High efficiency furnace(s)&multi speed blowers

[Aage]

Bummer.

[wmarsha]

yeah, I am bummed, too. well off to another source, I guess. Thank-you Hank for hanging with us and keeping us in the loop. I am truly sorry this didn't work for you and us, too. Back to the thinking loop for me. I did take the AS down to Salem this weekend and all was super dooper, didn't need the furnace, good thing, huh? I keep looking at that ugly A/C up on the roof and wishing I hadn't gotten that thing-kinda would like a nice unit mounted down low-gotta think on that for awhile. Roofline sure is cluttered up, tho....

[kejorg]

Trumatic makes a 2 speed furnace.... it is a german company.

I bought one of these on Ebay years ago to put in my boat but it ended up in my sons AS... honestly it works great and is dead silent it is about half the size of the Sub that it replaced it is a great unit but I do not know how efficent it is ... it does have a condsation chamber and drain. I think they have a website Trumatic.com.... goodluck

[wmarsha]

well, I didn't think I would fail you all, but I have. I sold my 1985 excella, with the suburban nt30 in it, so I did not replace it with the excalibur xt unit like hank has in his trailer. I bought a 1995 excella, and it has the atwood 8500 unit in it. If it gives me any trouble, the xt is on the horizon. I contacted atwood about controlling the blower speed independently, and having it run continuously. I asked for current drain info on start-up versus run. They gave no engineering info, and said it would be a bad idea to control the blower speed separately from the heat control. No reason(s) given, just "don't do it." So you are on your own, here. Sorry for the broken promise, and I will energize this thread if I do anymore measuring or any other furnace work. good luck to everyone, and again, thanks to Hank for being the guienie pig. how the hell do you spell gueiniea pig? the pig part I got...
ol bill

[Jammer]

Originally Posted by hshovic

On Tuesday I had a very cordial appointment with Chris Schmidt, an engineer with Atwood mobile products. He took the whole thing apart, replaced a blower wheel,
which made a slight difference in the hum. He also aligned the motor. But not much
difference. He finally said he thinks it is normal and is a result of the design of the "chopper" that reduces the voltage for the slow speed. We discussed putting more insulation, but there is no room. He has no other suggestions other than to return it.

Hank

If the problem is related to the "chopper" there are ways to fix it.

The first step is to determine whether that's really the problem.

If you are still interested in pursuing this, here's something to try. Get the fan motor running at the "low" speed while it's unhooked from the controller board using some kind of constant series resistance instead between the motor lead and +12v. Anything that will give you the same speed the controller does will work, maybe a turn signal bulb, or two of them in parallel. It may take some experimenting to get the speed right.

Do you still have the hum now? No? Then the culprit is probably the dirty waveform coming from the controller board, but let's be sure. Close up the furnace, and turn up the thermostat with it set on "low" and see what happens. Heat? No hum? Problem identified.

If that's really the problem, and I'm guessing it is, there are three ways to solve it:
1) Get help from Atwood
2) Put a filter between the controller and the motor to clean up the waveform
3) Put a circuit between the controller and the motor that switches the motor to a resistor when it's supposed to be on low, instead of using their fancy chopper circuit.

Neither 2 or 3 are especially difficult.

For (1) they should be using 25 kHz or more for the PWM (chopper) circuit so there's no vibration, and well yes they're going to have to do a little filtering so they don't create a bunch of electrical noise that leads to radio and TV interference. It's not hard. There are plenty of power supply geniuses out there who can help them with that.

For (2) what you'll end up wanting to do is put an inductor in series with the controller, and a capacitor between the motor side of the inductor and ground. A certain amount of experimenting would be involved to get the values right and you may end up finding that the fan speed drops too much when you do this. It depends on the details of the PWM circuit.

For (3) you would want to use a suitable dropping resistor and a relay. The relay would short out the dropping resistor whenever stage 2 is called for. If you measure the voltage across and current through your experimental resistance using a meter, those values can be used to determine the correct resistance and wattage.

Let me know if you're in the Minneapolis area sometime and want to experiment.

[hshovic]

Jammer,

Thanks for the tips! I agree, lowering the motor level with a resistance, rather than a chopper would be awesome. Though interesting, I think it might mess with something else, knowing that engineers sometimes do fancy things to save money, like using circuit components for other functions So without some fancy analysis, I would not be comfortable messing with a gas-fired appliance like that. Taking out the circuit might cause the entire thing to shut down for safety reasons.

I have generally gotten used to the hum...

[texascanuk]

Truma combi furnace and hot water combination specs at 90% efficiency. Has anyone installed one??

https://www.truma.com/us/en/press-ev...ruma-combi.php

[slowmover]

I think Switz had something like that installed.

[texascanuk]

I checked with my local distributor. Most of the Truma products are not available aftermarket due to their high quality...this is the reply that I got

"HI JAMES- UNFORTUNATELY, THE COMBI IS NOT A PRODUCT THAT IS AVAILABLE AS AN AFTERMARKET INSTALL AND IS ONLY AVAILABLE TO OEM’S THAT TRUMA WORKS WITH ON EACH FLOOR PLAN AND DESIGN TO MAKE SURE THAT IT IS BEING UTILIZED IN THE BEST POSSIBLE MANNER. THE ONLY PRODUCT WE ARE AUTHORIZED TO SELL IS THE AQUAGO WATER SYSTEM. I APOLOGIZE FOR ANY INCONVENIENCE THIS MAY BE FOR YOU."