Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Has anyone tried this Primary-Secondary variation?

Boilerpro_5 Member Posts: 407
if we are looking at oil fired condensing and non condesning I suspect the pricing is much closer for the two options. However, if looking at a gas fired mod con and a simple atmospheric gas fired unit, chimney vented, the pricing differential is much greater. Modcons are about 3 times the price of a basic, economy cast iron boiler, from my sources, depending on size. And while it will take more time to add the 3 way thermostatic for low temp operation, for me, at least, the rest of the boiler installation is much quicker than for a mod con.....no drilling 2 holes through a concrete wall for vents, very little time for installing venting (assuming chimney is usable), no condensate neutralizer to build.

I'm sure the great words of our wise host apply here like nearly all of life's other questions..."It depends.".

My own are of specialty, churches, gives me a unique perspective on this as the typical day load is often at most a 1/4 to 1/3 the peak load. This also applies to most commercial applications, where internal gains provide a huge amount of the heat necessary on the typical winter day. When I see 3 or 4 modcons installed in these applications, and only one is ever doing much work, it makes me stop and think about the wisdom of using such complex and expensive equipment for a load that almost never occurs.


<A HREF="http://www.heatinghelp.com/getListed.cfm?id=393&Step=30">To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"</A>


  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    We're looking at a boiler replacement

    it's the old Spencer in the article "A Hot-Water Stumper". This is a big old gravity hot-water system with a heat-loss down to about 180 MBH. The new system will include an indirect. We're thinking about splitting the load between two oil-fired boilers and controlling them with a 2-stage reset controller.

    We have the idea that if the two boilers are piped primary-secondary, some of the output from the first one will feed into the second one thus raising its inlet temperature. So there is no advantage in using a condensing boiler as the second unit. However, the first one would see all the cold water coming back thru those big pipes and would condense like crazy until it got real cold out. In this scenario, the condensing unit would always come on first, and the other one only if the water temperature had to be raised above condensing range.

    I've attached an attempt at a diagram of our concept- sorry, I'm no artist.

    Anyone try something like this? If so, what if any problems did you have?

    Thanks in advance.

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,479
    I've never

    tired this, but I like the idea of using t5he condensor boiler as the first stage. I'll be watching for experienced answers with great interest. WW

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Brad White_2
    Brad White_2 Member Posts: 188
    I like the

    "progressive temperature" sequence of things as Wayne also noted.

    I would ask if the main circulator should go after the 3-pass and there would be the PONPC. As sketched each circulator is in series to the primary and flows/pressures may differ (not knowing the loads and proportion of each). Also, yes a detail, but the air separator would benefit from being after the 3-pass and before the indirect.

    The other part of this is the dilution of the heating zone temperature by the indirect load. Can they be in parallel rather than in series? I suppose it is no different than a time-duration deprivation as in any indirect mode, but in this case the return from the indirect goes to the radiators not direct to the boiler.

    What controls the final temperature to the heating circuit?

    Will the ModCon be set up to protect the 3-pass? IOW: Fire to maintain 130 F minimum or the 3-pass circulator be locked out until 130 F is reached? Meaning a more positive lock-out I suppose.

    As so often I do, I may be missing something critical and obvious in what you are doing, so as with Wayne will stand by...
  • hr
    hr Member Posts: 6,106
    I'd still favor two mod cons

    What brand oil fired condensors do you have in mind?

    I'm not sure PS would be the way to pipe that so the condensor always sees the lowest possible tempersature.

    The trick to staging mod cons is the lead boiler fires till it approched non condensing temperatures. Then it starts backing off it's output and brings on the second stage boiler. The concept is to expose as much HX surface to the lowest possible temperatures.

    I think even as you get beyond condensing operation the modulation and reset controls built into mod cons would still provide a lot of bang for the bucks.

    Two 90K mod cons would provide a 10-1 turndown tracking the load from possibly 28K to 180K in steps. That's a nice control-ability feature.

    I'd wonder what % of the time the load could be met with return tempersatures in the condensing range. I suppose one could run the heatload numbers with different outdoor design temperatures to find that info?

    Be curious to see what you cook up for this install :)

    hot rod

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • rich pickering
    rich pickering Member Posts: 277

    Why not pipe the indirect off of the three pass with its own circ. Give the indirect priority, so it won't interfere with the primary system, and it will give the three pass something to do instead of sitting idle 90% of the time.
  • ScottMP
    ScottMP Member Posts: 5,884
    So Bob

    would you pipe the mod/cons with reverse return ??

    Interesting ?


    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    This is a big old gravity hot-water system with a heat-loss down to about 180 MBH.

    Decent thermal improvements? Still the original radiators?

