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Does anyone know the approximate cost to run an AHU blower?

W Ramsay | Posted in General Questions on

We are building a new house and is currently in mechanical rough-in. The ERV has been configured with the stale air fed from the return duct and the fresh air injected in to the return duct 5′ downstream from that. So the AHU blower will have to run along with the ERV and we’ll likely have to run the ERV more often to maintain good CO2/VOC levels in bedrooms at night.

We are talking to the HVAC folks about doing fully ducted. The numbers they are quoting are quite ridiculous. Is there a good way to guesstimate the cost savings for operating a fully ducted ERV vs what we have?

Thanks,

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Replies

  1. Expert Member
    Zephyr7 | | #1

    If you know what equipment you’ll be running, it’s easy to calculate out power consumption. If the unit gives a running rating in watts, just use that. If not, it should spec volts and amps. Multiplying volts times amps will approximate the wattage (not perfect, but close assuming a decent power factor, and you’ll err on the high side so you’ll get a safe estimate).

    Once you have the watt load, multiply that by 24 hours, then by 30 days. Divide that by 1000 and that’s how many kilowatt hours your device is using if running all day, every day, all month long. Your electric bill will give you your rate per kWh (remember to add in all the extra charges for distribution and the like, or just divide the dollar amount on the bill by the number of kWh you used to get a per-kWh price). It’s easy to determine your operating costs.

    A quick example: Assume your blower is used 350 watts to run. 350*24 hours=8,400 wh/day
    8,400*30 days = 252,000 wh/month. Divide that by 1,000 to get 252 kWh/month. Assume you’re billed at 12 cents per kWh, then your cost to run that 350 watt blower all day, every day, is $30.24/month.

    Note that many utilities offer demand metering, and usually the “on peak” time is 8 hours on weekdays with all other times off-peak. Off peak rates are usually substantially less $/kWh. You will usually save money with continuous loads like this on a demand metering rate since the time off-peak is enough cheaper to more than offset the increased cost during the on-peak time. Basically you usually save money overall, but you’ll have to look at the rest of your loads and the specific rate structure your utility offers to make a good decision if such a rate will save you money overall.

    Bill

  2. Jon R | | #2

    It will use very little power (~10W) if there is a way to turn it way down.

    https://www.buildingscience.com/documents/published-articles/pa-ecm-eficiency/view

  3. W Ramsay | | #3

    Thanks Z. The furnaces are Lennox EL296v. Not sure what specific model. I've not yet been able to find detailed electrical specs so might have to wait until I can get them from our HVAC folks on Monday and then try to run the calcs. It will be interesting to see how close this is to actual measurements once we move in.

    Jon, thanks for the link. Great article. It's sometimes easy to throw the baby out with the bath water. For me IAQ trumps energy efficiency so I'll run a blower on circulate or run an ERV more frequently for better IAQ. Given what the HVAC folks want to charge for this 'unusual upgrade' we may be better off financially to just run the AHU blowers on circulate all the time. Not ideal from an energy standpoint but given the efficiency of ECM's at these lower circulation rates might be much more efficient from a monetary standpoint.

  4. Jon R | | #4

    The iComfort S30 thermostat that you probably want to use suggests that continuous fan operation can be set down to 450 CFM (although EL296E vs EL296V isn't clear to me). Probably less than 50W (very roughly $50/year). I tend to agree that this is cost effective and will provide better ventilation - Co2 removal is easy to achieve without up-sizing the ERV.

    Note that furnace air flow rate may or may not change ERV balance. This is also a numbers question. For example, see the flat fan curve on the FV-10VEC1 - the effect of the furnace fan on ERV balance is insignificant.

    Separate ERV ducting as "best" needs to be reexamined. It's likely to cost more and may well provide worse ventilation distribution - the CFM numbers often don't add up if you want 15+ CFM per actual person in each breathing zone. Sharing ducts with a furnace also has issues. So perhaps distributed, on demand ERVs are best.

  5. W Ramsay | | #5

    "Separate ERV ducting needs to be reexamined."

    Possibly. If we leave ours the way that it is then during the night we will be wasting a lot of fresh air and energy (ERV, AHU blower, Furnace/AC, Humidifiers?, Filters?) on parts of the house where it is unnecessary in order to maintain enough fresh air intake and circulation for bedrooms to be healthy (CO2 < 600ppm).

    What is that cost and waste of energy? Is it fairly negligible? We often have nighttime lows well below 0°f (I think -42°f is our record low) so even with a very efficient ERV/HRV we'll still need to condition the air somewhat.

    If it costs $3k extra to do fully ducted and it saves $80/year then from a monetary standpoint that's not good - assuming the way we have it and with circulate running continuously provides as good or better IAQ than fully ducted.

  6. Jon R | | #6

    My last comment was meant to be general, not applied to your shared duct plan - I've edited it for clarity.

    I think you are on the right track. Don't waste money on dedicated ducts. Since the concern is very low bedroom CO2, use supplemental ventilation from a single room ERV (eg TwinFresh Comfo or perhaps something a little larger). Easy to calculate the CFM needed and less cost. If you want to, save energy by turning off the main ERV and the continuous furnace fan operation at night.

  7. Expert Member
    Akos | | #7

    Typical 1200cfm ECM blower is around 250W at full speed, at 400CFM power use drops down significantly to around 50W.

    Most furnaces have some jumpers to set the speed of the blower for fan only operation. You want this set as low as possible.

    One thing to watch for with continuous blower operation is de-humidification in the summer. Running the house fan after the A/C shuts down causes the water on the coil to re-evaporate. If you have a hard time lowering the humidity in the summer, you might want to change the ERV to timed operation (say 20min/hour).

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