I'm fully aware of 3-way valving, injection sysyems and primary secondary piping. There are ways to lower the temp on a cast iron boiler but none of those were mentioned in the suggestions given prior. Just looked like bad advice if someone took it at face value.

wasnt quetioning your knowlege just injecting some of mine

OK I just have to put my 2 cents in here. If your installing a Geothermal system large enough to assist the boiler with heating, what are you really saving? If you going to go thru the expense of drilling wells, or installing a closed loop and a water to water geothermal system, isn't it just more pratical to also install the supply and return air duct in the house and do away with the broiler setup entirely? How much more could installing air ducts cost given the cost of installing the rest of the system? If just seems to me if your going geothermal, you might as well go all the way to get the true cost saving geothermal offers, half measures is just asking for trouble. Even if geothermal could preheat the water to work with a boiler system, you just doubled the amount of equipment that could fail.

Carbon Monoxide only will be present if there is incomplete combustion. Just because there's a gas flame does not mean there is CO. However, because of the seriousness of the issue, and the possibility that it is present, measure obviously should be taken to mitigate any heat exchanger leaks.

TG,
Duct systems cost easily 6 or more thousand on a finished home + people like their baseboards. Either way I never suggested this was ideal the question was can it be done?
To those with the condensate issues, as I said before, that is more easily addressed than the geo question (mixing, buffer tanks, greater post purge ventilation......). The question at hand was have you seen or installed water to water geo tied to baseboard radiation.
E244, While I've taken exception to Engineer's comments from time to time, your characterization of him is unfair and shows that you've only read a small % of his comments. We all mis speak (type) ourselves from time to time (or in my case occasionally melt down).
Your insights might be most useful on new questions (not dead for 6 month threads). Welcome and good luck,
J

Just because threads are old don't mean people don't read them. And I truely think that some of engineers suggestions (in this post) would cause someone's system serious damage. This is the reason I questioned his knowledge of HVAC systems was curious to know his experience is this field. I do appologize for being critical of him and not of his comments.

I'll take up the flag again as he can hardly win by answering. What I know, I know from these threads, so it's all in here;
He is I believe an electrical engineer in FLA. Lack of local geo talent made him involve himself in the construction of his own home. He went on to procure a contractor's license and seems to know (unfortunately for local consumers) more about geo than most of the contractor's in my area. A recent thread has him reminding folks that "boots on the ground" are worth more than free internet advice. Curt (engineer) seems to be more practical and open minded than the tunnel visioned and detail fixated engineers I've met. He also has periodically explained his experience short comings. Again it's all here.
It is true that a reader who didn't put the information here in context could get into trouble (a topic for my most recent meltdown). But if you want to know wether setting back a thermostat really works or if your system seems to be operating in reasonable parameters, or the best way to pipe a DSH; Curt, Dewayne, Clark and Bill are sought out by homeowners and other contractor's alike. I know of no study showing me wether setting back my heat pump costs or saves, but they are running one here and while initial numbers may lack the purists scientific process, I'm eager to hear their results.
Or as Curt pointed out recently "free advice may be worth every penny". It's up to the reader to decide.
Joe

I know nothing about boilers so I can't say if there is bad advice on this thread. Those of us who post regularly on these forums occasionally disagree on how best to solve a problem mostly because we have different backgrounds, training and expertise. We never challenge each others intelligence or credentials. ( for which I am very grateful). Usually by asking questions, seeking understanding and reasoning we can learn from each other.

Like Joe, I have learned a lot from engineer ( Curt ) and am sorry that he has been given a bad rap here.

There are other forums out there where the tone is caustic and adversarial, let's not that happen here.

Electric244 , we will welcome you if you can help us aid the people who come here seeking solutions to their problems.

Cheers

Joe and DeWayne - thank you both for the kind words of support. I read the post questioning my advice and creds but let it ride owing to its scornful tone. Nothing is gained via internet flame battles and the easiest way to avoid them is to decline combat - it takes two to tango.

Everybody brings different experience and expertise to this site, and apparently I've been fairly honest about my background, since Joe recapped it accurately. I bring to the table a solid grip on the thermodynamics that guide and govern these systems and above average writing ability.

I don't (yet) have anything approaching a viable business doing geo HVAC work, but I'm transitioning in that direction. One of my training tools is to read and post here, knowing that misstatements of opinion and fact will be promptly countered.

