Advice on replacing both rooftop ACs with heat pumps

Topic: Advice on replacing both rooftop ACs with heat pumps (Read 3746 times) previous topic - next topic

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #50 – August 01, 2018, 07:32:06 pm

Final results are in. Dissapointing. The new ACs are only about 5dB quieter on the low fan setting. They are about the same noise level on the high fan.

Here is the table:

NOISE COMPARISON TABLE, 1997 PENGUINE I vs 2018 BRISK II AIR 15,000BTU AIR CONDITIONER
LOCATION      OLD AC NEW AC DIFFERENCE (db)
Couch         55      51            -4
Bath           63      58            -5
Bed           54      47            -7
AVERAGE                          -5
Note: Noise levels are rounded to nearest dB.

Some caveats:
1. The new ACs have up to 25% more airflow on high fan.
2. The new AC's noise is all fan noise, no annoying compressor hum.
3. No 'clunking' sound when the compressor starts and stops.
4. As mentioned before, there is an annoying 30HZ acoustic wave at regular locations throughout the rig. This may not be the AC's fault, but rather Foretravel's duct design.
5. To resolve the issue of return airflow interference with the control box, I pared down the box with tin snips and sheet metal formers. This allowed the box to be rotated 90 degrees, reducing the return air duct frontal surface area by 400% (16 vs 64 sq-in). What was Dometic thinking when they designed a huge control box that had to be fit inside the return air duct?
6. It's too bad the low fan setting is not as quiet as expected. Dometic should have a complaint files against them.

Would I do it again? Yes. Here are the reasons:

1. The old air-conditioners were leaking. There was caulking applied to the condensate pan, which was leaking. Someone also applied huge amount of caulk to the large 14x14 gaskets long ago, so torquing the four gasket bolts would not stop any leaking.

2. The heat pump is 2x more energy effieient than a heat strip. I care about saving money and the environment.

3. It's quieter, although marginally.

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #51 – August 01, 2018, 08:41:29 pm

Methinks the Gentleman damns with faint praise. I hope we are not faced with the A/C replacement conundrum anytime soon!

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #52 – August 01, 2018, 10:41:50 pm

What ya doing with the old ones?

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #53 – August 02, 2018, 12:02:43 am

https://www.youtube.com/watch?v=uCnsw9oRDsM
5 db is quite a difference. [to my old ears anyway]

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #54 – August 02, 2018, 01:26:14 am

You cannot Force 25% more airflow thru a duct system that was inadequate in the first place. Tim and others are correct it's air turbulence noise.. No magic fix here or anywhere else. Bigger units will make matters worse

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #55 – August 02, 2018, 06:05:14 am

Two replies.

1. Two old Penguin ACs are for sale for local pickup near the Pottstown, PA area. $50 each.

2. Our rigs are Hemholtz Resonators, causing that Booming sound. See attachment.
Example: Blowing over a bottle. The AC is the air source and our rigs are the resonance chambers.
Solution: I may experiment with reducing the booming sound by:

a. Design an anti-acoustic resonator chamber and install it out a window.
b. Modify the ductwork.
c. Close sliding doors.
d. Add dampening material.

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #56 – August 02, 2018, 07:53:46 am

Quote from: Old phart phred – August 02, 2018, 01:26:14 am

You cannot Force 25% more airflow thru a duct system that was inadequate in the first place.

So we who have no ducts can fit larger capacity units without suffering this booming noise? Extra points to the older coaches!

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #57 – August 02, 2018, 08:19:39 am

Sorry, but ducts or no ducts, we all travel in a big chamber with roof ACs that have a "neck' type air blower. Perhaps this is a case for a mini-split system, where the air handler is contained inside the rig.

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #58 – August 02, 2018, 01:09:45 pm

As long as your happy with them Tim, thats all that matters. I agree that 5 db is some reduction, nothing to write home about, but I would take it. Lets hope you have more longevity with these units than others seem to have had.

