Blower Fans Speeds

Problems related to residential installations.

- Blower Fans Speeds

Postby Freon » Fri Feb 08, 2013 4:52 pm

Slashes, dashes and question marks will generate an *. So avoid those punctuation symbols.

You have the right idea. More air flow through the heat exchanger will drop the temperature. The furnace should be well within the temperature rise if there is the 1200 cfm the high speed is designed to deliver. I have a feeling your duct system is restricting the 1200 cfm air flow. You need to find the return duct(s) diameter as well as all supply ducts. Previously I assumed 6" ducts for the supplies since that's a very common size. Each 6" duct can deliver 110 cfm. You can do the math. Your goal is to get the duct system to handle the 1200 cfm the blower is trying to deliver. Do you have metal ducting or flex. Also, what type filter(s) are you using and what are the dimensions.

If you can find the high limit safety, you may be able to read the temperature at which it opens. Some are stamped on the outside.
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- Blower Fans Speeds

Postby JTRFLASH » Sun Feb 10, 2013 11:20 am

Thanks Freon, your are also right the duct work is 6" so now I can complete the calcualtion.

I have tried the method of the HRV interlock to control the blower it has not worked out as I thought.

I managed to change the blower setting to low on the COOL terminal, (auto fan), however the cooler air coming in cools the house so much that the heat comes on, an endless cycle, so I have disconnected it.

I always believed that the fan always on was the best thing.

I can change the HRV inlet temperture to a higher setting, right now it is -5, perhaps if I set to zero. It is interesting as I want to bring in fresh air and use the furnace blower to circulate it, without the furnace fan what does the HRV do. The internal fans do not have enough power to circulate through the house.

Jim
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- Blower Fans Speeds

Postby Freon » Sun Feb 10, 2013 2:57 pm

Open all supply registers and return registers. The blower, unless the motor has been improperly replaced, is designed for your furnace. You problem lies in the duct work not being adequate to allow the blower to operate properly. Balancing air flow to each room is an art you can practice once you get the current system running a little cooler.

Either there are too few supply registers or too few returns. Or the ducts are sized improperly... too small a diameter. 90 degree elbows in a duct system kill air flow. Flex duct crimps easily if improperly installed and is horrible for 90 degree bends. I assume you have nothing more restrictive than a MERV 8 filter. The filter could also be too small (surface area) for the air flow you need. You need to make a picture of your duct system with all duct diameters, both supply and return(s) and get the dimensions of all filter(s).
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- Blower Fans Speeds

Postby JTRFLASH » Sun Feb 10, 2013 9:14 pm

So all supplyie are now fully open and there are no adjustments on the return registers. I am starting to wonder now if the problem is the return registers, not enough of them.

So of my supply ducts have 3 - 90s in them, remember this is a brand new home.

So I have 6 supply registrs in the basement and 13 up stairs, it is a bungalow, that is 19 x 110 * 2099. So more than enough duct for 1200 cfm, right.

I also beleive you asked me something about static pressure at 0.1 the CFM is 1574 as per the manufacture chart, at 1.0 it is 974.

My filter is a standard 3M filtrate 16x25 nothing close to a MERV 8.

So I have my job ahead of me to map out my ducts.

But now I am going to bring this issue up with the builder, we still have the house warranty, from what I am learning from you there is something very wrong here.

If we believe the blower is putting out 1200cfm and the supply ducts are 6", and there are enough of them, then the only thing left are the returns. To me this will also burn out the blower as it is fighting back pressure.
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- Blower Fans Speeds

Postby Freon » Mon Feb 11, 2013 12:09 pm

I agree you need to focus on the return ducts. With the 1 filter you have I can already see a problem with the returns. A 16X25 filter is 400 sq inches and will handle about 800 cfm of air flow. Check the return duct size. Here are some approximate air flow and duct diameter numbers. 14" round metal duct can handle about 700 cfm; 12" round metal duct is about 500 cfm; 10" round metal duct is about 400 cfm.

You may need to install another return with a filter grille. If your current filter is in the furnace you should consider a filter grille for the return and remove the filter at the furnace.

A simple test is run the heat with the blower compartment door off and check both the outlet temperature of the furnace and the air flow to the rooms. You will need to TEMPORARILY tape the blower door switch when the door is off. The temperature rise should decrease and the air flow to the rooms increase. If that happens then you need to add another return. 2@14" return ducts would give you 1400 cfm which would be a good target.

Another consideration is how the supply air gets from the rooms to the return opening. Many houses have a centrally located return in a hall. Therefore if doors to rooms are closed there is more restrictions for the air to flow to the return. One solution is make sure the bottom of the doors that are sometimes closed are 1 to 2 inches above the flooring material. That way air can flow under the door more easily to the return.
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- Blower Fans Speeds

Postby JTRFLASH » Mon Feb 11, 2013 1:46 pm

Thanks Freon, it makes sense in what you say, run the furnace with the blower door off, removes any air restrictions on the return side, more air in then we would expect temp rise to be lower. I will try this tonight when I get home and yes easy to tape off the switch.