    If so, I suspect that two condensing boilers would be highly preferable. Why? Because I suspect that the first would never need to produce temps anywhere near 140° supply.

    Say you have 2,500 sf EDR. Say you use two boilers with net output of 90 mbh (half the heat loss).

    90,000 / 2,500 = 36 btu/hr that each square foot of EDR must deliver. At low temps, I believe it very safe to estimate 2 btu/hr per sqft EDR per degree of temperature difference. So, you only need 18 degrees of temp difference. In a 68° room that means an average radiator temperature of only 86°!

    If a wide-open gravity conversion, you'll likely be moving at least 40 gpm through the radiation. If so, that's only 4½° of delta-t in the radiation loop. 86 + 2¼ = 88¼° required supply temp. For any reasonable room temperature, I'd guess that the one boiler couldn't even produce an average radiator temperature above 90°F!

    Now say you're moving 4½ gpm through the boiler to deliver its 90,000 btus. With the single boiler, I suspect that return temp from the system would never (again with reasonable room temp) rise above 85°F. You yourself suspect that the return temp will be extremely low, right?

    But for the sake of argument let's say you could get 85°F return temperature. Again, you've moving 4½ gpm through the boiler for a 20° delta-t. Again, let me assume 40 gpm through the system as a whole. 85° + 20° = 105° supply temp at the boiler. When it blends back into the big flow your supply temp (e.g. return to the 2nd boiler) is only 87½°F!

    Run with your own numbers, but if these estimates are anywhere near close, I believe you'll find it impossible for that first boiler to ever produce 140° to supply the return of the conventional boiler.

    You could use TRVs on the rads to reduce system flow down to reasonable rates (and thus be able to produce much higher temperatures), but the system still wouldn't need such high temperatures to heat the place to begin with and you'd still be better off with two condensing boilers.
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    All interesting ideas

    thanks a lot- and if anyone has any more I'm listening.

    Swampy, you may be right about radiator temps- I have a radiation listing for that house somewhere to compare the existing radiation with the calced heat loss. Yes, they did install all new windows but have not added insulation as yet.

    I did talk to the Monitor rep today. They've never done a job quite like this concept, but he sees no reason why it shouldn't work.

    What I like best about this idea is the redundancy it provides. Condensing boilers are by their nature more complex than cast-iron units. In this scenario if the condenser breaks down, the cast-iron unit could take over, keeping the house warm until we get there.


    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Rocky_2
    Rocky_2 Member Posts: 89
    ModCon/3-pass piping--Buderus website

    Go to Buderus website. Go to the SB series boilers. In their tech lit they show exactly how to pipe a ModCon to a 3 pass with indirect connected. A wealth of info.
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    Tried that

    but couldn't find the diagrams. Could you please post a link?

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    If you'll reply with an estimate of the EDR, I'll do me best with numbers for you. Not absolutely necessary, but an estimate of flow rate through radiation wouldn't hurt.

    I certainly appreciate the idea of redundancy, but that part-load sized non-condensing boiler won't be a happy camper without bypass. No big deal for an emergency situation, but I can imagine similar problems if significant daily setback is used and you drop in the second boiler for reasonably quick recovery.
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    Haven't found it yet

    but IIRC about 1000-1200 square feet EDR.

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Cosmo_3
    Cosmo_3 Member Posts: 845
    do a search for siggy

    He did an article about piping multiple boilers, and using a modcon as the lead boiler, since that is what gets the most use. I actually used that arrangement a while ago and worked great, I will have to see if I still have pics, I lost some pics forever when one of my memory cards went scuba diving in my coffee....

    If I find pics, or a link to the article I will post

  • Boilerpro_5
    Boilerpro_5 Member Posts: 407
    Worked with that Idea before

    The critical thing you have to watch out for is thermal shock to the 2nd boiler from hot water coming off the system into the cold boiler when it first kicks on. With your low system temps this probably is not a problem. However, you still need low temp protection.
    Basically you are base loading with high efficiency equipment (90% of your winter heating load) and picking up the last 10% with whatever is cheapest to install. The payback is just not there, I don't believe, to use all condensors on multistage space heating plants. Why pay for an expensive condensing boiler when it almost never runs.
    I always find the second critical heating load for good system design, the amount of heat needed on the Typical winter day. I design my heating plants to meet that load in the most efficient way possible. The rest of the boiler capacity is almost never used, so the efficiency of those lag boilers does not really have any signicant impact on the fuel usage.


    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Constantin
    Constantin Member Posts: 3,796
    I'd use two mod-cons...

    ... pipe them combined pri-sec and then stage them by output needs. That way both boilers always see the low incoming water temps they need to run at optimum efficiency and you can cycle them with an intelligent cascading controller so that they "wear" the same.

