Thinking of switching KAA2 for a warm air

Hello all, I am thinking about switching my KAA2 out for a warm air furnace, after 2 years of running. I know this goes against the thoughts of the boiler/water guys here, but here is why. My KAA2 is currently heating a water to air coil in new 60,000 BTU propane furnace, we have so much variation daily in Michigan it is almost impossible to keep dialed in. When the blower in the furnace starts, it will take about 10 or 15 degrees out of the water temp. in around 9-12 minutes of run time. The boiler is small, and it can not add heat to the water with the blower running, sometimes it can almost maintain it, but never increase. The aquastat is doing its job, but makes me crazy! I have tried all different settings of high and low, but in the end the same results. What ends up happening is the water temp. will either slowly work its way to above high temp., or slowly below low temp. (no change to settings) Lets use this as the model (160 low 180 high differential at 10 stoker, 11 turns out, 4 pins every 10 min.) At 190 the aquastat shuts down the stoker, until it reaches 170, circ. pumps still runs maintains heat in the house. Soon the fire is to small to recover with the blower taking heat out of the water and water temp. drops to 150. Now the circ. pump stops until water gets to 170, house is getting colder, that means blower has to run longer, which means the stoker is running longer. (Not to mention the lower the water temp. the lower supply air temp, stoker has to run longer.) Or it just happens in reverse, and first falls below the low temp.

I have tried with the stoker and combustion fan cycling at the same time, and the way Keystoker instructs combustion fan running continuous. 20 deg. and up does ok, below that a crap shoot. I am not sure how to solve this, any feed back would be great.

Outdoor Reset does the adjustment fight for you...but there really is no magic tweaks for too small of boiler for the load.

Is your Kaa-2 boiler wired to fire immediately upon a T-Stat heat call? If not, and if it is firing on the low limit instead, that may be a big chunk your problem. If a 60,000 BTUH propane furnace is adequate for heating your house, it's fair to presume that a Kaa-2 ought to be also.

Should of said additionally, slow down that fan speed.

McGiever, I don't know what you mean by outside reset? Which blower would I slow down?

Isayre when the stat. calls for heat it starts the furnace blower and the stoker. The aquastat stop the circ. or stoker with either an over shoot or under.

McGiever wrote: ↑

Tue. Jan. 22, 2019 10:52 am

Should of said additionally, slow down that fan speed.

+1

Coal boilers need some time to work their way up to full output (which is why it needs to be wired to fire immediately upon a heat call). They are not instant on like propane. Slowing down the (likely multi-speed) propane furnace blower will allow for less heat to be emitted over more time, which is what a coal boiler needs until it gets up and running hard. The Delta-T drop through the coil will be reduced, easing the burden of catch-up on the Kaa-2. Your homes overall comfort level may also benefit from slowing down the blower and (as a consequence) extending the T-Stat heat call cycle times. Slow and steady wins the race.

Tryingitout wrote: ↑

Tue. Jan. 22, 2019 11:12 am

McGiever, I don't know what you mean by outside reset? Which blower would I slow down?

How does outdoor reset work?

Outdoor Reset controls respond to changes in weather by changing the boiler water temperature circulating throughout the building. It sends out cooler water to the system during the warmer outdoor temperatures. And sends warmer water to the building in cooler outdoor temperatures.

Blower in propain furnace needs slowed, so not to smack the boiler so hard and fast.

A 3 speed blower has one speed wire connected and 2 wires capped or insulated...read the motor plate/tag to see what color does what speed and take it down a notch...

I can definitely try slowing the blower down tonight. I will have to check I don't remember if there is one or 2 steps slower left.

If your furnace blower is running while your circulator has been cut-out due to the coal boiler hitting or falling below 150 degrees, this needs to be addressed also. Your furnace blower must be wired to cut out in conjunction with the circulator being intentionally cut-out. Otherwise cold air is blowing throughout your house until the coal boiler recovers and the circulator comes back on.

Yes I have been dealing with that for 2 years, over a stupid $6.00 part. (Bimetal Disc Thermostats) One will be at my house today, and installed tonight. Dumb I waited so long, I know.

By being highly conservative, I've calculated that your Kaa-2 boilers BTUH output should be essentially identical to the output of an ultra-high efficiency 60K BTUH input rated propane furnace. With the main difference being instant full output vs. not instant full output.

60,000 BTUH x 0.95 efficiency = 57K BTUH output for the propane furnace.
76,000 BTUH Net x 0.75 efficiency = 57K BTUH output for the coal boiler.
(where Net/Gross = the boilers 'Pick Up Factor', which for hot water boilers is generally somewhere around 1.15 to 1.20)

What is the Delta-T (temperature drop) you are seeing from coal boiler outlet to coal boiler return for the water to air exchanger loop?

Well I can't answer that one! My plan this fall was to install thermometers on the domestic coil before and after the mixing valve, and on supply and return. The problem is this past spring we remodeled 2/3rds of our house, and in the fall expanded and remodeled the kitchen and dinning room. Do to time frame I was not able to work on the plumbing.

The good news is I have proper insulation in the walls and ceilings, which has helped greatly!!!!

Larry, the 76k net rating on the KAA-2 is the amount it should be able to deliver from the installed radiation (in this case, the coil). I don't think a 0.75 multiplier is needed. I therefore agree fully with your earlier statement that the KAA-2 should be able to replicate the output of a 60k furnace. That said, I don't understand the rationale for reducing the fan speed to somehow accommodate the needs of the boiler. Isn't the low limit of the aquastat doing the same thing it would do if there were radiators instead of the coil? Isn't that the same thing the fan limit switch might do in the furnace when the system started cold?

To me, the 2 things that seem important here are:

- verify the output rating of the coil approximates the 60k, and that the aquastat settings and flow rate approximate the values upon which the rating is based; and,

- make sure the feed and air rates are set high enough, and maximize efficiency by producing a proper (1-2") ash band after a long burn.

I dislike underpowered systems more than anybody, but here it seems like if the boiler is pushed even close to its capacity it should be able to fully replicate whatever the propane unit would do.

Mike

Mike, the boiler will heat the house, we had 14 below Monday, still 70-71 in the house. The coal was 3/4" from the end, and just screaming hot. The problem is control over the boiler, it doesn't seem to matter what I do, it works its way up to over shoot, or falls to far behind. Off the top of my head I can't remember what size coil installed, I know it was over 90,000, I can check the size when I get home. The 3 speed pump is on low, and with the piping I believe it is 6 or 7 gpm. The blower speed was correct for the coil size, and gpm. So I think the thought is slow down the blower to take the heat out of the boiler over a little longer time. It pulls at least 10 deg. in 12 or 13 minutes.

These low volume or low mass boilers can be touchy and not so forgiving as boilers of high mass...

Stopping the fan everytime whenever circulator stops will save a nice big slug of cooled off water from slamming the boiler on the re-start.

Knocking off all the highs and lows can get to some smooth sailing.