How much is your bath vent fan costing you in heat?

Ryland · 09-26-11, 11:48 AM

How much is your bath fan costing you in heat? I was thinking about this because over the summer we found that the window A/C was better then running a dehumidifier because it's dumping the heat outside to dry the air inside the house out, but in the winter you want to keep as much heat inside the house as you can, so why not, instead of pushing that warm moist air out of the house in the winter, dry it out at that point with a dehumidifier?
Average bath vent fan is 80 cubic feet per minute of venting and I would guess it stays on for 30 minutes at a time and the fans are cheap to run, 40 watts for most old ones and down to 20 watts for an energy star rated fan, but an 80cfm fan left on for 30 minutes is going to pull 2400 cubic feet of heated air out of the house in the winter, air that then is being replaced by fresh cold air from outside the house that is coming in from drafts and cracks in the house.

So my question boils down to, how much does it cost to heat 2,400 cubic feet of air? and is it cheaper then running a 600 watt dehumidifier for what I suspect is going to be about half an hour as well, just judging from using the dehumidifier to dry out my living room in the summer from 90% down to 60%.

Ryland · 09-26-11, 12:26 PM

My figures seem to say that 2,400 cf of air is going to weigh about 179 pounds at 70F and I think that a pound of air should have the same thermal properties as a pound of water there for each degree we have to raise a pound of air is going to take the same amount of energy, there for, if I did my math right each degree we have to raise that 2,400 cf of air is going to take 322 BTU's of energy or 0.003217845 therms of natural gas at 100% efficient to raise that incoming air one degree, I pay $0.54 per therm of natural gas, so to raise the incoming air 10 degrees it would cost me $0.017 so if it's -0-F out then it's costing me around $0.13 to heat that incoming air to make up for the exhausted air, right? or about what it would cost me to run the dehumidifier or almost two hours! at -0-F of course so at freezing it would be right around an hour, but at the same time I would be adding .6kwh or heat to the room, making the house warmer, not cooler, electric heat is expensive heat but so is natural gas heat in my mind and the dehumidifier is automatic.

Please feel free to challenge my math.

AC_Hacker · 09-26-11, 12:51 PM

Quote:

Originally Posted by Ryland

How much is your bath fan costing you in heat?

I am working on a similar question, having to do with bathroom venting and also kitchen venting (the two primary sources of humidity in a house).

I am studying how Passive House design principles address this issue, and I have found that both bathroom and kitchen air are the inputs to the HRV (heat recovery ventilator). The HRV extracts the heat (or cold and moisture) from the incoming air by passing it over an air-to-air heat exchanger, before sending it outside. The outside air is drawn in, passes through the HRV where it receives heat (or cold and moisture) and is distributed throughout the house. In the Passive House system, 100% of the air that is picked up by the HRV system is exhausted to the outside, and is replaced 100% by incoming fresh air. Passive House HRVs have an efficiency rating of better than 85%. Passive House air exchange rates have a target of about .3 house-air-volumes per hour but typically, the rate seems to vary between .3 and .6 exchanges per hour.

Quote:

Originally Posted by Ryland

So my question boils down to, how much does it cost to heat 2,400 cubic feet of air?

To answer this, you would need to specify

the outdoor air temp and humidity
the indoor air temp and humidity
how are you heating the air
what is the cost of fuel
what is the efficiency of the air heater

...if you knew all of these things, you could pretty much calculate the cost yourself.

But as an aside, I think that as handy as cost comparisons are, we need to think about these issues in a broader context. The reason being that the true cost of alternatives hasn't previously been considered, and we now find ourselves following least-cost alternatives while doing tremendous damage to our planet.

For instance, there is a wonderful Fuel Cost Calculator over at Build it Solar that allows you to easily calculate the lowest cost and also the lowest CO2 producing alternative to heating your home.

Regards,

-AC_Hacker

strider3700 · 09-26-11, 01:14 PM

I've heard numerous reports from people about their HRV's becoming clogged with mold due to the condensation from cooling the outgoing moist air from bathrooms. These are all 5-10 year old units and I doubt the owners were doing any maintenance on them. Does anyone have any experience with this?

I remember seeing similar concerns about making an HRV for dryers as well and it made sense that it could be a bad idea.