I have 20 years experience with industrial distributed controls systems, some of which run very large boilers and fired heaters. One of the largest I've worked on was the scene of one of my bigger foulups, which I'll briefly recount here in the spirit of self-deprecating humor:

Some stats I remember on this system: 1.4 Million lbs of steam per hour at 1600 psig, 200' high furnace, 400' stack approx 20' in diameter, 16 burners on 3 elevations. It fired # 6 heavy oil, so when I mis-tuned the oil flow controller (gain and reset off by a factor of 10, as it turned out) I turned an AWFUL LOT of # 6 oil into soot, and filled the entire furnace and stack with impenetrable black smoke. Camera feeds reminded me of Pearl Harbor images. A supervisor wryly radioed in that the smoke cloud was "thick enough that geese were landing on it"

Worse yet was the location - Brooklyn, near the old Navy yard. An east wind carried the smoke (and geese) on a leisurely tour of Lower Manhattan (Wall Street, etc). Our regional manager had family in from out of town, was touring Ellis Island (Statue of Liberty) recognized the source of the cloud and immediately knew who to blame...my only solace was that this occurred before 11Sep2001...these days someone would call in an airstrike.

I tell this (funny - I hope) story as a lead in to the underlying technical issue this thread wandered to - is it OK to reduce the high temp limit on a cast iron water-jacketed oil fired residential furnace? I do know that it is not unheard of to operate big units at varying temperatures and pressures based on load. To the residential question I bring the following points / observations.

1) Thermal shock - whenever a zone that hasn't called for awhile does call, approx 5-10 gallons of 50-60 degree water from ~200' of 1" copper baseboard rushes into the cast iron water jacket. If anything, having that jacket at, say, 150 vs 180 would seem to reduce rather than increase the shock.

2) CO leakage - two points - a) Furnace is waterjacketed and heats water, not air. The most common exchanger failure mode I'm aware of is water leaks, which are pretty easy to detect. b) I'm not completely clear on the underlying combustion chemistry, but CO is reportedly much less a danger with oil vs gas, even though oil has more carbon to begin with. Furnace and stack is hot year round, and continually drafts up the stack, even when not firing.

3) Corrosion - while I reduced the limit I did not reduce the firing rate, so stack temp stays in the upper 300s - enough to prevent condensation.

4) Service Co was willing to reduce the limit during summer to meet my concern of steep standby losses all summer when system's only job is to heat domestic water. Back of envelope calcs suggest that summer efficiency drops to 25% or less owing to standby loss. They are a decent, honorable long-established company. I'm sure one of their priorities is to avoid a troubling service call of not enough heat during a cold snap when they are saturated with emergency calls. However, I bet (and it has so far panned out) that replacement of hundreds of square feet of drafty single pane aluminum framed sliders with modern windows dramatically lowered the winter heat load, allowing lower limit operation year round.

The cost of a dedicated service call in spring would eat up most of the summer savings

Oil heat is a funny business - it is the only major method of heat where the equipment supplier often also supplies the fuel. It is easy to see where a conflict of interest could develop. I had an oil-fired air to air furnace in Pennsylvania and ran into a local practice where oil heating companies refused to service systems for which they did not supply the fuel (at a healthy markup). That didn't sit well with me.

(Constructive) criticism welcome, as always.

The biggest flaw in your logic is the part where you wish to leave a real job for contracting;)
j

The problem with lowering the return temp. to or below around 120 is the condensation in the heat exchanger. I always thought thermal shock was largely just an issue with steam boilers. I've also heard of reduced draft as it relies on a temperature difference between the flue gases and outdoors. This is why high efficient boilers have draft inducers. (So I was told at a boiler seminar) I'm not sure if the 300 degree stack temp. is above the condensing level, I would assume it is but I think the issue is more where the water first enters the heat exchanger at the lower temperature and it condenses at that point

haha I agree Joe, I've been taking night classes for years to get out of construction.

Has anyone ever seen/heard of geothermal radiant heating? It seems like a more realistic temperature for a geo.

If you are refering to in floor radiant, then yes, water to water geo works great if designed correctly. Lower operating temps than a boiler may require greater density to floor loops. Also tends to have lower COP's than water to air and obviously can't cool so installation is expensive if A/C is desired as well, but does a great job on domestic hot water.
J

Not leaving day job, just figuring on eventually getting pushed out of it. That and I married late and now have small children, so work travel away from home no longer appeals much.

I don't believe condensation is as much of an issue in the furnace with oil as it is with gas - gas (natural or propane) has many more hydrogen molecules that form water vapor during combustion than does oil. Similar case in point - gas engine car exhaust is white with condensation (and the exhaust pipe drips water) on cold days. That does not occur with diesel engines whose fuel is quite similar to home heating oil.