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #59 – August 02, 2018, 01:39:32 pm

Think its worth going with a 15000 btu unit if you have ducted air and need to replace your 13500 btu units. Only 10% more cooling, but unless really warm, you might be able to run only your back unit when sitting up front, and only your front unit when you go to bed. You do have to leave your bath/bedroom doors open a little bit to allow for return air flow. Little more flexibility in picking campgrounds because you probably can get by on just 30 amps.

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #60 – August 02, 2018, 02:27:26 pm

10db reduction is a 50% reduction in perceived sound. 5db should be quite noticeable

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #61 – August 02, 2018, 03:14:36 pm

Quote from: jcus – August 02, 2018, 01:39:32 pm

Think its worth going with a 15000 btu unit if you have ducted air and need to replace your 13500 btu units. Only 10% more cooling, but unless really warm, you might be able to run only your back unit when sitting up front, and only your front unit when you go to bed. You do have to leave your bath/bedroom doors open a little bit to allow for return air flow. Little more flexibility in picking campgrounds because you probably can get by on just 30 amps.

You can always open the vent under the ac just a little for airflow..Sorry but for me, larger is better when it comes to the AC

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #62 – August 02, 2018, 03:35:55 pm

"You can always open the vent under the ac just a little for airflow..Sorry but for me, larger is better when it comes to the AC"
Obviously if you need two ac's, you should run 2 ac's. but found that that "max cooling vent" on the bottom of the ac was a source of considerable noise even when closed, and air was going to the smaller vents. I cut a piece of reflectix and attached with double sided tape.
Maybe subjective, but seems quieter to me.

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #63 – August 02, 2018, 09:06:55 pm

In Houston, Texas we have to worry about humidity. If your ac unit is too big, the unit will not run long enough to lower the humidity

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #64 – August 02, 2018, 09:51:23 pm

Good idea about reducing noise caused by turbulent airflow around the cheap plastic diverter.
There is a lot of noise coming from this diverter because the full force of high-speed, 350 CFM air is being directed at it from the AC cool-air discharge duct. The diverter is molded on, so it can be overlayed with a piece of sheet metal that could be designed as an air splitter shaped like a curved inverted Vee. with, which would:

1. Smooth the airflow
2. Redirect airflow 90 degrees to the U270 ducts
3. Deaden noise

Give me some time and I'll experiment with this idea.

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #65 – August 02, 2018, 10:28:26 pm

I discovered on my 15K Penguin in that when the fan was in high speed it put out quite a bit more CFM than the old unit, also my sound meter measured an average 74 decibel. The airflow was high enough that I believe cause turbulence in banging off on the closed supply grill. I made an baffle (curved V Shape) that fitted into the small supplied plenum that spilt that airflow by directing it into each ducting opening. Sound meter went to 67 decibel and drop to 64 decibel in second speed after pull down.
Steve

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #66 – August 02, 2018, 10:42:12 pm

Quote from: Willy White – August 02, 2018, 10:28:26 pm

I discovered on my 15K Penguin in that when the fan was in high speed it put out quite a bit more CFM than the old unit, also my sound meter measured an average 74 decibel. The airflow was high enough that I believe cause turbulence in banging off on the closed supply grill. I made an baffle (curved V Shape) that fitted into the small supplied plenum that spilt that airflow by directing it into each ducting opening. Sound meter went to 67 decibel and drop to 64 decibel in second speed after pull down.
Steve

Steve, did you replace the supply grill after installing the baffle or use a piece of aluminum as Tim suggested?

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #67 – August 02, 2018, 11:08:25 pm

Shape an inverted curved diverter out of open cell flexible limp foam. At the speed of sound, it doesn't like to change directions. Low freq attenuation is really tough once you produce the noise. These units beat the air into submission by ultra compact design and fixed standardized roof opening size. Room for a small hemholtz chambers on the sides of the fan scrolls maybe, but I don't think that's where the sound is generated. It's velocity and turbulence generated past the roof unit due to the airflow CFM. Kill some CFM and kill some noise. Until the coil freezes, and you pay for removing too much moisture.

