# Forum Home Renovation Heating & Cooling  HEPA filter system for ducted heating

## paddyjoy

We have ducted gas heating system in our house with no filters, we all suffer from either asthma or allergies so I'm investigating some way of installing a HEPA filter to clean the air. Usually you would put a filter on the return air grille but because we have multiple return air locations I'm looking for something that could be installed under the house as part of the main return duct. 
Does anyone have any experience doing something like this? 
I found the following system online which would be similar to what I would want but I wouldn't need the unit to have it's own fans as air would be pushed through it by the heater.  Ducted A200D Air Purifier Home Office & Commercial Air Purifiers 
Would it be practical/feasible to get a steel unit fabricated with two duct connections and a filter housing in the middle as a tight @@@@ version of the product above?

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## justonething

There is a huge pressure drop across a HEPA filter. Air particles are slowed down as they pass through the medium and the tiniest dirt particles are thus trapped. If your heating system isn't sized with HEPA system in mind, then air flow will reduce so much that you will feel there is no air coming out of your outlets.
Also, you would want to install your HEPA filter on the supply side of your system as fresh air introduced in your system should also be filtered in addition to return air. You can always install a foam type filter on your return air grill which acts as a pre-filter to your HEPA.
I reckon your little air cleaner that you showed might actually be the right thing for you if you can connect it to your ducted system. I wouldn't just get a filter plenum built.

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## paddyjoy

Thanks yes it is a nice unit but unfortunately the cost for the box is $2.5k for the smallest one which is a bit out of my range. Might just try and put some basic filters in the return vents as a starting point and see if it makes a difference.

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## starraffy

> We have ducted gas heating system in our house with no filters, we all suffer from either asthma or allergies so I'm investigating some way of installing a HEPA filter to clean the air.

  Why not use an air purifier with HEPA filter? It removes particles as small as 0.3 microns in size,like fine dust, smoke, bacteria from the air and helps in respiratory issues as well.

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## paddyjoy

> Why not use an air purifier with HEPA filter? It removes particles as small as 0.3 microns in size,like fine dust, smoke, bacteria from the air and helps in respiratory issues as well.

  Had a look at this but the units are rather expensive and really only cover one room unless you get a large commercial type which costs even more and takes up precious space in our tiny house. 
After doing some further research I discovered that filtering is big business in america and there are heaps of places over there selling filters. They call them "furnace" filters and you can get them custom made to whatever size you require. They nearly all seem to be classified using a MERV rating Minimum efficiency reporting value - Wikipedia, the free encyclopedia  
I was worried about restricting the air flow of the system so I bought some MERV 11 filters which are on the upper end of the scale and filter down to about 1-3 microns. I had the filters made 4 inches deep, the material is pleated so the surface area of the filter material is reasonably large. I have 6 of these that will fit nicely into my return air vents (I have 6 small returns instead of one large return).   
Being the nerd that I am I have installed a pressure sensor in the return duct between the fan and the filters and will monitor to see what the pressure drop is once the new filters have been installed. 
As a benchmark I currently have some bog standard fabric filters installed, when the heater kicks in (orange peaks) the pressure (red troughs) drops by about 0.5hPa.   
While the numbers won't really give me any specific info on how much strain is on the fan it will give me an orders of magnitude analysis so I can compare the pressure drop between the bog standard filters vs the MERV 11 filters. If the drop is similar then I know that using the MERV 11 filters isn't adding any extra strain to the system, that's my logic anyway!

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## Uncle Bob

> Being the nerd that I am I have installed a pressure sensor in the return duct between the fan and the filters and will monitor to see what the pressure drop is once the new filters have been installed.

  Great idea! It will also tell you when the filters are clogged.  :2thumbsup:

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## paddyjoy

> Great idea! It will also tell you when the filters are clogged.

  Good thinking Bob that would be really useful, I need to graph the standard deviation over time I think. 
I have the filters installed now so I'll run it for 24 hours and see what the numbers look like.

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## starraffy

> Good thinking Bob that would be really useful, I need to graph the standard deviation over time I think. 
> I have the filters installed now so I'll run it for 24 hours and see what the numbers look like.

  that's nice Paddyjoy! nice solution.. yeah if you're after one room only , much better to use an air purifier but in your case.. wants to cover huge area that would be great. Though there a pretty less expensive air filters in the market .. like mine i have one and got it for around $250 only and yeah i'm satisfied, can cover one big room. Tell us your expererience with your newly installed filters  :Smilie:

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## paddyjoy

Have been running the new filters for a few days now, on each filter I glued a piece of standard filter on top to act as a pre filter so that all the bits of lego and other random items won't fall down into the pleats of the new filters. I shoved this down into the floor boot under the grill. It looks a bit gappy in the pictures but in reality the filter is 4 inches deep and does have a nice seal around the edges.   
The pressure drop with the combined filters is about 0.8 hPa compared to the previous 0.5. The system seems to be performing exactly as before, the airflow is just as strong and the house gets heated in the same time as it did before so my conclusion is that it's all good for now...

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## starraffy

That's cool ! you mean this one covers the whole house? and how many months to replace the filter?

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## paddyjoy

> That's cool ! you mean this one covers the whole house? and how many months to replace the filter?

  No I have 6 of them in total, our house is small and I wasn't happy having a big plastic return grille in the main living area so I split the return air up into 6 individual small returns. 
Hard to say how long they will last as I don't have much experience in this area, I'm hoping that they will last the full winter and that I will just swap them out next autumn.

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