is ERV best way to reduce CO2 in 1980s house, zone 3a?
acrobaticnurse_Eli
| Posted in General Questions on
I live in Durham, North Carolina climate zone 3a in a 2100 square foot 1980s two story house. There are four of us in the house, which per ASHRAE could indicate a need for ~75cfm of fresh air per hour. I recently purchased an Aranet4 CO2 monitor and realized that while CO2 levels drop to around 800 overnight in rooms that haven’t had anyone in them all night, my bedroom CO2 level hits 2000 with the door closed, 1800 with it open, and any room with people in it stays above 1100, which probably doesn’t help my wife’s migraines.
We don’t have any combustion appliances and I hadn’t thought of the house as especially tight, but I’m wondering if a Broan or Panasonic ERV might be in order. I mostly see ERVs/HRVs recommended for tight homes but I’m not sure what a better option would be in our case. It generally feels too humid outside to simply leave windows open, despite living in a forest with otherwise clean air.
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Wind and colder weather (enhanced stack effect) will ventilate a leaky house. You'll see this if you pay attention to outside conditions vs your interior Co2 levels. However, if the temps are warmer, and the outside wind speed low, then likely you would benefit from an ERV. The ERV is your best bet to moderate inside vs outside humidity and ventilate.
One thing that you can try now (best if you have a furnace with EC motor) is to place your HVAC system in constant air flow (fan mode) at night to help with the high bedroom CO2. This strategy will use a fair bit of power if your furnace uses a PSC fan.
Thank you. I'll experiment with running the air handler fan more often and monitor CO2 levels. I have not yet installed a circuit specific energy monitor but at least Duke lets me monitor my hourly energy use. My air handler is a Goodman ASPT Series Air Handler w/ Constant Torque ECM Blower.
Running the fan 24/7 is the answer. Also make sure you have clean filters.
Running the hvac in fan mode overnight made a big difference, keeping CO2 levels in the 900s for most of the night vs over 1500. I'm not sure of the exact power use, but it may be a few hundred watts per hour judging by my hourly energy use bump. At 8 cents per kwh it may cost 4 cents per hour to run the air handler. More expensive than an ERV per hour but without the additional equipment cost, and it definitely helps in the short term while sorting out what else makes sense. Now if only my thermostat would let me tell it to run the fan for more than 12 hours at a time.
If you can find the manual for your air handler, you may be able to set the power of the blower (CFM) in "fan only" mode to lower the power usage. For example, on mine, I can set it to about 30%, 50%, 75% or 100% of maximum power for "fan only" mode. I have found that keeping it at the lowest setting (30%) uses the least power by far, but provides adequate ventilation such that it's not worth running the fan only mode at a higher power.
At 30% my furnace only consumes about 50W. But at 100% it consumes like 500W. So you might be able to save a ton of power by cranking it down to a lower power for fan only mode. (or, it's default setting might already be for a pretty low power in fan only mode. But it's worth checking.)
Glad that helped out a bit :-)
As Pnw suggests, there might be jumpers on the control board of the air handler (see my comments below) to lower circ fan speed. Our Lennox has a touch screen control so fan speed mode CFM levels are set electronically. Accessing the “hidden” service settings allows you to set fan speed in 50 CFM increments so I’ve tweaked it as low as possible (given sensor readings) and only run it at night when we’re sleeping and power costs are lowest. The HRV is standalone and dumps fresh air into the main living space. The HVAC fan is needed at night to keep bedroom CO2 levels low at night.
On the EC fan air handlers at my commercial building, circulation fan speed is set by jumpers on the control board in the air handler itself.
On your Goodman unit, it may be power taps only, so no way to change the fan speed for fan only mode: https://www.goodmanmfg.com/docs/librariesprovider6/default-document-library/ss-gaspt.pdf?view=true&mobiledevice=true
If this is the case you may want to look at a stat like the Ecobee which can dial in circulation fan times that take into account the heat/cool demand run times. You can use the same stat to manage an ERV so that the air handler fan runs at the same time as the ERV if doing an integrated install…as you would likely be doing.
Thank you. I will look into the Ecobee as it would be great to have more control over when the fan runs and to have the option of easier integration with an ERV.
I researched my current nest thermostat more and finally found the right settings to have the fan run a certain amount of time per hour. I'll experiment and see how often I need it to run in order to have air quality where I want it. Thanks again.