Ryland · 09-26-11, 01:15 PM

I looked at a few air to air heat exchanger bath fans that are out there, they tend to cost $300 or so and they shut down the heat exchanger once the temp drops below 20F to prevent ice up.
But I agree, we need to look for the best solution, not just the cheapest, that is why I was wondering about just keeping the house sealed tight and making sure it's conditioned properly to prevent damage to the bath room, also because bath rooms tend to have moisture problems even if there are vent fans.

My neighber has moisture issues in their master bath room because of the complex shape of the room and a large walk in shower that takes a while to dry out and I think I've talked him in to trying the dehumidifier in the bath room to dry it out, paired with my kill-a-watt we will find out how much per month it really would cost to keep a good sized bath room dry, the hard part is figuring out the resources saved by not having to remodel the bath room due to mold and mildew over the years.

Piwoslaw · 09-26-11, 02:53 PM

This got me wondering: If you have a pound (or cubic foot, or whatever) of moist, post-bath air, and you separate the moisture from it, then which part has more (thermal) energy: the moisture or the dry air, both at the same temperature? I'd think that the amount of heat that can be reclaimed will come mostly from the phase change of condensing the steam, than from dry part of that unit of air.

I'd do the math to get a rough estimate of the proportions, but it's late here, and I've been fighting a cold all day, so what brains I had were blown into my handkerchief

AC_Hacker · 09-26-11, 07:21 PM

Quote:

Originally Posted by Piwoslaw

...which part has more (thermal) energy: the moisture or the dry air...

This doesn't really answer your question directly, but the rule-of-thumb increase in efficiency of a condensing vs. non-condensing gas furnace (or water heater) is about 15% greater efficiency, which is actually a lot, especially considering how good the efficiency of a modern non-condensing furnace already is.

-AC_Hacker

AC_Hacker · 09-26-11, 07:38 PM

Quote:

Originally Posted by strider3700

I've heard numerous reports from people about their HRV's becoming clogged with mold due to the condensation from [5-10 year old units in] bathrooms...

I've looked at quite a few diagrams of HRVs from patent drawings and also installation and service manuals, because I'm interested in doing a DIY HRV project.

In all the drawings I have looked at, the heat exchange cell is removable and is either made of plastic or aluminum. The aluminum cell could easily be run through a dishwasher every few years.

Ditto the plastic one, but care would need to be taken to defeat the heater coil in the dishwasher, else the cell would deform in the dry cycle.

I have a long, sad trail of wilted plastic salad tongs that attests to the hazard of the dry cycle.

-AC_Hacker

Xringer · 09-26-11, 10:39 PM

I know that my bath room vent is pulling cold air in from outdoors,
but, it's not something that we really notice.
The amount of air that's infiltrating in from the light switch plates etc,
is not having much, if any effect on the thermostats.

Just going in and out of the front door, bringing in the groceries,
seems to have more of a cooling effect than running the bathroom fan.

I know my house isn't super tight. It was assembled by college kids working part-time, in Oct 1956.
Plus, my ears don't pop when the oil burner comes on.. That thing really sucks in the fresh air.
Maybe this year, I'll get some fat PVC pipe and build it an air supply.
Might warm up the basement a bit this winter.

09-26-11, 11:48 AM	#1
Ryland Master EcoRenovator Join Date: Mar 2009 Location: Western Wisconsin. Posts: 913 Thanks: 127 Thanked 82 Times in 71 Posts	How much is your bath vent fan costing you in heat? How much is your bath fan costing you in heat? I was thinking about this because over the summer we found that the window A/C was better then running a dehumidifier because it's dumping the heat outside to dry the air inside the house out, but in the winter you want to keep as much heat inside the house as you can, so why not, instead of pushing that warm moist air out of the house in the winter, dry it out at that point with a dehumidifier? Average bath vent fan is 80 cubic feet per minute of venting and I would guess it stays on for 30 minutes at a time and the fans are cheap to run, 40 watts for most old ones and down to 20 watts for an energy star rated fan, but an 80cfm fan left on for 30 minutes is going to pull 2400 cubic feet of heated air out of the house in the winter, air that then is being replaced by fresh cold air from outside the house that is coming in from drafts and cracks in the house. So my question boils down to, how much does it cost to heat 2,400 cubic feet of air? and is it cheaper then running a 600 watt dehumidifier for what I suspect is going to be about half an hour as well, just judging from using the dehumidifier to dry out my living room in the summer from 90% down to 60%.

