Questions re undersized return ducts and locking out High Heat

EBEBRATT-Ed

@Hogan
I agree with cutting in a return grill in the floor. But don't use it as a transfer grill duct it to the furnace.

As far as dirt and dust etc the flor return grill just drop into place without screws. If it gets dirty (an it will) you pull it out 1/year and clean it

Hogan

Another interesting thing I found was I took the pressure above the AC coil and it was 0.06 WC.  Basically the same on both attic and basement furnaces.  So that indicates no constraint on the supply side right?   That is a very healthy number.  So this is all a return duct issue

Hogan

Zman said:

Your furnace is not making the building pressure positive or negative to the outdoors so the power vent "shouldn't" affect the furnace. That being said, where is the water heater combustion air coming from? I don't like appliances that are competing for combustion air to be shoved in a closet together. Is your furnace pulling CA from the outdoors? Is it possible to make your water heater pull it from the outdoors?

Re the water heater, it must pull combustion air from the utility room.  It is a brand new unit that is quite sealed at the top exhaust motor so that is good.  My old water heaters had a Powervent motor yet also had an inch or so of open space at the exhaust so a large negative pressure in the room could potentially compete with the Powervent motor.  Here, the blower motor is sealed down on top so I don't think there is really any way for that exhaust to go anywhere other than up the PVC.   This isn't a direct Powervent though where it takes combustion air from outside via a second PVC.  It must just take it from the ambient utility room. 

This isn't a "closet" but a 12 x 12 x 9 ft ceiling utility room.  It has a door out to the finished basement.   I don't think there are issues of the furnace and water heater competing now.    Are you saying that if I just opened up a vent on the return right near the furnace that the water heater would then potentially have a combustion air issue?   I guess it would be interesting to see to what extent that whole room went to negative pressure.   There is actually a 14 x 8 sheet metal panel at the top of the return trunk that is screwed and taped from when the last guy relocated something on the return and I could just take that off for kicks and see how much it changes the return static pressure and also to what extent it makes that room negative pressure.   I would guess that having that be a vent though would be against code even though the water heater exhaust is sealed to the top and uses Powervent but I might be wrong

Hogan

Hey I also have a THRESHOLD QUESTION here. This often happens when I start pursuing something in my mind and then I need to take a step back and ask WHY

WHAT AM I TRULY SOLVING WITH LOWER STATIC RETURN PRESSURE? Is it primarily just easing stress on the blower motor? I sort of think in terms of cost/benefit sometimes and the facts are 1) my house is warm in the winter and cool in the summer. 2) I seem to be getting good supply airflow from the vents. So for sure there is a benefit in trying to improve return airflow, but it becomes a question of cost to retrofit. If we are saying that the blower motor may be working harder to pull the air through, maybe that means the motor may die after 15 yrs instead of 20 or something, I dunno, and it costs a few hundred dollars to replace the motor. I'm still considering some solutions but what I'm thinking is, one wouldn't spend $7500 or $10,000, let's say, to improve return duct airflow for the primary benefit of adding a few more years of service life onto a $300 or $400 blower motor.

pecmsg

With a blower motor the lower the static the more work or energy it uses. Less slippage of the squirrel cage blade. 

pecmsg

How to measure TESP
https://www.nciblog.com/measure-tesp-2-piece-ah/

SuperTech

Hogan said:

Hey I also have a THRESHOLD QUESTION here. This often happens when I start pursuing something in my mind and then I need to take a step back and ask WHY WHAT AM I TRULY SOLVING WITH LOWER STATIC RETURN PRESSURE? Is it primarily just easing stress on the blower motor? I sort of think in terms of cost/benefit sometimes and the facts are 1) my house is warm in the winter and cool in the summer. 2) I seem to be getting good supply airflow from the vents. So for sure there is a benefit in trying to improve return airflow, but it becomes a question of cost to retrofit. If we are saying that the blower motor may be working harder to pull the air through, maybe that means the motor may die after 15 yrs instead of 20 or something, I dunno, and it costs a few hundred dollars to replace the motor. I'm still considering some solutions but what I'm thinking is, one wouldn't spend $7500 or $10,000, let's say, to improve return duct airflow for the primary benefit of adding a few more years of service life onto a $300 or $400 blower motor.

