no thermostat?

k

Tim,


I'm confused. You have a system that is a 200k BTU burner that heats up what in 15 minutes?

A 200k BTU boiler can theoretically consume 50k BTU's in 15 minutes. While that can make the first emitter in the system pretty darn hot, you can't possibly heat 200k worth of heat emitters in 15 minutes unless it is a warm day and you only need 50k BTU's. And I'll assume we're talking about a conventional boiler so the best you can put out in 15 minutes is only 30-35k BTU's.

If a conventional 200k boiler is satisfying a thermostat in 15 minutes isn't that the very definition of short cycling?

Tombig_4

no thermostat?

New mod/con (WM Ultra 3 - 105BTU) installed.

As I understand it, and confirmed with Weil Mclain rep, mod/con boilers work best when they run almost 24/7 and modulate up or down to meet load requirements.

I have a high mass system with very large cast iron radiators throughout my house. To heat them requires a large amount of gas (about 400k btu's on this warm fall day). When we fired up the boiler I was surprised to see how long it took to raise the supply side temperature 65 degrees (65 to 130). It took four hours of full burn. What ended up happening is that the temperature at the thermostat was met before the system could ever modulate down. The system never reached the target temperature of 130 degrees because the thermostat was satisfied before achieving 130 at the supply sensor.

I watched the boiler over the next several hours and found the firing cycle (controlled by the thermostat) was too short on the burn and too long on the 'off'. It was previously set to cycle for 'hot water boiler' (old Honeywell chronotherm thermostat with poor cycle time descriptions). The result was the boiler would come on for an hour or so and then it would turn off for an hour or so. During the off cycle I lost so much heat that the system comes back on with full 100% gas burn and never really have a chance to modulate down.

aside--
Now I had convinced myself a few years ago that the cycling was never right on the system and that the old cast iron WM boiler constantly short cycled. I am convinced the system never achieved the temperature it was supposed to and I ended up cycling this big old dragon of a boiler way too many times. To pay me for my abuse the gas company hit me with a 7000 gas bill last year and in 2006.
---back to story


So this new boiler is designed to run almost constantly and is well controlled by its included outdoor reset. As the temperature points are adjusted the supply temp can be matched perfectly to my home (I think).

The problem is that every thermostat I look at lacks any real cycling control. Most offer a simple method to change the boiler from 1-12 cycles per hour, but given my high mass issue I really need a cycle of 4 hours of burn time or more and minimal (if any) down time.

The outdoor reset on this system effectively sets the desired supply temperature target. This is adjustable by qualified persons and allows changes to high heat output, low heat output, outside temperature to fire high heat (0 deg), outside temperature to fire minimum heat (70 deg) and a warm day reset (shutdown above 65 outside). It also has setting for modulate on temp difference at target temp minus supply and modulate off temp difference.

Default settings for cast iron radiators:
180 (supply temp) high heat at 0 degrees
120 (supply temp) low heat at 70 degrees
modulate on when supply drops 5 deg below target
modulate off when supply is 5 deg above target

By changing these settings, you adjust the heat curve. I adjusted mine down to a low heat setting of 100 deg when the outside temp is 70. At the current temp (50 deg) the system is trying to hit a supply temp of 124 degrees.

So here is my theory:
I intend to run the system only on the outdoor reset and eliminate the thermostat altogether. I've done this temporarily by setting the thermostat to 88 degrees and let the system run 24 hours. My inside temp hit 72 degrees, so I turned down the low temp by ten degrees and I expect the inside temp to drop by a degree or two through the night.

Keep in mind the system shuts off when it's 65+ outside (setable) and it shuts off if it goes a few degrees (setable 2-10 deg) above the supply target temp. Anybody see a problem with this?

Timco

you may want to post pics of the boiler & piping. I have 200K BTU systems that all heat up in 10-15 min on full fire....

Tim

Timco

Not satisfying the demand, but making rads 140*.(hot) Load is all CI rads piped in 2-pipe method...That is on high fire. I will program the curve tomorrow so it will modulate and it will take longer, but not hours.

EJ hoffman

do it all the time

I set up most of my systems to run off outdoor reset only For those customers that insist upon thermostats I give them one but I don't usually wire it up. The key to making it work is to get your reset curve set right, and your pumps also need to be sized correctly. I have some munchkins set up on single pump constant circ, and some prestige set up like a buderus two pump system. Once you balance the system you won't feel or register more than a 1 degree bounce in temp.

k

What boiler are you installing?

On mine it didn't start to modulate until the supply temp was equal to the target temp it picked. I am thinking that cast iron is such a bad heat conductor that the initial 4 hour high-burn (mine only outputs 94k btu) "charged" the radiators. Then once they are up to temperature they just need to be maintained.

So if I maintain them at a temperature based on the outdoor reset calculation I should be able to run 24/7 with no thermostat, right? And then I'll get maximum modulation, condensing & steady heat with no temperature swing and never a short cycle. Right?

k

Thanks, you have confirmed my beliefs.

I love the non-attached thermostat. I learned of that two years ago at my office. The architect, who works in the next office, explained that the reason mine seems to have no effect is that it wasn't hooked up and that was a common practice in buildings to allow the tenants to 'feel good'.

Your post vindicates me and made me ROFL.

Jamie_5

reset curve

May I ask why your reset curve is bottoming out at 120F rather than 70F?

jp_2

sounds like

your target temp is too high. 100F water temp at outside temp of 70F seem high.

k

Wow 70!

The default on the Ultra 3 for cast iron radiators is 120 low and 180 high. I have been steadily moving the low downward and like the way 100 feels. 90 felt too cold.

I'm in a drafty 100 year old house with several missing storms, I wonder though if 70 might be perfect for a low loss modern house.

What is the ideal setting for a drafty old house? Should I change the top and bottom of the curve or just continue to fiddle with the minimum target temp?

Anyway I look at it, I'm already saving a ton of gas; literally.

k

ok, I'll lower it. Does your suggestion change if my house is old and drafty?

Jamie_5

reset temps

Well, heat loss formulas are models, but according to those models, loss is directly proportional to the difference between the higher (inside) and lower (outside) temperatures. The formulae do not account for delay in heat loss occasioned by the mass of the house, but they are a good first approximation.

So if the emitter's (radiator's) heat output is also proportional to its supply water temp, you would want to reset the water temp all the way down to the indoor setpoint. The output of cast iron radiators is probably not directly proportional to water temp because of the convective component of their heat output, but that's where I'd start.

The fact that your thermostat was satisfied before the supply water temp was reached demonstrates that you don't need that high a water temp for that outdoor temperature.

A friend and I married a Munchkin to a mix of cast iron and panel radiators with TRVs, set the central thermostat to 72---two degrees higher than setpoint---and let the reset run down to 70. It works fine.