I have a quick question about using buffer tanks with a Mod con. I have a Munchkin T-50 boiler with an outdoor reset. I use it for both space heating and for heating an indirect tank for dhw. So the min modulation in space heating mode is around 16,000 btus. Now the house is heated with baseboard. The baseboard was designed so that that at 160 deg water temp, baseboard could deliver ~21000 btu’s including a fan coil kickspace heater in the kitchen. I have never hooked up the electrical connection to the fancoil unit because I have not needed it. So the actual output at 160 deg water is ~ 15000 btu’s. The unit easily keeps up with demand with water temps leaving the boiler of around 130 degrees when the outside temp is around design temp. I don’t have a return temp gauge, so I don’t know what the delta is.

Clearly the unit is never delivering more than 15000 btus, more likely around 12,000 btus. Now since the heatload maxes out at 12,000 btu’s, most of the time it really only needs say 4,000 btu’s and the min modulation is 16000 btu’s. It seems like it is short cycling, I.e. the burner is only firing for 3-5 minutes at a time. The only time it fires for any length of time is when it is heating the indirect tank, which is a superstore 50. Would an inline 10 or 20 gallon buffer tank help out in this situation? I am not so much worried about efficiency, I currently only use 150 gal of propane per year. I am worried about burner life. Am I over thinking this?
Eric

How many cycles per DAY or HOUR is it running? Is it running 3-5 on/1-5 off at outdoor design conditions, or is more like 3-5 on/25-75 off?

If you have an infra-red thermometer you can get a good handle on the delta-T with either a masking-tape wrap or a splash of paint on the relevant measuring points (bare bronze or copper isn't emissive enough to read true- even less-rusted iron can read low. PEX is usually emissive enough though.)

OK, you could use a buffer tank. Less than 20 is a waste of time and money. I use condensing water heaters with a plate and sub-system for such small loads.

I do. because I can run a boiler at 100 degrees or lower, but I can't run a water heater that low, and I may well have a very high max load but low micro loads in the shoulders.

I prefer zone sync, but what can you do.

A week or so it warmed up to around 50F after it had been in the teens for a while. I was away so the house had dropped down to ~53 deg. The thermostat was set to 50° so it did not fire. I came home and wanted it warmer and was too lazy to start the woodstove so I turned up the thermostat to 65. I noticed the house was not warming up very fast so I checked the baseboard. They were luke warm so I checked the boiler. It was firing to keep the water ~100-105 deg. It would fire for about 2-3 minutes water temp would go up to~115, then shut down for 10 minutes circulator still running.
When it was around 12 deg outside I turned up the thermostat so the boiler fired and I could watch it, (design temp 7) it ran around 130 water temps, the burner seems to run about 10 min on, 10 min off until temperature is reached. The house is almost never kept at a steady temp so I don’t know how often the boiler would run to just maintain a temp. When I don’t burn wood I set the thermostat to 63 for an hour 5 am in the morning and 62 for an hour in the evening(5 pm), just before I get home, and 50 deg the rest of the time. The house temp drops 2-6 degrees in the 11 hours between heating cycles at night.
The more I think about it, the more I think I might just have to have the set points adjusted on the vision controller .
I should buy an Ir thermometer and figure out what the return temp of the water is.

Someone should start building a gas boiler that modulates down to about 3000 btu, but can still heat an indirect tank for dhw.
Cheers,
Eric

Baseboard works reasonably with 130F water, but it's output is limited at 110F with a fairly non-linear response to water temp change. To recover from a deep setback when it's not-so-cold outside the outdoor reset function is screwin' ya. With fin-tube baseboard I'm not sure you can get a reasonable curve for the outdoor reset either. Setting it up for fixed temp output of 130F (rather than trying to fit an outdoor reset curve to a non-linear emitter) you would get the response time you need from setbacks, and likely get less short-cycling. Then, either tweaking the fixed temp down a bit (or the flow with a ball valve) to get it to run at or near min-mod would maximized the burn time, but be mindful of keeping the delta-T within spec, if the controls don't already self-protect for that on the boiler. A 50% duty cycle under design conditions suggests that it's running above min-mod.