I think everyone here is trying to prevent you from catastrophic system failure that happens when you need it the most. It's not just the lifespan of the blower motor that is affected by high static pressure and poor airflow.  The heat exchanger will certainly fail prematurely as well.  When you need a replacement system at the worst possible time what's the cost/benefit of that?

Some of these new systems don't last very long under ideal conditions. They aren't as robust as they once were because of the government imposed efficiency mandates.

pecmsg

SuperTech said:

Hogan said:

Hey I also have a THRESHOLD QUESTION here. This often happens when I start pursuing something in my mind and then I need to take a step back and ask WHY WHAT AM I TRULY SOLVING WITH LOWER STATIC RETURN PRESSURE? Is it primarily just easing stress on the blower motor? I sort of think in terms of cost/benefit sometimes and the facts are 1) my house is warm in the winter and cool in the summer. 2) I seem to be getting good supply airflow from the vents. So for sure there is a benefit in trying to improve return airflow, but it becomes a question of cost to retrofit. If we are saying that the blower motor may be working harder to pull the air through, maybe that means the motor may die after 15 yrs instead of 20 or something, I dunno, and it costs a few hundred dollars to replace the motor. I'm still considering some solutions but what I'm thinking is, one wouldn't spend $7500 or $10,000, let's say, to improve return duct airflow for the primary benefit of adding a few more years of service life onto a $300 or $400 blower motor.

I think everyone here is trying to prevent you from catastrophic system failure that happens when you need it the most. It's not just the lifespan of the blower motor that is affected by high static pressure and poor airflow.  The heat exchanger will certainly fail prematurely as well.  When you need a replacement system at the worst possible time what's the cost/benefit of that?

Some of these new systems don't last very long under ideal conditions. They aren't as robust as they once were because of the government imposed efficiency mandates.

Newer ECM motors are very unforgiving with high static pressure. At almost $1000.00 for a replacement that’s a lot of electric to save. 

Most systems I saw suffered from oversized equipment on undersized ducts!

Hogan

Thanks for the above, understand. Will see if I can find any good contractor that might have a look and be creative. The ones I've come across in the past sort of just shrug and say "it should be fine" or "it works right?"

I'm gonna open up that flap on the return just to see how much that lowers the return ESP, just to get an idea of how much benefit there might be to trying to cut in a new vent either in the basement or on the first floor.

Hogan

WELL THIS IS INTERESTING

I opened up that little flap at the top of the down ducts into the furnace (it was there already from when the installed modified one of the downdrafts to line it up more compactly).

It is an 18 x 6 hole. When opened, of course it lets a good suck of air into the system.

ESP before the fiberglass filter measures around 0.18 to 0.20 with that opened. And after the fiberglass filter, 0.3. SO opening that up results in a drop of fully .20 WC right there, and puts the system down to around 0.6.

So the question is, leave it open and see how we go? (ultimately would put a grille over if I kept it open like this) I would of course need to measure somehow to make sure the water heater isn't backdrafting but I would think since the top is sealed and it using a powervent motor, it would certainly overcome whatever negative pressure is in that utility room yeah? Of course I now feel some mild suction at the door to the utility room (if it is closed) and also at the top of the stairs to the basement, but nothing major. But for sure I would be altering the return airflow to bias the basement. I do still feel return airflow at the return ducts on the first floor.

I guess I need to think about whether it's worth it to leave it like this or not. Must decide if the lower suction on the blower motor is worth this questionable placement of a sort of quickie solution. Obviously easy to change it open or closed (it literally holds itself on there with suction haha). But I have confirmed that opening a decent sized return vent does materially drop the ESP on the return side.

My guess, though, is that having this return air in the mechanical room is not up to code here....just "feels wrong" to me even though it drastically improves the static pressure.