On road diesel fuel max sulfur content was lowered from 500 PPM to 15 PPM last year. If that rule applies to home heating oil (maybe it does already or will soon) we might see a new generation of lower flue gas temperature home heating furnaces as fear of acidic flue gas condensation is reduced.

I wonder if (correct me on this) draft inducers have more to do with horizontal venting and the desire to avoid a continuously open flue during off cycle, thus reducing standby losses. I wonder if a draft inducer performs the same function as a 'power vent' on a high efficiency storage gas water heater.

~122F is the dew point at sea level for sealed combustion NG exhaust.   It's a few degrees higher for propane exhaust & several degrees higher for oil exhaust (and artifact of the higher CO2 & SO2 content, but I digress...)

But the amount of dilution air in both the combustion path makes a difference in where the dew point is in the heat exchanger, and most atmospheric-drafted boilers mix dilution-air prior to the stack with a draft-hood to further lower the CO2 (and in oil burners, SO2) concentration, lowering the dew point of the exhaust.   With lower dew points you can operate at lower temps without condensation hazard, but drafting excess dilution air into the combustion path beyond that necessary for complete combustion loweres the  oeverall combustion efficiency. 

In typical boiler heat exchangers, regardless of what the stack temperature is, the flame side of the heat exchanger is a few degrees warmer than the temperature of the cooler water entering the water jacket over a significant portion of the surface area, and not much more than the temperature of the output water anywhere.  At 140F out, assuming a typical 30F delta-T on the radiation... well...

Many boiler manufacturers will not warranty boilers operated continously with return water temps lower than 130F (despite the fact that the AFUE test critera are for 120F return water, 140F output water, which will destroy most cast iron boilers in a single heating season.)  If the return from radiation during the heating season is expected to be lower than 130F (and with 140F loop water, that will almost surely be the case) boilers can/should be plumbed with a bypass loop to mix boiler output with the return water to ensure the return water stays above 130F.  But with only a 10F delta-T at the boiler you may run into short-cycling issues (usually tame-able).

But the heating sysem hydronics could just as easily already be plumbed as a primary/secondary loop system with all zones mixing return water into the primary loop, which can also protective of the boiler by keeping return water temps considerably higher than the radiation return.

Where engineer's mom's boiler & heating sytstem is on the spectrum of all possiblities is unknown.  If a competent tech has tweaked it to run at 140F output without condesation issues, that can be (and is ) done.  But it's good to actually KNOW what the return water lower-limits are, and MEASURE the return water temp, eh?  (Measure the temp, find out from the manufacturer what their limits are, then decide which corrective measure, if any to take.)

Most modern oil-fired boilers are cold-start tolerant- any condensate that forms on startup is re-evaporated & scrubbed by the longer burns, but continuous operation in condensing mode is more corrosive than NG or propane due to the oil exhaust's sulfuric acid content, and tends to burn it out faster than NG or propane boiler.

Good information - I will check return temps. I do recall observing that after baseboards get warm, return water line was almost imperceptibly cooler than supply line, (way too hot to touch for all but the briefest moment) which I can only attribute to fairly high circ pump flow rates.

If dewpoint is high for oil combustion why don't we see condensation of vapor in diesel engine exhaust? Is it that diesels have much higher excess air than a oil-fired home furnace? Why don't we observe condensation when oil furnace flue gases hit winter cold outside air?

Diesel air/fuel mixtures are leaner than an oil burning heating appliance (if you leaned out your oil burner that much it would be an explosive mixture at atmospheric pressure!) so it starts out with a higher dew point than you'd find at the boiler's heat exchanger in the first place, and the exhaust temps (at the manifold) are also higher.   This is by design- were it to stay at or near it's dew point in the exhaust manifold you'd have some serious reliability issues!  But at some very cold outdoor temperature you might still get airborne condensation.  (It's a different mixture, and very low temperature but you've probably seen vapor trails behind jet engines in the coolth of 25000' altitudes... ;-) )

Oil furnace flue systems are typically designed with added dilution air (either draft-hood or barometric damper) specifically to lower the dew point of the exhaust well below the exhaust temp, limiting the possiblity of flue condensation.  But on some systems it's sometimes possible to observe in-the-air condensation misting above the tops of oil burner flues on very cold days.  (It's just more common to see that with NG exhaust.)  But that may be a sign that it's running too cool as well- there may be condensation eating away at your flue liner under those circumstances.