I'm sure you realise this already, but distributing air inside the envelope will work great if it's windy outside (so differential pressure on the house exterior is high), and it will help to even out Co2 levels at night, but you will likely see a need for ventilation still in other conditions. The fact that you have efficient heat/cooling and dehumidification should make integration of an ERV pretty straightforward, ducted and interlocked to your air handler.
Agreed. While running the air handler 15 out of every 60 minutes has substantially decreased indoor CO2 levels there is still room for improvement. I initially thought about doing separate ductwork for the ERV but I'm seeing more and more support for using the ductwork I already have.
I'm considering a Broan ERV, specifically the B210E75RS since the 210cfm capacity would give me plenty of room to go up or down. I'm also realizing I could use a supply and return duct kit to tie my current Santa Fe Advance 90 into the mix, having it further dehumidify fresh air from the ERV before passing on to the HVAC supply ductwork. Having air go through the ERV first would also decrease the work on the dehumidifier.
In order to make ERV maintenance smoother I'm inclined to use large low pressure drop in-line filters for both the exhaust air before it gets to the ERV and the fresh air intake before it gets to the ERV so there should be much less to clean up in the ERV itself.
See my comment #6 for why running the fan actually increases infiltration.
Running the fan causes a slight increase in pressure in the rooms with registers and a slight decrease in pressure in the parts of the house with returns. This leads to more air leaking through the building envelope. In a house built in the 1980's I would expect this leakage to be significant.
It may well be that by running the fan you're just ventilating the house more.
After running the HVAC fan for a couple hours the air felt much fresher throughout the house. With temperatures having been fairly moderate the past week the air handler hadn't run for multiple days.
I finally scheduled a blower door test for later this month to see just how tight/leaky my house is since Duke Power offers them at a subsidized price of $50 and so many calculations for how much fresh air is needed are based off of knowing how much air leakage a house has. It will help to get an approximate number.
In my two story house there is a 16 by 20 inch return at the top and bottom of the stairs that are in the center of the house, and every room has 1-3 supply registers. The ducts are rigid metal and insulated. For heat I have a basic Amana/Goodman ASZ14 heat pump that was installed right before I purchased the home last year. There very well may be leakage around outlets and windows, though I found the walls themselves have a plastic vapor barrier directly behind the drywall which might make leakage less than otherwise expected. The walls are more apt to dry outward as the sheathing is mostly celotex with plywood at corners.
I have an in-wall Santa Fe Ultra MD33 that has done a fair job of managing interior humidity, and a Santa Fe Advance 90 in the encapsulated crawlspace. At this point I wish I'd installed a ducted Santa Fe Ultra to manage the whole house rather than the somewhat noisy MD33, and I may still do so at a later date as part of providing fresh air. Adding humidity sensors to the crawlspace, attic, kitchen, and bedrooms prompted much of my early work on the house and now it's CO2 monitoring.
My dryer is a Miele heat pump, so no dryer duct needed and the old one is plugged up. We have a fireplace but it remains plugged as well. I added a ducted range hood and bath fan as before there were no bath fans and the range hood was recirculating.
I agree that an increase in infiltration is likely at least part of what's decreasing CO2 levels, and at best it's only dispersing it throughout the rest of the house until it can leak out. I would rather not be drawing in lots of pollen and humid air at other times of year, so an ERV seems like a good way of controlling the fresh air intake unless you have a better suggestion for a probably at least moderately leaky 1980s house. There are probably less energy intensive ways to increase infiltration in the mean time, including maybe just opening a window or putting a fan in a window.
8 cents per kw… consider yourself lucky, very lucky…
I do. I wouldn't so readily consider running a ~500 watt HVAC fan if electricity cost 25 cents per kwh. I'm still looking for options that aren't just spreading CO2 around/increasing infiltration, whether it be AirKing's Qufresh, AirCylcer, a ventilating dehumidifier, and/or an ERV.
It would be very surprising if your HVAC fan uses 500 watts in fan only mode. Are you sure?
Since you have already invested in a dehumidifier, I'd go the ERV route if you really have the need to bring in fresh air. A ventilating dehumid is not as efficient as an ERV when in venting only mode, and in that mode it just brings in air without any energy transfer or exhausting of stale air.
I'm trying to not cry in California. Cost vary between 25 cents and 59 cents per kWh here for me depending upon season and time of day.
