Your thoughts on my OWB performance

[MVrana]

I'd appreciate hearing your thoughts on what I'm seeing with my OWB. For background, it is plumbed to a water/air heat exchanger in my gas furnace. Water jacket setting is at 190 F. Temp ouside is in the mid 30's. Area being heated by this unit is about 2000 sq. ft, built in 1924, moderate insulation, many windows (some new, some old), 10 ft. ceilings.

When the furnace first kicks on in the morning (t-stat setting change from 62 to 68), the air coming from the vents is a toasty 136 degrees. After running continuously for about 30 minutes, house temperature has risen to about 66 degrees. But, now the air temperature coming out of the vents has dropped to about 103 F. What does this say about my system design? Does this suggest water capacity of the boiler is too small? Or, is this just a typcial response curve as the boiler fires and tries to heat back up? Note that since we keep the thermosat set low at night (10 pm to 5:30 am), the boiler probably hasn't fired very much during the night and it may take a little time for the fire to build up (bed of coals may not be very hot/active). The OWB does have a fan for combustion air.

Thanks for your input.
Mark

 

[flyingcow]

Must be a pretty good load of wood left in the boiler? Also be curious as to how your return water is piped in for mixing.

 

[MVrana]

Thanks for the reply. Hot water is piped to the heat exchanger from the top right side of the water jacket. Return water enters the jacket at the bottom left corner.

There's plenty of wood left in the firebox in the morning when the temps are mild. Filled it at 5:00 last night and it was still nearly full this morning.

 

[stee6043]

A better indication of "performance" is your actual water temps going into and out of both your boiler and HX. Can you add some thermometers? They don't have to be too fancy to get the job done.

Let me throw something else at you - the AM jump from 62 to 68 is too much in my opinion. I would suggest this is hurting your overall efficiency. Your house has a lot of thermal mass and when you let the whole house cool that much at night you spend a fair amount of time re-heating it in the AM. I'd suggest you bump your overnight temps to the 64-65 range to help with the big surge in the morning. I'd bet overall you'll have better efficiency.

I used to let my house cool significantly more at night too. Late last year I begain toying with overnight temps to see how it affected my storage temperatures. For my situation I found that 64 at night and during the day when nobody is home is optimal. I bump to 67 at 4:30 (just before the wife and I get home from work) and then I bump to 69 at 6:30 for post dinner relaxing. Back to 64 at 9PM. When I was dropping to 60/62 at night I drained a lot more heat from my tanks at 4:30 to get everything back up to comfy temps....

 

[flyingcow]

When your getting 103 temp from your HX, whats the temp of your water boiler?
-
I'm assuming your going to need a mixing valve to maintain temp, but I'm getting in over my head, you need some of the more advanced members to chime in.

 

[MVrana]

Stee6043, thanks for the reply. Temperature loss from the boiler to the heat exchanger is less than 2 degrees F. Temperature loss in the heat exchanger is about 32 degrees F, give or take. Since the air temp at the vents goes from 136 down to 103, I can only conclude that the water going to the heat exhanger is dropping in temperature with time. I need verify this by checking the temperature at the boiler after it's been running a while.

You may very well be right about overloading the system by dropping the house temperature too much at night. When the weather gets really cold outside, we do keep the thermostat set to at least 64 at night. When temperatures drop below the 20's , it can take more than an hour or two to get the house from 62 to 68.

 

[MVrana]

Thanks Flyingcow. I still need to measure the boiler temperature when the vent temp drops to 103. I'm assuming it is much lower than the 190 degree setting that initally produces the 136 degree vent temps.

There are no mixing valves on this system. Hot water flows directly from the outside boiler to the water-to-air heat exchanger in the plenum of my gas furnace.