BTW: Many boiler manufacturers draw the line at 140F return water (but that's likely an arbitrarily large margin of error.)  At 130F return water you can be on the edge of issues with oil burners, yet still have margin with NG burners.  But if it's reliably above 130F, even if it's close, you're likely to be OK (at least as far as the boiler is concerned. The flue may be another matter.)  Check the boiler manufacturer's specs & warrantys.  When you measure the return water temp, be sure it's right at the boiler, not at some arbrary point on the radiation return.  If there's boiler-bypass plumbing it should be obvious, and the return temp will be higher than the radiation return. 

There's plenty of online info about protection schemes to keep boiler return water temps high enough to avoild condensation, yet low enough to not kill efficiency.  A short sheet that hits most of the standard approaches lives here:

http://www.comfort-calc.net/Bypass_Piping_Explaination.html

But there are other methods of boiler-bypass & return water temperature control as well- google 'em!

FWIW: The indirect HW tank may have originally been set to 140F exactly for boiler-condensation protection reasons.  Setting it to 120F may cause the coil return water temp to be too low.  Mind you, the raw volume of condensation in hot-water heating mode will be quite low compared to what happens during the heating season. (Unless she's heating the hot-tub with it. :-) )

If you think the burner is oversized for the heat load on the house (probably is, most are.) And if  it's cycling yet still delivers enough heat on design-day to keep the house warm with 140F water the whole system SURELY is oversized.  Having more than the necessary radiation can be a good thing- delivering the heat at lower temps is always good for efficiency as long as it doesn't induce short-cycling on the (oversized) boiler.  More than just turning down the output temp you can probably get fuel savings out of derating the burner with a smaller jet.  (This MUST be done by a competent burner tech who measures and adjusts the combustion air mixure and the flue's dilution air, stack temp, overall combustion efficiency, etc.  If he can't tweak it up to at least 75% CE, you may want to consider replacing the boiler.  If it's really ancient and doesn't aready have a flame retention burner- at least replace the burner!))  A lower burner output will result in longer burn times/higher duty-cycle allowing the boiler to approach it's full steady-state combustion-efficiency, with fewer cycles overall.  Burns much shorter than 5 minutes can seriously knock back the efficiency of a medium-mass boiler considerably. 

If it's short-cycling even during the heating season that 85% AFUE boiler is probably delivering no better than 65-70% as-installed.  But when it's running at more than 2/3 duty cycle with 15-20minute minimum burns it'll likely meet or beat it's AFUE test performance (assuming 130F return water temp.)  IIRC, AFUE tests pre-stabilize the boiler with a 30 minute burn, then run a series of 9 minute burns with 21 minute off intervals @140F output, 120F return water. (And it ignores jacket losses, only measures flue losses.  Sometimes insulating your boiler better can make a measurable improvement- is the boiler room always hot?)  A medium-mass boiler that tests out at 85% efficient under those conditions probably has a steady state combustion efficiency in the 87-88% range.  But if it's idling most of the time, only spittin' out 2-3 minute temperature burns every 20-30minutes, it's way over the efficiency cliff.  The higher the duty cycle and longer burns, the closer it comes to it's raw steady-state thermal efficiency.

And conversely, the shorter the burns & lower the duty cycle, the further down the cliff it slides... (probably about 50% average efficiency during the summer water-heating-only season, but at least it's 50% of a much smaller number.)

(Seems strange to be discussing hydronic boilers in this much detail on a geothermal thread...  SOMEBODY STOP ME!!! :-) )

Great info - thanks for taking the time to post all that. I learned about AFUE method, among other things.

I don't have access to system right now but I believe it is running a 0.8 jet, and the house Manual J came in at 55k Btuh, so it is oversized. I'll ask about derating it, but it may be as small as it can be while maintaining good stack temp. It tests out at 85% efficiency each year. It's a fairly new Weil-McClain. I dimly recall they may have dropped the jet size when she tightened up the windows.

The summer efficiency is horrid - it short cycles a few times per day all summer just to make hot water. I believe the indirect heat exchanger is fouled because the return line is almost as hot as the supply line even when the indirect tank is completely drawn down and full of 55 degree water. It uses most of a tank (~150+ gallons of oil) through the summer just for hot water - house average occupancy < 2 people.

I asked about switching to an electric storage water heater for the summer months and take the oil burner out of service for the summer, but, strangely enough the folks that sell her that tank of oil think its a very bad idea - claim boiler will corrode if not run 12 months per year.

I'd really like to replace the whole setup with a closed loop geo system, but the house is a 3 level cube with an open stairwell and no present ductwork, so it would be the project from Hades.