If that is the case above, the temp rise is less, I am not sure where I could add the extra returns, I think if too close to the furnace, which I really only have access to, would that not limit the return air from the other returns that are much farther from the furnace. I mean the air would take the least path of resistance and perhaps not heat the upstairs as much as it did before? or is that why you suggest filtered as it would provide more air but be slightly restrictive.

For the entire bungalow I found 5 return metal grills, 2 sets are from a within the common wall, these grills are back to back and common to the same inside wall space for the return. The other does not have back to back grills. So when you think about it the returns occupy a 3.5" stud depth by 16" wide * 56 sq in., which I think is about 289 CFM, times 3 * 867 CFM for the entire house. So the total area of space for the return air is 56 sq in x 3 * 168 sq in how can that be enough space for the air to return, if the furnace is pulling to provide 1200 CFM. I guess I will find out tonight.
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- Blower Fans Speeds

Postby JTRFLASH » Mon Feb 11, 2013 10:27 pm

Update - I was wrong about the filter, it is a lot better than I thought.

It is a 3M Filtrate Alergen, an MBR rating of 1500 which is apparenty equivelent to a MERV 12.

Is that too much filter?

I believe if I go to an MBR 1000 that is equiv to a MERV 8.

I have UV lamps in the ducts so perhaps MBR 1500 is not needed.

I won't have time to do my open door test tonight.

jim
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- Blower Fans Speeds

Postby JTRFLASH » Tue Feb 12, 2013 11:37 am

Hi Freon, just to add, I didn't understand what you meant by installing a filter in the return ducts.

In Ontario Canada I have never seen return grills installed to accomodate a filter. Our return grills are permanently screwed into the studs and a filter of any type cannot be added, well not easily. I did see on the internet return grills that had built in frames that you could drop down as they had hinges and you could mount standard sizes of filters in them, I see they have carbon ones too. But again I see this as more restrictive to my system.

Any suggestions on my previous post. Is my filter too much at 1500 MBR / MERV 12, 3M say their filters are made not to restrict air flow, however one time I tried to pull out the filter while the blower was running, it wasn't easy.

I just thought I should make 2 tests. One with the air handler door open, then with it closed and the filter removed. That would be a good test.

Jim
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- Blower Fans Speeds

Postby Freon » Tue Feb 12, 2013 3:44 pm

Google filter grilles so you understand what I am referring to. Most furnaces were designed for a single spun filter... the blue kind that block very little particulate matter. Newer filters are more and more restrictive to air flow. That's why you go to filter grilles placing many filters in the return system compensating for the flow restriction of newer filters. Your current filter is very restrictive and may be a big part of the problem. However to have the small particles removed from the house air you need that kind of filter. Therefore your only choice is remove the furnace filter and have several filters located in filter grilles at the returns.
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- Blower Fans Speeds

Postby JTRFLASH » Tue Feb 12, 2013 8:58 pm

HI Freon, ok I have looked around but I don't understand what the differnce would be where the filtration is located in the system, whether at the furnace or at the grills does it still not give the same restriction. When I think about it if I had 6 grills with 6 filters that would that be a lot more restriction in sq inches than a 16 x 25 at the furnace, wouldn't it.

Here are the results.

I did 4 tests, gaugeing the temp rise for 5 mins for each test from blower start.
No change in thermometer placement.
Based upon a thermostat temp of 22C

Air Handler Door off - 50C - 28C rise
Air Handler Door On using MERV 7 filter - 56C - 34C rise
Air Handler Door On using MERV 12 filter - 57C - 35C rise
Air Handler Door On using no filter and filter cover is close - 54C - 32C rise

I think this proves for the filters at least what 3M states, that no matter the MERV value the air is not severely restricted, which is the point of a pleated filter design, which of course provides more surface area for the air than a flat surface, like the surface area is doubled to accomodtae the denser filtration, to get the same air flow. The MERV 7 is definitely short of filtration material but still creates a restriction from an open air handler door in a rise of 6C.

So taking the filters out of the equation there is a restriction using no filter at 54C vs the door open at 50C, however all within specs as stated by the manufacture which states a rise of between 19C and 36C.

It is interesting as I called the manufacture and asked what MERV filter they recommended and they said no larger than MERV 8, well I just proved only a 1C temp rise difference between the MERV 7 and MERV 12.

I think I am done here, I have learned a lot, I may have to put a return in the basement as I am finsihing it off with additional interior walls.

Any other thoughts?
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