 

[bowsky64x]

I am also using water to air heat exchangers with my gasifing boiler. I found that the heat exchanger will heat up all the duct and metal around it when the fan is off, when the fan kicks on it is not only pulling heat through the exchanger but it is pulling off the warm duct, once the ducts cool the air temp drops some. I have my boiler set at 160 right now but due to warmer temps my water usually sits at 200 , I can have both the house and shop pulling heat and my return temps to boiler never drop below 150 . I did install a mixing valve due to everyone elses advise on this forum , spent $120 on it and never use it. In my opinion you would have to be pulling some serious btus off your system to need a mixing valve or maybe my piping and system size is overkill for what I am heating. i have a econoburn ebw200 with 1 inch pexalpex pipe running 200 ft to the house wich is 1600 sq foot of 100 year old farm house and then back to a 1500 sq foot insulated shop. The radiant heat alone from the boiler in the shop keeps it 65 and I keep my house 70 all day and night, With this setup I have a hard time keeping the temp of my water cool enough , my aquastat is always shutting my fan completey down causeing some cresode issue in my stove. i definetly need storage but am perplexed on how exatly to hook it in, many of the diagrams I have seen do not seem to be the best way to do it.

 

[Mid Michigan]

My OWB is homemade so keep that in mind.
So for my 2 cents:
My supply and return temps can be as much as 30* different with my HW coil in furnace. If I am correct I am moving 7 GPM through the 1" pex. After 1 hour that is 420 gallons. My stove hold approx. 100 gallons. I have completely replaced the water 3 more times plus some. The stove cannot put the heat back into the water as fast as I can take it out. Especially when it may take 15 to 30 minutes for fire to get rolling again. I think in time it catches up but by then no one may be home. When I added some more storage this straightened out some.

 

[MVrana]

Thanks MidMichigan. My stove only holds 130 gallons. I think you are right that it just takes some time to catch back up after pulling out most of the heat.

 

[sdrobertson]

130 gallons is the problem as you can only store so many BTU's per gallon. In the am when you start pulling BTU's your quickly using all that are available and the boiler is trying to catch up. During the night your not pulling any so the boiler is happy just sitting there waiting for the water to cool. Two things will help...both things suggested above...1. More storage to draw from in the AM when your demanding allot of BTU's to raise the house temps so the boiler has time to catch up, or 2. Decrease the temp difference during the night so the boiler can keep up with the BTU demands over a longer period of time.

 

[Duetech]

MVrana, Your opening question is "Thoughts on my OWB performance" and it seems it is performing well. As stee6043 stated the big jump could be the problem. Programming hourly incremental Tstat jumps (1*-1*-2*-2*?) to allow your boiler to go "active" and establish a fire cycle might go a long way in actually getting the most out of your temperature recovery and energy costs. Trying to heat with water that has cooled to 120* mostly causes my blower to run constantly with little if any temperature gain and that is a lot of electricity for so little gain. Letting the boiler (gasifier) gain a few degress (160*) throws a noticeable temperature rise into the air. Getting your boiler to go active before a big drain on storage should cause better efficiency by not sapping a large btu drain out of it before it can respond to replace them. Realizing your system sweet spot really can't be known until the boiler is at peak output when the btu load is triggered and an evaluation is done. Like maybe doing a 1*-1*-4* series once you know the swing or effect of the series mentioned earlier . The swing may of course vary with actual weather conditions but that's the OJT of the boiler being used.

 

[MVrana]

Thanks sdrobertson and cave2k. I think you guys have hit the nail on the head. I checked the boiler water temperatures this morning. 183 degrees F when I start heating. 30 minutes later the water is 143 F. Air temps at the vents go from 136 to 103. When the air is at 103, the house temp moves very slowly. The blower just runs and runs and runs.

You are right that the system works well once the house is at temperature. Depending on the outside temps, the furnace kicks on a couple of times and hour and typically runs for about 5-10 minutes.

I'll probably either go to small step settings on the t-stat as you've suggested, or just leave it at one temperature. Thanks again for your input--very helpful.