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Old 11-18-14, 07:32 PM   #21
Fordguy64
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been away for a while but 2 weeks ago i replaced my 30 year old 40-60% eff (per the only info i could find on it) with a new 2 stage 95% eff furnace with ecm blower. i also replaced the lux thermastate that allowed me to adjust the swing with a nest learning thermostat.

well its been very cold the last few days down into the single digits at night and upper 20s low 30s during the day. its hard to tell how long its actually run but i know it has only kicked into the high stage once (on its own) but i did force it before just to make sure it worked


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Old 11-19-14, 06:42 PM   #22
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Quote:
Originally Posted by Ormston View Post
I was once asked to look at a device that had been retrofitted to an oil boiler heating a swimming pool.
The device had been sold as a fuel saver for what seemed like a lot of money at the time, I think it was something like £200 (around 23 years ago). The owner reconed it had paid for itself in the first year and was very keen for me to fix it as it had stopped working.

This £200 box contained a small hand soldered board with a 555 timer switching a relay. The fault turned out to be a failed relay so was an easy fix.

The device worked by delaying the startup of the burner when the thermostat called for heat, there was a pot on the fromt of the box labeled low to high saving. The higher the saving selected the longer the delay before starting the burner. I think it was something like 20 to 30 mins delay.
This delay has the effect of increasing boiler run times by forcing longer off periods.

I have no idea how well this works in a domestic application, it's on my to do list to try with my heat pump.

Steve
This type of control would definitely put an end to a system that was short cycling for some reason. It would definitely work better for a hydronic setup, though. Just think what would happen if it were rigged to a short-cycling air handler or blast furnace! What would the homeowner say if they turned up the thermostat and the furnace didn't immediately start blasting hot air???
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Old 11-20-14, 01:50 AM   #23
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Quote:
Originally Posted by jeff5may View Post
This type of control would definitely put an end to a system that was short cycling for some reason. It would definitely work better for a hydronic setup, though. Just think what would happen if it were rigged to a short-cycling air handler or blast furnace! What would the homeowner say if they turned up the thermostat and the furnace didn't immediately start blasting hot air???
Probably not suitable for every system/install, UK heating systems are almost exclusively hydronic or electric storage heaters.
I,ve not seen or heard of any hot air systems being installed in my lifetime(must have been some but certainly not the norm), the only houses I,ve ever seen with hot air system were hated by their owners for the sudden cold feeling as the blower stopped and for their inneficiency.

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Old 11-20-14, 11:26 AM   #24
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As long as the system spends most of its cycle time burning gas and blowing hot air, it isn't short cycling. Many apartment and condo units have furnaces that have very little "dead" time waiting for heating calls in between run cycles. I lived in one a number of years ago that I swear cycled fifteen or more times an hour. Vent 1 minute, light burner, blower kicks on, burner shuts off 2 minutes later, blower shuts off 1 minute later. 3 Minutes later, vent kicks on and here we go again. Not a well designed heating system.
I'm not sure this gathers the whole efficiency equation though. Let me explain. What I've done is stuck an analog dial-type stick thermometer through the rubber sheath above the AC evaporator coil and also put a digital thermometer probe in the duct with the shortest run to the furnace.

What I found was the first 5 minutes of blower run time the temperature is still ramping up. Once I hit 10 minutes it is mostly there and starts to settle off but it doesn't reach the full temperature rise(supply temp subtract return air temp) until 15 minutes.

This also doesn't include the 75 seconds that the furnace runs the burner without blowing air. My furnace runs the blower for 2 minutes after the burner shuts off but it is still putting out a 40 degree temp rise by the time it shuts off and this is with the highest (800CFM nominal) blower setting for a 60k nominal output furnace. I found that manually leaving the blower on until I hit 3 minutes would allow the temp rise to drop to about 20 degrees. Keep in mind that the heat exchanger and metal ductwork is absorbing some of this heat before the full temperature rise ends up coming through the ductwork and out the register and likewise after it is shutdown.

I figure efficiency would be boosted if I could get a cycle time of at least 20 minutes and modify things so the fan kicks on after 30 seconds of burner time and doesn't turn off until 3 minutes after the burner shuts off.

My thermostat has a maximum span of -1f/+2f, so if I have it set to 70 degrees, it will turn on at 69 and shut off at 72. With that 3 degree span I get a 12 minute call for heat which equates to a 10 minute burner time.

If anyone ever finds a thermostat that has a customizable minimum run time setting that would allow me to choose 20 or 30 minutes or a span that I could push out to get such a run time, I really want to know about it. When I manually run a 30 minute cycle, the temperature in the house is comfortable before and after the furnace runs so I'd like to configure the furnace like that. It would be especially nice to have a larger span during setback when I'm away from home too so I'd get a more efficient run.

