, but about 16 hours.

Correct me if I'm wrong : Increased flow rate through the heat exchanger would reduce head pressure which will cause lower suction pressure, thus lower evaporating temperature, thus quicker frost build-up? It's capillary injection into the condenser, not TXV.

Thanks for your help so far. I was speaking to my manager this morning about it, and with the current economic climate, doesn't think they would spend money anyway.

[mad fridgie]

Sorry i did not realise is was a capillary, (I think you mean the evaporator, the outside coil)
Needs to be TXV.
Needs (to be able) to run 24hrs.
Within your 16hrs of running it still has to produce enough energy for 24hrs of heat loss, due to the slow reaction time the client is not really saving money, only making them selves uncomftable.
One think you need to be carefull of is there is not heat transfer between water enter and exiting the floor (giving false readings) (increase flow rate, no detriment (well abit more power), only benefit)
Does the floor have coverings (carpet) on it?

[acnerd]

Yes I meant into the condenser coil in heating; as you said the evaporator. You've got me thinking now, perhaps there are two TXV's in the outdoor unit with bypass on them for heating or cooling. Will check next time I go there.

There are no carpets on the floor, plain tiles.

Perhaps I will suggest running the system for 24 hrs a day in the winter and see how we do. If the floor stays warm, then less run-time. I see your point. After being off for 8 hours, it then has the laborious task of reheating the floor. Of course the heat-pump is available to run 24 hours a day, so it is always keeping the water at 50'C. (I have set the indoor 3-way valve to start opening if the outside air drops below 18'C.)
But when we suddenly need to heat a lot of floor area, it will run a long time .... maybe that is where the problem starts.

[goshen]

hi there it is smart to leave the unit running 24-7 on cold days this will make a big diffrence in preformance!!!

[mad fridgie]

You really can not use any form of air temp (indoor or outdoor) as a control, your physical reaction time is to slow.
It could be as much as 12 hours before you see the results of any changes made.
The best way to think of this, is the concrete is the heater, keep this at a constant. If the ambient is warm then little energy will be released from the floor.
If you have a TXV then reduce your water temp, from 50C, you will get more nett heat energy and less input power

[acnerd]

Thanks for the help Mad Fridgie, I'll have a look next time on site. For now though, fly back to South Africa for two weeks holiday, so the work can wait!!

[mad fridgie]

Thanks for the help Mad Fridgie, I'll have a look next time on site. For now though, fly back to South Africa for two weeks holiday, so the work can wait!!

Enjoy, and have a cold one for me, Mad

[icecube51]

did you try to give it a nice warm blanket?
if i have trouble on a frozen unit,i give it a blanket,so the passing air is less cold.simple,but effective sometimes.
Ice

[Sledge]

Couple of questions...

is the unit used for AC as well? I dont remember ever seeing a system that chills the floor for cooling, but when you mentioned a second txv...

I am thinking the best suggestions

;is to leave as is
;to install a ground source loop (mucho $$$)
;install a second evaporator, pipe to use second evap as condensor when defrost is needed

[tonyhavcr]

There are some in my area that dill a large well and ues the water with a flat plate heat-exchanger for a out side coil to get heat they need it is -10 deg. in winter in my area.
I will say that I never tried this nor do I know the laws about this.