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Does a kiln create a negative air pressure at home and what is the impact


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Hello,

I am installing new 10 q. ft kiln(Cone Art) in the basement. I hired a technician to install a vent. He explained that running motor over 2-3 days of kiln operation will suck out a lot of air and create negative air pressure in the house. He advised to install make-up air vent. Could anyone advise how crucial it is to have air make up vent supply? I have 3000 sq ft home.

Can air be drawn from existing HVAC air supply?

What negative impact will I experience if I do not have air make up?

What are pros and cons to open a window tiny little bit instead ?

Could anyone with kiln installed in the basement advise on your experience regards this matter?

Also HVAC technician advised to install air return in the kiln room to circulate it through the house to save on heat. -  - is it ok to do?

 

Thank you very much!

Alla

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Also HVAC technician advised to install air return in the kiln room to circulate it through the house to save on heat. -  - is it ok to do?

really bad idea in so many ways-no its not ok to do

I would crack the window or have a make up air vent-not part of your HVAC system.

I'll let others with basements answer the other questions

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4 hours ago, Alla said:

Also HVAC technician advised to install air return in the kiln room to circulate it through the house to save on heat. -  - is it ok to do?

 

Hmm, does not know a bunch about kilns ...... or makeup air actually. No return, only will distribute kiln fumes nicely through the house. (Very bad idea) Depending upon your kiln vent, if it’s a downdraft then home infiltration is fine and makeup air likely not required.

If you have a larger exhaust for your kiln, such as a hood, then yes design for it and interlock so it closes when not in use. Furnaces and hot water heaters have required  combustion and dilution air supply over the years however as these appliances have grown more efficient they now have their own outdoor air supply and discharge. Dryers generally exhaust about 200 cfm, bath fans 30 - 100 cfm yet there is no requirement for makeup air. Actually energy codes started to suggest that even with relatively tight houses getting combustion air from the rest of the volume of the house was becoming more preferable than the old outdoor air supply duct.

A downdraft exhaust will remove less than 100 cfm of air at best. Actually I have never measured any fully configure that discharged more than 20 cfm. If you are installing a downdraft I am not aware of any codes that require makeup air for that exhaust so I think his statement is pretty baseless and find his recirculating statement to be outright concerning.

Interesting in that kilns are unique and often confuse many in the trades. I always mention to electricians to forgo the old 3% acceptable voltage drop and limit it to 1% or less as practical. Kilns are only designed with about 110% excess capacity so in essence as a kiln wears by 10% it approaches a need for new elements. This is often 100-150 firings for midfire cone six stuff. If ya don’t know kilns, starting out with 3% less capacity would be a bummer.

For all the rough and tumble,  experienced potters often end up to be pretty ingenious actually

To answer your question assuming you have a downdraft exhaust in my view there should be no negative effects or need. Now that bathroom fan and clothes dryer .......... just kidding.

Edited by Bill Kielb
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Thank you for your response. Yes, we are installing downdraft exhaust - kiln comes with a fan that will exhaust 65 CFM. This fan will be running for 3 days during kiln operation. So it will draw a lot of air out. There is no bylaws, nor kiln manufacturer recommendation on installing air make up vent. Bathroom fan does not run continuously so I do not think we can compare the two.

 

The kiln surface temperature will be up to 300F. It is going to be located in 8 x 8 ft room. It may be hot in the room - this is why it was discussed putting air return to circulate this heat through out the house. I do not understand why it is bad idea. The manufacturer states it is safe to have kiln in the basement, there should  be no poisonous gases as they are being exhausted.  if it is safe in the basement - why not to reuse the heat and distribute it through out the house.

 

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There is silica dust all over a pottery studio. Best to keep it contained in the pottery space. Please do not circulate the dust throughout the house!

As for using the heat, my kilns are in my basement, and the room above them (guest room) does get warm when the kilns are on. In the winter, I keep the guest room door open to let the heat spread around the first floor. In the summer I keep the guest room door closed. So take advantage of the passive heat, but don’t force it. 

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The HVAC technician may be thinking of a heater reclaimer which is installed on a chimney and exhausts fumes outside and returning heat inside. They are common on oil and gas burners and wood stoves.

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Personally I don't think this item is necessary. I've seen the temp in a room rise an easy 20°+F when the kiln is running. Maybe install registers at the top of the walls or ceiling to exchange heat into different rooms

And Yes, there should be a way for fresh air to be returned into the room when using an updraft or downdraft vent. A cracked window is fine.

Edited by Smokey2
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47 minutes ago, Alla said:

with a fan that will exhaust 65 CFM. This fan will be running for 3 days during kiln operation

If it’s a 65 cfm rated fan it won’t be after all the suction and discharge losses but ok I have only measure several of the common models out there. So no worries at 65 cfm running for weeks, months, years. There are plenty of leaks in the tightest house. Remember the 200 cfm dryer? But having said all that put a makeup air source in the room, not sure what your climate is but at this point it could be  just about any size pipe you like. Not sure why you will run it for three days, but ok I guess. Fan electric and 4000 cfm of outside air to heat and cool doesn’t seem energy conscious to me but at least it will be a cold or hot room. The question was is it necessary and the answer is still no.

The kiln will exhaust fumes, especially with a downdraft it will not get all fumes so  do as you please but a return is a bad idea, period! Call Frank Tucker at Coneart and ask him if he thinks it’s a good idea, maybe he will. I doubt it.

To remove most of the heat generated and fumes you will need a hood, 400 - 600 cfm and definitely local makeup air at that point to ensure you do not suck natural gas appliances draft back down their own flues. . L&L has some nice kiln loss charts on their site if you guys want to design this. Just figure out your typical makeup air temp, or average air temp and size away.

As far as potentially spreading dust and fumes, just don’t do it would be my final comment.

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In an eight by eight I'd go downdraft kiln vent and secondary hood as well, for it'll be both hot and smelly/fume-y in there - hence, adequate make up provision required. Methinks I'm just echoin' Bill here, eh?

Suggest adding smoke and CO detectors and fire extinguishers - both in and on the way to said room.

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No vent system will exhaust 100% of the fumes, so you don't want to spread those fumes around the house, or spread any studio dust. I've never had a customer have issues with negative pressure due to their downdraft vent. I have, however, had customers that were not getting enough makeup air when using an overhead hood. But those use much larger fans, 265cfm or more, and cracking a nearby window an inch fixes the problem.

In an 8x8 room you're going to need to figure out a way to exhaust the heat, otherwise it's going to get really hot in there. If the kiln controller gets too hot it will shut down. Unless you have windows that you can open that are not located near where the downdraft vent is located, you'll want to install some other method of venting heat to the outdoors. Or if it's a small kiln, just moving cool air into the room with a fan or having a fan blow out a window may do the job, but removing that hot air is the best way to go so your AC isn't working overtime to cool the space and you don't have to have a window open during bad weather. And if you have another room above the kiln room it's definitely going to get warm up there if you're not venting the kiln heat to the outside. Vent-A-Kiln hoods work really well, and only need a 5 or 6 inch penetration to the outdoors, just a little bigger than a clothes dryer vent. If you use a hood, you don't necessarily need the downdraft vent, as the hood will remove fumes too. Using both would be the best of both worlds, though, as downdraft vents bring benefits that hoods don't, like improved element life and better looking glazes.

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