I'm realizing how much the cost of energy can affect building decisions. If energy costs were 25 to 59 cents per kwh here it'd be a much easier decision to find a way to fit a heat pump water heater and add exterior insulation vs maintaining a 24 year old electric water heater, and painting and replacing a few pieces of siding as needed. Replacing everything with more efficient options would have a higher carbon cost without much financial savings. I'm still trying to make sure I'm not being wasteful.
If you are trying to "fit" a HP WH, please start a new post about that.
There are now some terrific options both power (120), size and cost wise.
With a 24 year old WH it is only a matter of time - you should be prepared with a plan for a HP WH
Over the past several days the weather has been nearly perfect, staying about the same temperature during the day outside as inside with minimal wind, low humidity, low UV and low pollution, which has coincided with indoor CO2 levels climbing as the air handler continues to run 15 minutes per hour. Even rooms that have had no one in them all day started to have CO2 levels of 1200, and our bedroom started hitting 1500 again when we were in it, rarely dropping to the 900s anymore.
This afternoon I realized I should take advantage of the rare weather and bring in some fresh air. I placed a small ~5 inch high and ~2 foot wide fan in a bedroom that is mostly used as a playroom and set it to blow fresh air in, and then across the hall, past the upstairs air return, I turned the bathroom fan on to bring at least some of that fresh air into our main bedroom on the way to the bathroom. So far it seems to be working. In about an hour it brought CO2 in the bedroom down from 1200 to 900, and the room with the fresh air fan is in the 600s and dropping, feeling much more comfortable.
This isn't a long term solution in and of itself, but it's a nice release for the moment, and I can see leaving the fan in the window for a while as we used to leave it in the kitchen to run as an exhaust fan before I installed a vented range hood. I'll turn it off when it's cold, as it will be down to the 30s overnight, and back on when the weather is nicer. The window has a screen for bugs and the short fan doesn't require the window to be open more than a few inches. I'll leave the aranet4 in that room overnight to see how cold it gets given that the room will also be getting warm air as needed from the heat pump.
You should take advantage of the nice weather to take a few CO2 readings outdoors. Basically get an idea of what zero on the scale is.
When I first got the aranet4 I saw CO2 levels in the 430s outside so I was surprised today when I tested multiple locations 2 to 15 feet away from the house and got readings from 450 to 475. A friend pointed out that having a ton of leaves in various stages of breakdown may be boosting outdoor CO2 levels. Indoor levels today have been between the 600s to 900s. I kept the fan in the window overnight but left it off until this afternoon. Given that low speed is rated for 73cfm I may experiment with leaving it on low to approximate what a fresh air supply to ductwork would provide, though the fresh air intake would disperse it throughout the house first and filter it.
See my post, #1, low wind and moderate temps will likely coincide with high co2 inside a leaky home. This is where an ERV, particularly one that ramps with CO2 levels, will shine.
On your proposed setup, remember the KISS rule :-)
As I dig through the manual for my santa fe dehumidifier I see one option for ducting it in is to have it take over an hvac return if a house has more than one. My air handler has separate returns for the upstairs and downstairs despite not being zoned, each with 12 inch round ductwork. The dehumidifier is designed to be able to run the fan continuously to purify a whole home's air, even when not dehumidifying, and has a 10 inch return and supply for managing 300+ cfm. When I next check on it I'll use a kill-a-watt meter to see how many watts it needs to just run the fan, as up to now I've only had it turn on when dehumidifying.
Per the manual, I could have the upstairs return run directly into the dehumidifier and have that then connect directly to the supply with a backdraft damper in place. I can use a T or wye to have the 6 inch duct with ~65 cfm of fresh air connect to the duct coming into the dehumidifier, with it then going through the 2 inch thick merv 13 filter of the Santa Fe unless/until I opt to add an in-line filter. Sante Fe's filters seem good and are not overpriced, so we'll see. A motorized damper could be added to the fresh air line but if I'm happy getting the full 65 cfm it's described as optional. I could try this before adding an ERV to see how it works, and then if I still want an ERV I could use a 12 inch 10 6 wye from the upstairs return and have the 10 inch duct go to the dehumidifier, the 6 to the erv, and then have the ERV fresh air supply for coming back into the house connect to the T going into the Santa Fe. That way the dehumidifier isn't limited by the CFM of the ERV and can circulate air throughout the house even when the air handler is off.
A nice bonus to this initially would be not purchasing more equipment, just the duct connectors and ductwork to connect the dehumidifier.