    Under the circumstances, you get the same degree of redundancy, excellent, year-round efficiency, etc. and lower fuel consumption, as the standby losses of the 3-pass boiler (if it's not a cold-start) will be sizable.
  • Guy_6
    Guy_6 Member Posts: 450

    I like the idea of the second boiler being piped directly to the indirect as well. With that , you could utilize outdoor reset on the first boiler, which would aid the condensing. The second boiler could remain a single high temp boiler, as that would be pretty much what the heat side of the system would need upon it being called on.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Wow. Less than half the radiation I was expecting. Maybe the dead men in MO just went crazy with radiation... I was though assuming that decent insulation had already been added.

    Again assuming 90 mbh part load boiler:

    90,000 / 1,200 = 75 btu per square foot.

    I'll drop output to 1¾ btu/hr per square foot per degree of temp difference so, 75 / 1¾ = 43° temp rise. 68° + 43° = 111° average rad temp. Let's take flow down to 23 gpm (single Taco 007 or B&G 100). That gives a delta-t of 8°.

    So, the one boiler should max out at around 115° supply and 107° return.

    Again I'll assume 20° delta-t (9 gpm) for the boiler. (107 * 14) + (127 * 9) = 23x

    1,498 + 1,143 = 23x

    2,641 = 23x

    x = 115° supply from the 1st boiler (return to the 2nd).

    I really wouldn't expect the one boiler to be able to produce more than 120° (and likely less in actual operation) with any rational indoor temps. If the basement piping is not reasonably insulated, think significantly less than 120°.

    Unless that second boiler can deal with sustained low return temps, I'd suggest thermostatic bypass for it.

    Here's an idea for the simplest control I can think of:

    1) Fire first (condensing) boiler to a fixed internal aquastat setting of about 100°. (this assumes typical ±10° differential). If less differential up the setting to try for about 110° at cut-out.

    2) Mount a strap-on aquastat on the main supply to the radiation. Use setting of about 125°. Wire it to control BOTH boilers. Use a relay to "replace" the internal aquastat for the first boiler.

    3) Drop in the 2nd aquastat in two different ways: First as the 2nd stage of a two-stage thermostat (this for reasonable recovery after setback). Second only when outside temp drops below some level (start VERY low and go up only if 1st boiler cannot maintain desired temp).

    4) Again, you'll want bypass around the conventional boiler to keep return temp up if it cannot operate with sustained and very low return temps. I know that condensation problems with gravity conversions are very rare, but this just might be a way for such to occur.

    5) This arrangement should keep the 1st boiler as the "primary" with the 2nd cycling should the aquastat setting ever actually be achieved during a heat call.


    I once fired my old boiler (170 mbh net output) for about 12 hours straight in 50s weather with all TRVs wide open. 1,049 sq.ft. EDR and pipes uninsulated. House 85°+ with the windows and doors open. Supply maxed at about 167° and would not go above. Not sure about the return temp, but am pretty sure it was somewhere around 140°.

  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Please excuse error for boiler flow in the above. I used 4.5 gpm for 20F delta-t with 90 mbh input as opposed to 9 gpm--sorry--had a brain fart when working fast...
  • hr
    hr Member Posts: 6,106
    P/S, Scott

    Not sure I would worry about 135 water hitting the cold cast.

    You might fire the second cast boiler without running the primary loop boiler to "preheat" and prevent hot shock if it were a concernburner without the boiler circ until the preheat temperature was reached, deprending on the type of boiler. Still I don't think 135 to a 65 degree boile would be a problem.

    hot rod

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Cosmo_3
    Cosmo_3 Member Posts: 845
    The more mod cons.....the better!!!

    I was assuming that he was set on one mod con, and one not. But obviously as long as the money is there upfront, two mod cons are better than one!!!

  • Trouble Frank...

    If my assumption is correct that the main (large)circulator moves heat through the former gravity system, you won't be moving water past the two tees serving the DHW tank unless there is a call for space heat.

    Other than that, what you have drawn would work, except that it is essentially a parallel series system serving the boilers. I'd much rather see the boilers served by one set of input secondary tees, and then pipe the DHW in parallel to the high temp boiler.

    The reliability issues of older mod cons are pretty much a thing of the past. I have less problems with the mod cons than I do the atmospherics these days. The nice thing about most of the mod cons is that of you do have a problem, with most of them you can hook up a lap top to them and see exactly what their problem is or was!

    If my assumptions regarding the main loop pump are wrong, please tell me so and I will generate a drawing for you when I get a chance.

  • Frank Fix...

