Dehumidifying a Residential Basement

[supaman79]

Guys,

As some of you might know already, I'm knew to the HVAC/R trade, and I've come across a simple problem for you guru's (haha).

A residential building (near the shore)that I've been working on needs dehumidification in the basement. Our ductwork is already sweating, since we haven't insulated yet, and also there's a definite presence of moisture in the space.

Using a pychrometric chart, I can easily find out what my delta gr/lb will be, but where do I go from there.

Also what are comfort conditions in a basement space? any applicable codes? What are avg. DB/WB conditions before dehumidification in New England (Massachusetts specifically)?

I plan on using stand-alone units, a complex system is not needed...I just need a little direction so they are sized properly.

Any input gentlemen?

 

[CESSNA1]

SUPRAMN79: THERE ARE A COUPLE OF WAYS TO DEHUMIDIFY A BASEMENT.
1. USE A DEHUMIDIFIER
2. WATER PROOF THE WALLS
3. THE METHOD I USED IN MY HOUSE WAS TO TAKE DOWN THE RETURN AIR DUCTWORK AND DUMP THE RETURN AIR INTO THE BASEMENT. THE HEATER/A/C THEN TOOK ITS SUCTION OUT OF THE BASMENT. IT WORKED BEAUTIFLY. BE AWARE THAT BUILDING CODES MAY NOT ALLOW THIS, SO IT MAY BE NECESSAY TO PUT DAMPERS IN THE RETURN AIR DUCTWORK CONNECTED TO FIRE/SMOKE SENSORS SO CLOSE AND SHUT OFF THE HEATER/AC BLOWER IF SMOKE/FIRE IS DETECTED.
4. DO ITEM 3 AND WATERPROOF THE WALLS

ANOTHER CONCERN IS CORROSION SINCE YOU ARE NEAR SALT WATER. THE A/C CONDENSATE MAY BE SALTIER AND MORE CORRSIOVE THAN NORMAL SO YOU MAY WNAT TO USE CORROSION RESISTENT MATERIALS IN THE A/C COILS.

REGARDS
DAVE HALL

http://WWW.WSEC1.COM

 

[GBAILEY]

Very first thing: I would identify the method of water entry to the basement and correct this issue. Obviously, it's impossible to dehumidify with an active source of water entry. If it's a leaky basement, waterproof the space as best possible. If it's infiltration, seal the holes in the building envelope. A continiously damp space/basement calls out for a nasty mold party.

BTW, is this a crawl space or a "real" basement with concrete floors and finished walls?

Once this issue is delt with, you probably can use your normal residential sizing and estimation methods to get the right equipment for comfort control. Residential equipment can usually handle reasonable infiltration/humidity problems. I design below grade spaces here in KY 70/50% in most cases, your mileage may vary.

It'll probably be difficult to find a residential unitary product that specifically addresses dehumidification (i.e has some kind if humidity control/sensing ablility and reheat capability) so a stand alone dehumidifier may be required if you can't completely eliminate abnormal water in the space.

If you have a good heat/cool load calculation program and some engineering experience/gumption you could oversize the equipment a bit and use baseboard reheat or something of the sort for dehumidification. Note that this option would NOT be very efficient or cost effective or easy to control, etc. , but it's doable.

 

[friartuck]

Hi Supaman

I used to work for a company that specialised in the removal of moisture in housing. In particular, damp caused by moisture produced by processes such as showers, kettles etc.

The method applied was much more effective than using ac dehumidification units which were ok to an extent, but didn't give particularly good coverage.

The system worked in a very simple way and a guarantee was given to the householder that if the system did not work, they could have a 100% refund. Not bad eh.
In my 10 years with the company, we installed probably well over 10,000 systems and had to refund about three customers.

So how did the system work.

Air was introduced (it was taken from the loft space which generally was warmer due to solar gain) Outside air is also drier (look on a psychrometric chart). This was then trickled into the building (only through one grille situated at the top of the stairs).

The dry air percolated throughout the house and diluted the moist air which in turn became drier and therefore dried out the rooms.

I recently did my secretary a favour and got her a unit. She lives in a bungalow with single glazing. They used to get damp in the corners and on all the windows. She hasn't had damp for 2 years now. 100% successful.

The units have temperature sensors to modulate the fan speed and ensure that freezing air is not introduced. but you could achieve this yourself.

The airflow does not need to be great, just 24hrs a day. 1 ac/hr is usually sufficient.

Hope this helps



Friar Tuck of Sherwood

 

[YardDarg]

How do you calculate air changes in a room?

 

[supaman79]

YardDarg

CFM = Volume x ACH/60

ACH=AIR CHANGER PER HR
CFM=CUBIC FEET PER MINUTE

 

[HVAC68]

For the benefit of those guys who are into SI units - I am struggling to change over

L/s = Volume in m3 x ACH / 3.6

HVAC68

 

[friartuck]

Room vol (m3) x ac/hr (i.e. 1 or 1.5 or 2 etc)/ 3600=m3/sec

the 3600 is secs in an hour. The 1000 correction is to convert m3 to litres i.e. 1000 litres in a m3.

Some fan manufacturers work in l/sec particularly for small domestic units up to 100l/sec. Over this, m3/sec is usually used. Also, m3/hr is common.(but not SI)

There is an amusing thread on SI units. The World of SI is complicated by peoples inability to stick to the basics, hence m3/hr.

Once you get into it, its fairly straight forward, but you will always find somebody changing the rules slightly.

Stick with it.

Friar Tuck of Sherwood