Today was the day I was supposed to have a blower door test to see just how leaky my house is, but when the energy assessor recommended by my power company arrived he said they're currently only doing thermal imaging and blower door tests are on hold. It was still nice having him look at my walls, fireplace, attic, and crawl space, though as I followed along with my own thermal camera he suggested I was already equipped to seek out further leaks. I didn't get an air leakage number, just validation that I was already on a good path to improvement, with no glaring thermal holes noted.
I've been trying to research what I can and make sure I don't rush into getting something that isn't the right fit, but it seems that as long as I get a large enough ERV I can just plan on adjusting the CFM based on comfort/CO2 levels without knowing the exact amount of fresh air needed per ASHRAE. If I were doing this for someone else and I wasn't going to be there on a daily basis it would become more useful to have exact data, and at that point I'd also be more justified in either getting my own blower door or connecting with someone that does.
I finally set up an emporia vue 2 energy monitor in the sub panel with air handler so I can monitor the fan only mode energy usage and it appears that the estimate I got from looking at Duke Energy's app that lets me see my whole house energy usage down to 30 minute intervals was accurate. The fan clocks right around 500 watts, 4 cents per hour. Not too bad financially at 8 cents per kwh, but as discussed throughout the thread if the fan is mostly improving CO2 via increased infiltration I'd rather be controlling that via an ERV.
I finally have everything I expect to need for installing the Broan 210cfm erv including in line filters and hopefully within the month I'll be able to get it fully installed and post photos.
As others have commented, 500W in fan-only mode is actually pretty bad. For comparison, my furnace fan in fan-only mode uses 58W to circulate about 500 cfm. In my old house, which had better ductwork and a newer system, my furnace fan in fan-only mode used only 20W to circulate about 400 cfm.
I'm not saying it would be cost effective to change anything. Just be aware that 500W in fan-only mode, especially for a 2100 sq ft house, is actually very high.
I was finally able to run my Broan ERV today, having to wait for a break in rain to add the inlet and outlet to the attic. With a single stage constant torque ECM air handler for my heat pump that takes 500 watts to run, I opted to give the ERV its own basic ductwork.
I have 6 inch ducts pulling stale air from the two upstairs bathrooms, joining into an 8 inch duct that then goes through a 2 inch thick 14 by 20 inch merv 8 inline filter. For filtering fresh air into the house I used an insulated in-line filter with two 14 by 20 inch MERV 13 filters that are 2 inches thick and in a V configuration. Had I not already purchased the filter boxes I'd have been tempted to see if I could do anything like Dennis had built, but I'm simply glad to have everything working now. If outdoor smoke becomes much of an issue I may experiment later.
Rather than duct to each bedroom for the fresh air supply, so far I have an 8 inch duct supplying the upstairs hallway which is between the three bedrooms and by the upstairs hvac return so when the air handler is on it can also help distribute to the rest of the house. I've added a Tamarack Perfect Balance return in the bottom of one bedroom door so far and will be taking care of the others shortly.
I have not yet installed the optional control panel or the bathroom sensor that can ramp the ERV up to max settings upon sensing increased humidity. I had it run at max for an hour and watched the bedroom CO2 level drop from 1600 to the 700s. After being 17F last week it's now in the 70s outside so opening a window stopped doing much to drop CO2, but it also made it a prime time to ramp up the ERV. CO2 hit 2100 last night with the window open so I'm looking forward to see what it's like tonight with the window closed and ERV running.
While it uses 240 watts at 188 cfm, turning it down to 95cfm uses only 44 watts and is of course quieter, so I will likely leave it in the 90s most of the time.
Sounds like a well thought out system that is performing well too. Certainly power use on the ERV in a standalone ducted setup makes more sense vs running a 500 watt system to distribute fresh air. With a family of four, two cats, 2400 square feet total, tight’ish house, our system spends a lot of time in that 60-75 CFM range to keep co2 levels 600-800 ppm.
Thank you. We're also a family of four, in a 2100 square foot home but no pets at the moment. I decided to try turning the CFM down from 97 to 70 two days ago and watched the energy use drop from 44 to 24 watts, and our CO2 levels are still good.
It's nice being able to adjust the exact desired cfm from a control panel in my bedroom and then look at the emporia app on my phone to see how many watts the erv is using at that cfm, and monitor the CO2 trend on the aranet4 throughout the week.