Below ~120F the output of fin tube isn't so reliable or predicable- the number of dust-kittens or bugs that have accumulated on the fins since the last vacuuming can affect the convection loop too much. It's pricey to buy it new, but cast-iron baseboard such as Burnham BaseRay or the Weil-McLain RC works great at low temp. (At 4-5x the price of fin-tube per linear foot.)

http://www.usboiler.burnham.com/pdf/baseray_io.pdf (The 100F spec is ~125BTU/linear foot compared to 590 BTU/foot @ 180F. )

http://www.weil-mclain.com/en/multimedia-library/pdf/weil-mclain-pdf/products/baseboards/snug-baseboard/snug_baseboard_lit.pdf (no 100F spec, but it pretty much tracks the competition.)

Recycled cast iron baseboard can sometimes be had at scrap rates, if you're good at scrounging. It'll add some (but not a huge amount) of thermal mass to the system as well. Fin-tube's thermal mass is pretty much just the water, but with cast iron baseboard there's enough to more than double or even quadruple the system water thermal mass in a tiny mod-con & baseboard setup. Despite having only 1/9 the thermal mass per pound of water, there's a LOT more pounds per foot of cast baseboard than water-weight. To deliver 21KBTU/hr @ 160F you're talking 45-50' of the stuff, which get's to be pricey at retail ($40-60/foot depending on the distributor & volume.) Recycled/used I've seen it for $20-25/ft, sometimes less. (eg: http://hartford.craigslist.org/hsh/2081760001.html http://hartford.craigslist.org/hsh/2070962551.html http://southcoast.craigslist.org/for/2059062995.html http://worcester.craigslist.org/mat/2082135375.html http://providence.craigslist.org/hsh/2117294272.html )

Setback has always been overrated. More than a couple degrees makes for discomfort and little fuel savings (if you actually live in the home).

Reset on the other hand increases comfort, system efficiency and fuel savings especially when coupled with a condensing boiler. As for fin-tube, response time can be slow at low temperatures but if you let reset rule you will not have problems. Many condensing boilers also have programmable boost cycles, again if you can stay away from the controls.

If you don't work 10 hours days away from home and sleep for 8 (or you want to be comfortable the few hours you are home) get reset and forget your troubles.   Of course fin-tube is not really radiation, so Dana's recommendation for CI radiation is good if impractical.

CI baseboard is only 1.5-2x the cost of fin-tube if your scrounging or cobbling. (Shopping at the scrap yard is even cheaper than craigslist, and there are literally tons of this stuff leaving New England ports bound for Chinese smelters these days- if eric really wants to, he can get it down to a few hundred out of pocket.) If it's under a grand (~$20/foot) it's probably worth it. Even a cheap 20-gallon buffer tank will be $250, and won't fix the low-temp loss of emittance issues with the fin-tube, nor will it raise the comfort level. At scrap yard rates it might be close to a wash.

But if you're only using the boiler in heating mode to be able to come home to (or get up in) a warm house before stoking the woodstove, outdoor reset is useless. If you're there most of they day keeping the place up to temp THEN it makes sense, but it's going to burn more fuel keeping an empty house warm, even with the efficiency of outdoor reset and decent radiation. In eric's single-guy wood-burning mode of living, setting it up at fixed temp to deliver something between 120-130F and forgetting it is probably the "right" thing to do, even if cast iron baseboard would be more responsive & comfortable. A propane burner will deliver well-over 90% efficiency on long burns with 130F output (a point or two higher than a natural gas burner.) Setting it up for outdoor reset in his case buys nearly nothing even WITH cast-iron radiation. With fin tube it buys LESS than nothing (actively costs!), since at low temp the inability to get the heat out of the fin-tube and into the room aggravates the rapid-fire short-cycling behavior.

If the lowest practical limit is on the order of ~120F with fin-tube, and the design-day requirement is only 130F a reset curve is nearly-meaningless- it's flat. If it's been tweaked to the boiler's min-modulation the difference between 120F & 130F for a propane-burner is the difference between 94% vs. 95%, assuming a ~ 20F delta-T. It's on the knee of the curve, not the steep part. Set it to a fixed 125F- see it it's enough on design day, if not bump it a few if yes, back off, but not below 120F. And see what the minimum burn time is on not-so-cold days- you'll probably get a similar duty cycle, but somewhat longer burns since more heat is actually leaving the fin tube DURING the burn than with 100F water. 2-3 minute burns with a ~25% overall duty cycle could turn into a less-abusive 6 minutes on, 20 minutes off if you're lucky. It'll probably take more mass to get it consistently over 10minutes though.

Come to think of it, turning on the fan-coil blower might suck enough heat out even at low temp to lengthen the burns- there's more than one way to skin the cat, eh?