09-26-11, 02:53 PM	#6
Piwoslaw Super Moderator Join Date: May 2009 Location: Warsaw, Poland Posts: 961 Thanks: 188 Thanked 110 Times in 86 Posts	This got me wondering: If you have a pound (or cubic foot, or whatever) of moist, post-bath air, and you separate the moisture from it, then which part has more (thermal) energy: the moisture or the dry air, both at the same temperature? I'd think that the amount of heat that can be reclaimed will come mostly from the phase change of condensing the steam, than from dry part of that unit of air. I'd do the math to get a rough estimate of the proportions, but it's late here, and I've been fighting a cold all day, so what brains I had were blown into my handkerchief __________________ Ecorenovation - the bottomless piggy bank that tries to tame the energy hog.

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09-26-11, 12:26 PM	#2
Ryland Master EcoRenovator Join Date: Mar 2009 Location: Western Wisconsin. Posts: 913 Thanks: 127 Thanked 82 Times in 71 Posts	My figures seem to say that 2,400 cf of air is going to weigh about 179 pounds at 70F and I think that a pound of air should have the same thermal properties as a pound of water there for each degree we have to raise a pound of air is going to take the same amount of energy, there for, if I did my math right each degree we have to raise that 2,400 cf of air is going to take 322 BTU's of energy or 0.003217845 therms of natural gas at 100% efficient to raise that incoming air one degree, I pay $0.54 per therm of natural gas, so to raise the incoming air 10 degrees it would cost me $0.017 so if it's -0-F out then it's costing me around $0.13 to heat that incoming air to make up for the exhausted air, right? or about what it would cost me to run the dehumidifier or almost two hours! at -0-F of course so at freezing it would be right around an hour, but at the same time I would be adding .6kwh or heat to the room, making the house warmer, not cooler, electric heat is expensive heat but so is natural gas heat in my mind and the dehumidifier is automatic. Please feel free to challenge my math.

09-26-11, 01:14 PM	#4
strider3700 Master EcoRenovator Join Date: Dec 2008 Location: Vancouver Island BC Posts: 745 Thanks: 23 Thanked 37 Times in 30 Posts	I've heard numerous reports from people about their HRV's becoming clogged with mold due to the condensation from cooling the outgoing moist air from bathrooms. These are all 5-10 year old units and I doubt the owners were doing any maintenance on them. Does anyone have any experience with this? I remember seeing similar concerns about making an HRV for dryers as well and it made sense that it could be a bad idea.

09-26-11, 01:15 PM	#5
Ryland Master EcoRenovator Join Date: Mar 2009 Location: Western Wisconsin. Posts: 913 Thanks: 127 Thanked 82 Times in 71 Posts	I looked at a few air to air heat exchanger bath fans that are out there, they tend to cost $300 or so and they shut down the heat exchanger once the temp drops below 20F to prevent ice up. But I agree, we need to look for the best solution, not just the cheapest, that is why I was wondering about just keeping the house sealed tight and making sure it's conditioned properly to prevent damage to the bath room, also because bath rooms tend to have moisture problems even if there are vent fans. My neighber has moisture issues in their master bath room because of the complex shape of the room and a large walk in shower that takes a while to dry out and I think I've talked him in to trying the dehumidifier in the bath room to dry it out, paired with my kill-a-watt we will find out how much per month it really would cost to keep a good sized bath room dry, the hard part is figuring out the resources saved by not having to remodel the bath room due to mold and mildew over the years.

09-26-11, 10:39 PM	#9
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	I know that my bath room vent is pulling cold air in from outdoors, but, it's not something that we really notice. The amount of air that's infiltrating in from the light switch plates etc, is not having much, if any effect on the thermostats. Just going in and out of the front door, bringing in the groceries, seems to have more of a cooling effect than running the bathroom fan. I know my house isn't super tight. It was assembled by college kids working part-time, in Oct 1956. Plus, my ears don't pop when the oil burner comes on.. That thing really sucks in the fresh air. Maybe this year, I'll get some fat PVC pipe and build it an air supply. Might warm up the basement a bit this winter.