air to air HP question

hot_rod

I installed a new Goodman HP in office/ shop this summer. A friend that is an HVAC tech did the install and fired it up for AC.

I had him add a HW coil with the unit and I can supply that from the wood boiler or the Phoenix LP fired unit.

I want to add an outdoor thermostat to enable the HW coil.

1) at what temperature do you switch the HW coil? I've heard anywhere from 0- 20F.
2) when I switch to the HW coil should the compressor power be interrupted? Or should the compressor run when back up HW coil is running?

JUGHNE

Where is the AC coil in relationship to HW coil. Would the HW air pass thru the ac coil?

NYplumber

HR here is my novice answer.

Contact the manufacturer for a graph/table on the COP. Calculate your electric rates. And calculate the btu output as the temperature outdoors dives. I would assume you will have the HP turn off at around 40*f outdoor temp. You can use a differential controller with NO and NC relays to open the low voltage wires to the HP and close the low voltage to the relay/pumpg control that will tell the boiler to flow.

You should also configure the HP to trigger the boiler to heat when the HP is in defrost mode. If i recall there is a dwyer defrost control that minimizes unnecessary HP defrost cycles.

Looking forward to hear how you go about the project.

njtommy

Well being that you have two hot water heat sources it really depends on you. If you where running the wood boiler I would not even being running the heat pump do to close to free heat. But I'm assuming you have propane for the other boiler. If this is the case I would recommend finding the balance point of your system/ heat loss. Once the Hp can't handle the load any more just switch over to the HW coil. I would just wire it in as W2 or second stage and have an strap on aqua stat bring the fan on. This is pretty much the same set up I have in my own home. I use a Honeywell t-stat with out door air sensor. The T- stat will switch over from hp to HW once the out side temp drops.
On the Honeywell t-stat it will also give you the option if you want to be able to run the heat pump and HW coil at the same time or not.

hvacfreak2

I used 35 degrees on dual fuel installs years ago and I know that todays systems can go much lower. Start at 30 would be my answer for a starting point and see how it does.

I would say to break the Y terminal to the outdoor unit on the change over ( balance set point making ) . If the HW coil is on the leaving air side of the air handler you can use that as the
" Auxiliary " heat portion ( terminal W1 on thermostat ) , in which case the compressor and HW coil are operating at the same time ( as far as Y and W1 ) until the balance point stops the heat pump operation. Bryant / Carrier thermostats are great for this
( " Edge " is the model for the stat ) .

SWEI

Is this an inverter-driven design or a standard single speed type?

The COP/output table together with the LPG/electric rates are the best route. If it's a single speed compressor you will probably hit the low ambient capacity limit before the dollars cross over.

hot_rod

great info, thanks.

The coil is down stream, downwind I suppose, of the AC coil.

Standard single speed compressor.

I assume the balance point is when the unit runs non-stop and can no longer meet the load, that would be the lowest COP?

The manual shows what looks like a Ranco cap. tube "outdoor thermostat" control 0- 50f adjustable, mounts right in the outside unit.

I have the mid level Ecobee, so I can configure the call to the zone valve for HW backup. I can easily configure the LP fired Phoenix as back up.
The wood fired boiler is still somewhat interactive, it only runs when i'm around to tend to it.

The office had in slab radiant, but with this warmer weather, and wide temperature swings from day to evening, also a lot of south glass it is less controllable compared to the FA HP.

This is the first season with forced air used for heating, so far it has been more comfortable for the gals in the office. Maybe the noise of the blower lets them know they are warm and comfortable.

JUGHNE

Having danced with a few air/air HP's for about 30 years, my conclusion is that the fewer defrosts it has to suffer thru the longer it lives. It is all a temp humidity issue. All the savings seen in COP can go quickly if the compressor goes south. If warranty gives you a new complete unit, not bad. But only the compressor is a costly band-aid, should change rev valve? add suction line filter, etc. I'm in northern NE, close to SD so that is a consideration. I know 1 or 2 states south they are the cat's meow. And I am spoiled by in-floor anyway

njtommy

I will say these scroll compressors seem to take a hell of a beating.
I switch over at 35f to limit my amount of defrosts.
The other thing I noticed is that when you go into defrost you most likely won't get hot water fast enough to the coil to over come the cooler air.
Yes humidity comes in to a big play. The higher the humidity the warmer you will feel.

SWEI

Very interested in this from a controls perspective, so help me learn here. Most of our systems are equipped with outdoor temp/humidity sensors, from which we can easily calculate dewpoint. Is there a strategy we might employ to optimize this?

TIA

hvacfreak2

the " balance point " in my post was referring to the outdoor change-over set point that stops the heat pump operation .

edit sorry , re -read your post. The lowest COP is technically correct I believe ( as to how this is " calculated " ) . As jughne pointed out we calculate it to stay out of defrost in the field , lol.

http://hvac-talk.com/vbb/showthread.php?437432-Calculator-for-heating-balance-point-for-heat-pump-with-gas-furnace-(dual-fuel)

hvacfreak2

SWEI said:

Very interested in this from a controls perspective, so help me learn here. Most of our systems are equipped with outdoor temp/humidity sensors, from which we can easily calculate dewpoint. Is there a strategy we might employ to optimize this?

