Is my GMVC951155DX short cycling? Advice please
I am looking for some advice on whether or not my furnace is short cylcing - or if there are other suggestions?
Immediately below is the set up and some context. The house, is 3500 sq feet excluding the basement (1500 sq feet), located in Canada's capital - with weather extremes of -35C to +35C. Currently -20C.
- A Goodman GMVC951155DX furnace (2007)
- Honeywell Vision Pro IAQ (2011)
- White Rodgers Humidifier (2006)
- Bryant ERV (2012)
- I recently had the ERV installed, along with a cold air return to the basement (there wasn't one before) and the installation of a furnace direct air intake pipe from outside (previously it was drawing air from the basement - which i suspect, now, may have been drawing/removing all the cold air that fell to the basement).
The new cold air return is 90% blocked given that the return is close to the furnace, and was drawing too much air - but it is still enough air even though I have partially blocked the return). If I unblock the new cold air return (completely) so that it sucks up the cold basement air, the furnace comes on and off 6 times in 20 minutes.
Prior to the new basement return, the house only circulated air from the first (main) and second floors. I realize there may be some insulation challenges in the basement, however the basement was much warmer before the changes.
The added difficulty, even with the current circutances (new cold air return largely blocked) is that the furnace is coming on for short periods. The temperature on the VisionPro thermostat aligns with what is called for, but the furnace only comes on for approx 5 minutes, and does not seem to be sufficient to warm up the house (it feels cold). As for gaps in between calls for heat, there are long gaps at high temperatures, but small amounts of time at low termperatures).
If I keep the house at a high temperature (I tried this, say 23C), the temperature in the house is maintained. However, if I keep the temperature at say 21C (70F), then the house gets cold despite the frequent calls for heat. I would have thought that if the the basement cold-air return was drawing more air than before, that the furnace would have come on more - and the house would have been warmer, not colder.
The technicians, as I understand, did not make any changes to the thermostat setup etc or change any settings on the HVAC, except to install the ERV controller next to the thermostat. Currently, the ERV is off in the above scenario.
What could be causing the furnace to short-cycle (on for short periods)?
Last edited by cliff_steve; 01-15-2012 at 03:26 PM.
Reason: contradiction in question; clarification made.
I can think of several issues that can cause a furnace to short cycle but extra return air is not one of them. Has a valid Manual 'J' load calculation been done on the house? Perhaps the furnace is over sized. You've stated, I believe, that the t-stat is maintaining set-point so over sizing is the most likely candidate. As to the basement return making the basement colder or the house colder, I doubt that's the cause. The ERV being installed is a concern. Things seem to have changed since that addition, is that a fair statement? If so, then the overall temperature rise of the furnace would be lowered, resulting in a lower supply temperature to the rooms. This certainly could cause things to feel cooler, even though the room temperature is satisfied. Certainly you understand that the HRV (I would hope it's an HRV rather than an ERV in your climate as an ERV can freeze up internally in cold weather) is pulling in outdoor ambient air. The purpose of the HRV should be to create a forced air exchange in the home. This would be necessary for a very tight house that has insufficient natural air exchange. I'll make a giant leap of faith here and assume that the installation of the HRV was the result of a blower door test determining that your natural air exchange rate either recommended or mandated mechanical forced air exchange? If not and you just arbitrarily elected to have the HRV installed, you may have upset the apple cart of comfort. Only the appropriate testing and/or load analysis can really answer your questions. As I stated at the outset, I can think of several situations that can cause a furnace to short cycle.
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The heating cycles per hour are factory set at 5 on the IAQ stat. Which in itself will cause too short of cycles on a 90% furnace. The setup needs to be gone through completely, including the cycles per hour, should be 3. That's assuming the furnace and ductwork are all sized correctly...
possibly related to - temperature sensitivity of honeywell vision pro iaq?
Thanks, it seems the auto-discover feature has moved from 5 to 3 already - presumably today, when the weather averaged -24C. I had checked it very recently, and it was at 5 (I had thought to change the number manually then, but did not).
Is it possible, that it has something to do with the preciseness of the temperature maintained by the Honeywell Thermostat? For example, I believe the thermostat calls for heat after 0.3 degree drop in temperature, which is easily satisfied (and yes, probably an oversized furnace, purchasd by previous owner). However, if I increase the temperature by 0.5C, the furnace stays on longer, and the house feels warmer. I've noticed this through manual tests - for example, initiating the increase in temperature - and when fully satisfied, dropping it back down by 0.5, and then increasing it again.
I have had the super-precise thermostats previously, but I am wondering if this works in this house (my last house was much smaller, very different set up).
Is there a way to modify the temperature sensitivity setting on the Vision Pro IAQ?
I am not certain the issue is related to the ERV - which is currently off (and not in use given extremely low temp). I imagine the use of the ERV increases the need for additional heat, compared to other ventillation systems, but it supposed to average 70% transfer, even at -25 which is comparable.
Yes, there was an energy audit completed on the house by the previous owner, of which I have a copy, and an HRV was recommended.
Also, I have run the ERV on cold days no problem (it is apparently designed to recirculate house air to defrost the core based on outdoor temps - so that the erv core does not freeze up). Admittendly, it does blow some cold air - which is to be anticipated to a degree.
I am leaning towards either the thermostat - or the move to the outdoor ventillation (air intake goes outside the basement wall to draw in fresh air) compared from the indoor air intake (indirect intake)? The latter used to draw a lot of basement cold air for the furnace needs (and at the same time create negative pressure in the house); cold air which is now being left in the basement and being recirculated throughout the house? Could this be, if not the thermostat?
Last edited by cliff_steve; 01-16-2012 at 12:09 AM.
Reason: incomplete original posting
The intake for the furnace. Was it ran directly to the furnace. Or was it just ran into the room.
The fresh air intake now connects directly to the furnace from outside.
The only way to do that would be to adjust the cycles per hour even lower. If you set it to 2 per hr it would force it to run longer cycles, thus creating a larger temp swing. You could even experiment and set to 1 cph.
Originally Posted by cliff_steve
Yep, dunkman that is what I would do.
Originally Posted by dunkman
In much colder weather you might have to let it cycle 3 times an hour or the temp swing might be over 4-F degrees
My thermostat has a SWING setting from 1 to 9, I have it set at 7 now with the temp to be 20-F tonight; it cycles-on for about 18 minutes & stays off for more than an hour. The temp swing is around 4-F; drops 3-F below setpoint & cycles off 1-F above setpoint, - which is usually set at 65-F.
Tomorrow night it's predicted to be zero, so I might set it at a swing of 5.
This is my first winter with a 60,000 propane very-high efficiency furnace, so I am experimenting with it.
You will get more efficient use of fuel when the cycles are longer with longer times between cycles. It takes a while to heat the HT-EX, plenum & duct system each time it cycles on, if it short cycles it wastes too much fuel reheating everything before it reaches steady state conditions.
I've had run-time cycles from around 12 minutes to as high as 30 minutes with different settings, & due to the temp-swing it never got too hot or too cold.
I am alone & dress fairly warm & am comfortable with those settings & temp swings.
I am an efficiency nut & want to squeeze out every bit of it I can...
Last edited by udarrell; 01-16-2012 at 11:35 PM.
Reason: misplaced word...