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #68 – August 03, 2018, 02:32:03 pm

It's slightly louder with the new diverters. My hypothesisis is that the ACs are pumping out slightly more air, which makes the air exiting the ducts louder. Attached is a photo of the one AC cover with a diverter, one without, and the dB reading in the bedroom with the door closed.

I would recommend not doing this modification.

TABLE OF DECIBEL READINGS BEFORE AND AFTER INSERTING A SPLITTER IN THE OUTLET DUCTS OF TWO DOMETIC BRISK II AIR CONDITIONER IN A 1997 U270
              SPLITTER
LOCATION      BEFORE AFTER DIFFERENCE (db)
Couch         51      53            +2
Bath         58      60            +2
Bed           47      49            +2
AVERAGE                        +2

Two possible positives:

1. The dB readings may be within a margin of error for the app.
2. There may be a slight improvement in efficiency.

There is no change in the Hemholtz resonance. On high fan, the 30HZ vibration is quite pronounced at regular locations throughout the rig.

Thanks for putting up with my project.

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #69 – August 03, 2018, 04:08:11 pm

Having used the Atwood unit with 2 separate fan motors for the condenser and evaporator for a few months now, I think the noises you are complaining about are endemic to Dometic's use of a single motor for both fans. It is unlikely that Dometic puts much effort into balancing out the two fan blades and I think that is what causes the problems you are referring to. The Atwood units don't have the low frequency vibrations that you are talking about (my old Penguin units did though).

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #70 – August 03, 2018, 04:54:57 pm

Noise on my Penguin 2 appeared to be coming from turbulence from the air from the blower directly hitting the irregular surface of the "max cooling vent". Rather than try to make a diverter, I cut a piece of 1/2" plywood and sealed it in place above this vent. Air from the blower now hits the piece of smooth plywood and is directed into the left and right vent plenums with little obstructions. I have lost the use of this vent, but because it allowed most of the cooled air to exit into the coach at a single point, it really did not allow even cooling. Noise from 3 ft below ac at low speed went from 67 to 58 db. Not a very sophisticated mod., but can now turn tv volume down from 80% to 30%.

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #71 – August 03, 2018, 10:53:03 pm

Nice fix Jim. I like your overall setup. 4 AGM's? Solar max helps, what alternator do you run with this setup?

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #72 – August 03, 2018, 11:09:15 pm

Quote from: Caflashbob – August 03, 2018, 10:53:03 pm

Nice fix Jim. I like your overall setup. 4 AGM's? Solar max helps, what alternator do you run with this setup?

Thanks Bob, still running the LN160, but with the big solar, don't worry too much about engine charging. Am considering wiring the fwd ac [with easystart] into inverter panel, to see if I can use it running down the road without using gen. Probably okay when sunny, but might be pushing it if no sun. Did dump the isolator and use a battery combiner.

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #73 – August 04, 2018, 12:19:35 am

Old bird ran everything through the double inverters.

Ac's would come on on battery. Couple of hours in testing.

I ran a similar load for several hours to test everything on a run into Vegas. Gauges moved a lot when the convection oven came on. Three 8G8d's rebuilt 160 LN.

Combiner on. Batteries up. Convection oven on. Chest freezer on. 13.5 driving. Big drop when the oven cut in.

May have had the refer plugged in to the inverter side of the plug also for more load,

Gauges will jump around. Settled down in a minute or two, nothing blew.

Magnum 2812. True 3000 watt.

I would not try this with a 2200 watt inverter.

Re: Advice on replacing both rooftop ACs with heat pumps

Reply #74 – August 04, 2018, 08:52:28 pm

The 30HZ Hemholtz resonance also happens when rolling down the road over 20MPH with the windows open. I should design an air scoop to resolve this. It would equalize the low pressure waves created by the open windows.

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