    Frank, Here's how I'd do it. Other than redundancy, I see no benefit to your way of piping. I certainly don't see a need for simultaneous operation for very often, and by that time, you're probably already approaching the mod cons less condensing range, and the non mod cons non condensing range.

    I got to thinking about it the other day. I've not sold anything BUT mod cons for the last 3 or 4 years. I just don't believe in using anything but the most efficient appliances any more.

    Oh sure, the non mod cons DO have their place in life, I guess I've just been lucky enough to not have to use it as often as I used to.

    B1 would be the mod con, and B2 could be either.

    Crazy thought here, but knowing you and how you like to work with gravity as much as possible, how about eliminating the main circulator and replacing it with a fat spot (low loss header) in the main line and letting gravity do its thang. Do a full outdoor reset with indoor reset influences and let 'er rip!

    A leap to the past...


  • Boilerpro_5
    Boilerpro_5 Member Posts: 407
    But where's the payback!!!

    Why use a 2nd modcon boiler when it is only used 10% of the heating season? You put out alot of extra cash upfront for nearly no gain in overall seasonal efficiency. Utilities have been using this type of reasoning for years. You build a few very efficient generating plants for your base load and then throw together some cheapo peaker plants for those extreme days. The modcon is your baseload provider, and a cheap cast iron boiler is you peaker unit. As I understand, many governmental bodies in Canada and the US military use this concept for heating plant design. In any larger structure I always calculate 2 loads, the peak load and the typical day load. Its critical you meet that typical day load with the most efficient system possible, the peak load doesn't matter very much becasue it only occurrs a few days a year. This is why modcon can work well with higher temp systems.....They are running in the condensing range for most of the winter. Only during a few days a winter will they run noncondensing.

    I am going back to clients that I installed noncondensing multistage heating plants in the last 10 years and suggesting they pull one of the installed units and swap in a modcon to get very quick payback on the investment. Most of these installations saved 35% to 65% over the previous boilers, but a single baseload modcon can save even more. I have one of these already in place (the owneres were particularly long range thinkers) and the condensing boiler is the only boiler they ever see firing unless it is extremely cold or the system is coming out of setback.


    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • hr
    hr Member Posts: 6,106
    What is the actual $ difference Bpro?

    Suppose a mod con in that size could be had for 3 grand. What is a cast in that size going to cost. Let's say it is half as much 1500. Then you add and build return protection as a t3 way thermostatic, perhaps 4 hours of labor to fool it into thinking it's a low temperature appliance. Add the fittings and misc to make the conversion. I'd bet you come within 5-600 bucks of the plug and play mod con?

    Plus the mod con gives you DHW priority built in. Outdoor reset built in. Most data log, most give troubleshooting error codes. Most modulate, which none of the cast iron do, pretty clear the advantages for not a whole lot of additional upfront cost.

    If you agree the rest is a salesmanship issue. Should be a fairly easy upgrade when all the benefits are explained.

    hot rod

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Must agree HotRod, but it seems that oil is the preferred fuel with Monitor the anticipated boiler. Condensing yes (and supposedly no problem with US fuel oil), but modulation no.

    In that light, such an arrangement [might] prove efficient and practical with good, simple backup. My real idea is that the condensing boiler will provide the heat with the traditional exercised (an efficient with proper heat transfer) via the indirect. The 2nd boiler should rarely have to run if moderate room temperatures are maintained with at most slight daily setback...

  • Constantin
    Constantin Member Posts: 3,796

    ... but I how far to take the first-cost vs. operational cost debate? Should a client start installing electric water heaters to cover the last 10% of the load since it occurs so infrequently?

    I guess the debate is somewhat academic since Frank is looking for a oil-fired solution. No mod-con oil-fired boilers have yet been sold to the public in the USA, AFAIK, though Brookhaven and Alan Mercurio have/had units to play with.

    Anyway, if a twinned mod-con system is used in a home and each boiler is rated for 50% of the design load, the second boiler will be running more than 10% of the total runtime due to IDWH, heat loss, etc. A second mod-con would have much less standby losses than a cheap atmospheric that has to be kept at temperature year-round.

    A dual mod-con setup would give you a very fine modulating response to a load instead of the slugs of hot water that a bang-bang cheapo atmospheric boiler produces. Add mixing valves to the atmospheric to make it behave better and be protected, and I would be surprised if the added material and labor does not come close to the marginal cost difference between a mod-con and a regular boiler.

    Plus, you can take advantage of the second mod-con to minimize recovery times on the indirect. Lastly, with the right cascading controller, you'll get even wear across both units while minimizing standby losses. As a client, I'd look at the marginal cost and the marginal benefit of going with a powerplant as you propose vs. the other options. As you correctly point out, we do not have all the data to know what the right answer is...
This discussion has been closed.