So basically my opinion is that based on the experience from my furnace, I'd like to see at least a 20 minute cycle time to be reasonably efficient. If I could go for a 30 minute cycle time, I'd drop the thermostat down a degree or two to get roughly the same shut off temperature. I think the Honeywell default of 6 cycles per hour(at 50% load) is an atrocity to the lifespan of a furnace and to efficiency. I'm not sure how anyone could want their furnace to fire up every 10 minutes and run for 5 minutes each time at half load.

Oddly enough at half load, which I could get to before really starting air sealing work my furnace will fire up for a 30 minute call for heat and shut off for 30 minutes when it was -20f outside and set to 70f inside. So under all conditions my furnace will never fire up more often than once per hour. Now that I've done air sealing work and some extra insulation I've got my heat load down to 17k BTU/hr at design load(-11f). Definitely less run time on the furnace but I'd still like to enforce a minimum run time anyway to stretch out the lifespan of the furnace and get a little extra efficiency.
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Old 11-20-14, 12:34 PM   #25
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Looks like I'll have to measure vent outlet temperatures.

The cyberstat has a bunch of settings for temperature variance. I've never heard of any other thermostat with a +/- 3F swing on it. There is no setting for minimum run time, but it will control a heat pump, so you can probably control a minimum off time.

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Old 11-21-14, 07:52 AM   #26
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I just ran a little test.

There is 1 minute 15 seconds of burner run time prior to the blower starting.
Blower Time, Temp rise, percentage of max temp rise, %*57000BTUhr
1 minute, 25 degrees, 33%, 18810
2 minutes, 40 degrees, 53%, 30210
3 minutes, 50 degrees, 67%, 38190
4 minutes, 60 degrees, 80%, 45600
5 minutes, 65 degrees, 87%, 49590
6 minutes, 70 degrees, 93%, 53010
9 minutes, 75 degrees, 100%, 57000 (or about 10 minutes of the gas burner running which is actually 12 minutes of call for heat since my furnace takes 2 minutes from the call for heat until the burner kicks on)

After shutting down the furnace
1 minute, 50 degrees, 67%, 38190
2 minutes, 35 degrees, 47%, 26790

I'm not sure how to calculate all of this but if I add all of the figures together and average them I suppose I'll get somewhat close. It seems that I'm getting 67.75% of the steady state heat output in the first 10 minutes of operation including the cooldown period or about 38618 of the 57000. Which is 51.49% of the 75000 input rate. Seems a little hideous to me. What's worse is the previous owners had a thermostat that only had the gas valve running for 5 minutes which looks like an even worse proposition considering the temperature through the registers is still rising. If I take the 10 minutes and change it to 20 minutes, things look better at 91% of the heat output versus 67.75% of the 10 minute figure and 94% if we make a 30 minute run.

Granted, this math is rough but I think it illustrates the point.
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Old 11-21-14, 08:33 AM   #27
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Interesting numbers. Ill have to do some data collecting of my own and share it.
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Old 11-24-14, 09:11 AM   #28
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Installing a SMALLER FURNACE solves a lot of this. Cut the furnace size in 1/2 and run time each cycle triples. The time in between cycles doubles. Instead of running (4) 3minute cycles per hour, you run (2) 9 minute cylces. 12 minutes per hour @ 100K vs. 18 minutes @ 50k.

I found this when I changed my own furnace from 88k to 44k (yeah it was THAT oversized). The furnace actually runs long enough to get to to the full 45f rise instead of cycling off before reaching steady state. My funace runs 45 seconds before the blower kicks on, blower runs 3 minutes after burners switch off. There is about a 10f rise once blower kicks off. Even though the furnace cycles HALF as much thereis LESS overshoot/droop compared to the old furnace. Old furnace would overshoot 2-3 degrees, new only 1 degree.

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Old 11-28-14, 11:42 PM   #29
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Try moving to a lower fan speed for heat. This should allow you a faster warm up and higher output tep. I improvised a " variable speed profile" for my blower interrupting the board output with a spdt fan relay and an adjustable delay on make timer. NC on relay was low speed and NO was high speed.
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Old 11-29-14, 08:13 AM   #30
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Try moving to a lower fan speed for heat. This should allow you a faster warm up and higher output tep. I improvised a " variable speed profile" for my blower interrupting the board output with a spdt fan relay and an adjustable delay on make timer. NC on relay was low speed and NO was high speed.
Blower is single speed, one of my previous mods was replacing the OEM 3 speed 1075PRM motor with a single speed 825RPM. PSC motors have awful efficiency on low speed, old motor used 355W, new uses 150W. Temp rise did go up about 7 degrees when I changed motors. Furnace is rated 25-55f, 45f is on the high side, but still 10f from max.

I did do the mod you are talkind about on my old furnace. It had to be on HIGH speed to stay off limit, and the giant blower produced a cold draft when it 1st kicked on. A sequencer was my "time delay" of choice, had a fixed 30 second delay. Long enough to let the blower clear the cold air out of the ductwork while still on low speed.

Dumping the oversized beast and installign a furance 1/2 the size fixed a lot of comfort issues. No need for a low speed anymore since the blower isn't overpowering to begin with.

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