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Comparing Mod/Con propane to oil boiler

mac_5
mac_5 Member Posts: 11
Ted,

Thanks for the replies.

You are right that is a confusing statement. What I have investigated and thought I wanted was a low mass/three pass oil fired unit. Then someone mentioned that using it as as supplemental to the heat pump is not an efficient use of that type of boiler and with an oil burner. They suggested I look into a mod/con as it may end up being more efficient the way that I would be using it.

So that is what I am trying to figure out. As supplemental to the heat pump which would work better and cheaper?

Comments

  • mac_5
    mac_5 Member Posts: 11
    Comparing Mod/Con propane to oil boiler

    I am trying to figure out the difference in operating cost between a mod/con propane fired boiler and an oil fired unit. In either case, the boiler will be a new low mass, three pass unit with outdoor reset (Tekmar 260?) that will provide for supplemental heat through a hydro-coil for the heat pump which will be the primary heat source. The boiler will also provide for DHW through an indirect tank.

    Because the boiler will be supplementing the heat pump and not the primary heat provider, it will have a more limit fire time. Does that impact the effective efficiency of each type burner differently?

    Currently, propane is about $2.20 per gallon with oil at about $2.50. I have been partial to oil due to the higher net BTU output and the lower net cost per usable BTU.

    Is there an easy way to calculate the operating cost of each in the way that I will be using the boiler?
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    Not sure about the comparison

    but one thing to consider- if this is to be a "hydro-air" setup, used only in colder weather, will you be able to install a coil that can handle the load with no more than 140-degree water temperature? If you go higher than that, a mod-con won't condense, and its efficiency will drop.

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  • mac_5
    mac_5 Member Posts: 11


    Steamhead,

    Not sure what the water temp will need to be for the hydro-coil. How do I determine that?
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    You have to know your house's heat loss

    then, check the manufacturer's charts to see if the coil will emit that much heat at 140 degrees.

    If you or your contractor haven't done a heat-loss calculation, stop right now. Don't do anything more until the heat-loss calc is done. Otherwise you might be buying bigger equipment than you need.

    Click on the "heat loss calcs" button above. This takes you to Slant/Fin's site where you can get a free, easy Windows calc program. If you still use dial-up, I'd order the CD since the download is VERY large. Then do the calc and see where you are.

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  • mac_5
    mac_5 Member Posts: 11


    I have done a heat loss but I will also have the installing contractor do one so that the system he puts in is supported by his numbers. I don't want to dictate what the heat loss is and then end up having a finger pointing contest if there is a problem.

    Anyway, I have calculated the loss at 70K. Can you refer me to some coil manufacturers' web sites?
  • Ted_9
    Ted_9 Member Posts: 1,718
    Boilers

    I had posted Unico's hydro coil numbers here recently. I'll look them up.

    BTW, what do you mean by this? "In either case, the boiler will be a new low mass, three pass unit with outdoor reset"

    You do know that a Mon/Con is not an oil fired three pass right?
  • Ted_9
    Ted_9 Member Posts: 1,718
    Unico Hydro air

    Here it is. This is just for Unico. This was just for the 3 ton cooling and with heating module. Refer to the link below for the right size heating coil.


    This is from the Unico book.

    HW-3660

    700 cfm, 140 degree entering water temp.

    4 gpm= 45,100 btu's

    6 gpm= 48,800

    8 gpm= 49,500

    10 gpm= 45,100(that may be a misprint)





    BTW, here are some nice hand books from my friends a Capco Supply:



    http://www.capcosupply.com/unico_handbook.htm
  • Joe Brix
    Joe Brix Member Posts: 626
    WHat region are you in?

    I'm not a believer in HP in the snow belt. Usually our electric rates can't justify it. I would never calculate a hydro-air config as supplimental but be able to carry the full heating load requirement if needed. Nice to have when the HP dies in January. I would stick with a duel fuel config. : lockout the HP below 35 degrees. Since you're not running any low temp radiant, go with a good oil boiler with outdoor reset and DHW priority. Get the
    smallest boiler that will meet your heating load. The 2 or 3 times a day the Indirect calls for heat is not worth oversizing a boiler for. Not worth the added expense of a Mod/Con LP boiler unless you need the direct venting.
  • mac_5
    mac_5 Member Posts: 11


    Joe,

    I live in Virginia with a winter design temp of 8*. I like heat pumps as they are rather inexpensive to heat and cool. My electric rates are $0.11. With demand defrost I can heat my space cheapest with a heat pump with supplemental needed at around 30*. At the design temp, a 3 ton heat pump is still contributing 14,000 BTU's at a COP of about 2.2. Still cheaper than oil.

    The boiler will be sized for the entire heat load.

    Your overall recommendation is pretty much what I had thought I wanted: low mass/three pass with indirect DHW and good controls. I need about 70K BTU, so there are several that can fit that need.