TIA

With a programmable controller you could use outdoor dew point / temperature to reset a discharge air temperature ( or water temp. ) if that is what you were referring. If this was referring to changeover ,


http://hvac-talk.com/vbb/showthread.php?437432-Calculator-for-heating-balance-point-for-heat-pump-with-gas-furnace-(dual-fuel)

JUGHNE

IMO the outdoor humidity increases the defrosts because the more frost/ice developed from higher humidity reduces the air flow thru coil, lowering coil temp and asking for more defrost cycles even if outside temp is not that cold. FWIW

hvacfreak2

JUGHNE said:

IMO the outdoor humidity increases the defrosts because the more frost/ice developed from higher humidity reduces the air flow thru coil, lowering coil temp and asking for more defrost cycles even if outside temp is not that cold. FWIW

I lost a couple of posts trying to post a link , but yeah, plus one to this also.

njtommy

I have a buddy who changes his out door set point depending on the weather. He will run his HP down to 20f as long as we have no snow or rain. If we are expecting bad weather he just runs his 2 stage gas furnace gas furnace. Like every thing else you can go really crazy with these setups.

JUGHNE

Everyone needs a hobby.....but if he has NG I would go down to 30 and enjoy warmer discharge air in the house.

Techman

Finding "the Balance point" is a little diff than "knowing"what the balance point is. The BP used to be typically around 35-40*. I'm not sure about the BP being in the 20's. I think that would depend on the "sizing"of the heatpump(AC load) and that is kinda related to the structures construction. Way back there were "outdoor temp sensors" that brought on the 2nd stage of heat(1st stage of electric) the depending on the setup there were two more "outdoor"sensors.There are a couple of good suggestions here, so pick the 1 that suits your setup. Merry Christmas .

John Mills_5

To avoid defrosts, better shut off the pump at 45 or so. The evap (outdoor coil in heat mode) runs 10-15° below ambient so if the temp is in the lower 40s, the coil is running around freezing. Since most units have time/temp defrost, they will see the 30° coil and defrost, which probably won't be needed. With demand defrost, that won't happen. Been meaning to change my timed board with the new Emerson universal. I hate timers!

njtommy

Any up dates @hot rod ?

hot_rod

I did get it all working with a hand from my son Max, he is an Ecobee expert and it had plenty of adjustments inside for this.

Ecobee uses weather data, not actual temperature sensor at the shop, I have it set for 35° to start, I may bump to 40, I'd like to get SOME heat from the thing.

Ironman

I'm an ODW dealer in addition to being an HVAC contractor that specializes in hydronics. Here's how I've been integrating ODWBs with FA and hydronic systems for a long time:

1.Use an ETC that senses the supply water temp of the wood boiler.
2. With your setup that has a gas boiler, I would let the ETC choose between the wood boiler or the gas boiler depending upon whether the wood boiler SWT was hot or not.
3. Run the W2 wire from your stat to the "C" terminal of the ETC, the "NC" to the gas boiler and the "NO" to the pump on the wood boiler. A relay may be necessary to energize the wood boiler circ. I'm sure you'll know the particulars, I'm just giving the concept.

With this setup, you select the the balance point that suites to switch over to fossil fuel on then Eccobee. When the stat is set to "HEAT", the HP runs until the balance point and then switches to fossil fuel. If the wood boiler is hot, that will be the source; if not, the gas will be. When set to "EMERGENCY HEAT", only the fossil fuel will run, but the ETC will automatically choose the source.

hot_rod

Ironman said:

I'm an ODW dealer in addition to being an HVAC contractor that specializes in hydronics. Here's how I've been integrating ODWBs with FA and hydronic systems for a long time:

1.Use an ETC that senses the supply water temp of the wood boiler.
2. With your setup that has a gas boiler, I would let the ETC choose between the wood boiler or the gas boiler depending upon whether the wood boiler SWT was hot or not.
3. Run the W2 wire from your stat to the "C" terminal of the ETC, the "NC" to the gas boiler and the "NO" to the pump on the wood boiler. A relay may be necessary to energize the wood boiler circ. I'm sure you'll know the particulars, I'm just giving the concept.

With this setup, you select the the balance point that suites to switch over to fossil fuel on then Eccobee. When the stat is set to "HEAT", the HP runs until the balance point and then switches to fossil fuel. If the wood boiler is hot, that will be the source; if not, the gas will be. When set to "EMERGENCY HEAT", only the fossil fuel will run, but the ETC will automatically choose the source.

Thanks, I like that simple logic, I have about the same with a couple I-solar controls, since I get a good deal on them!

The solar control gives me timer, data logging and VS function all in one box. It also has some wood boiler functions, and my drawback solar feature.

Ironman

Awesome. I should have realized that you'd have something like that on hand.