    So, overall you don't think that a mod/con would be worth the cost to purchase or operate?
  • Ted_9
    Ted_9 Member Posts: 1,718
    OK

    I see clearly now, thanks Mac.
  • scott markle_2
    scott markle_2 Member Posts: 611
    Controls


    What is the supply temperature of the heat pump at design temp and 14,000btu output?

    14,000 does very little if it's not warm enough in a high temp application. I'm wondering how these heat sources would be integrated if they are to run together in the way you describe, as opposed to the oil burner taking over below a determined outside temp.

    The modulating ability of a condensing gas boiler could perhaps be utilized to match the deficiency of the heatpump. Sounds complicated though.I never seen any schematics or control descriptions on that.

    Perhaps you could design a two stage system where the heatpump is used for radiant and when the radiant is not capable of meeting the load a hydro coil kicks in powered by a small oil burner. The heat pump would continue to operate in these conditions,contributing what it can.Probably working at a even higher C.O.P because of it's low temperature application.
  • mac_5
    mac_5 Member Posts: 11


    scott,

    I'm not sure what the supply temp would be.

    The hydro-coil would be installed in the duct work just upstream from the heat pump air handler. Instead of the typical electric strip heaters coming on to supplement the heat pump output, the hydro-coil would cycle in to supplement the heat pump output.

    I envision the boiler controls raising the output of the boiler as the outside temps drop and more dependency is placed on the boiler. I hope that I won't have to raise it much over about 140* until outdoor temps reach down to about 20* then it would move the boiler temps up as needed beyond 140*.

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


    I don't know the loads involved, but suspect that your DHW load will be significantly higher than required for boosting the output from the heat pump.

    In other words, you'll likely need a decent sized boiler for DHW (just make sure that the indirect can INSTANTLY ABSORB THE FULL OUTPUT OF THE BOILER) and a small boiler to supplement the heat pump. In that regard, I suspect that the "mod" (modulation) of a gas mod-con is what you really need. You're unlikely in this situation to benefit much from the "con" (condensing) of the mod-con, but the "mod" is just what you need...

    The ALL CAPS part of this message are HIGHLY important! If your indirect cannot INSTANTLY ABSORB THE FULL OUTPUT OF THE BOILER you have utterly wasted capacity FAR above that required to supplement the space heating. Consider a Viessmann indirect. The 43? gallon (smallest) stainless steel horizontal indirect is the bargain of a Cadillac line.
  • scott markle_2
    scott markle_2 Member Posts: 611
    Air temp

    Perhaps instead of modulating the temperature of (combustion heated) hydro coil by out door temperature, you could use a sensor in the duct work to modulate boiler output based on supply duct air temperature. With a mod-con this might be fairly simple and not even require a mixing valve. This would ensure that supply air temps. were sufficiently warm to avoid discomfort but also not so hot as to cause short cycling of blower and scorched air effect. Not sure how (specific control),but target duct temp could perhaps be regulated by out door reset to increase boiler and heat pump efficiency in lower load conditions.

    I realy don't know much about air heat, Are variable blower speed controls available that can be used to achieve the air equivilant of hydronic constant circulation?

    I guess I'm kind of biased against air heat,I have allergies and air heat seems to bother me a bit more. Maybe it's just dirty ducts and cheap filters.

    As you get further south central ac becomes more or less expected,you have all this investment in duct work so you want to use it. personally I would prefer mini-ductless system and hydronic heat .
  • scott markle_2
    scott markle_2 Member Posts: 611
    instantant absorb

    Mike, I'm not sure about the emphasis on instantly absorbing the full output. With a mod con if return temperatures from indirect rise, boiler output will be reduced. Why must the indirect absorb the FULL rated output? How would this work in a structure with a high heat loss and modest dhw requirements.Are you suggesting that the indirect would have to be over sized in this case? Looking over some Viessmann tank literature I noticed that hx surface areas do not increase in direct relationship to capacity, on the larger tanks the ratio hx:tank volume is lower.

    My concern would be a means of suppressing air handlers during dhw production to avoid uncomfortable insufficiently heated air. If the boiler was sized just to match the deficiency of the heat pump and dhw requirements are high,or an overly large indirect was used, then there is a risk that the air handlers could be shutdown for to long, effecting comfort.
  • mac_5
    mac_5 Member Posts: 11


    Scott,

    Yes, the blower will be an ECM variable speed blower and I could have it set to maintain whatever CFM I want in both the duty cycle and not.

    Interesting idea to monitor duct temp.

    I've never lived in a house that didn't have A/C and ducts. There is always a lot of talk about low discharge temps with a heat pump and how it makes the home feel drafty and cold. I have never experienced that situation and I have had many heat pumps now in many houses. With the ECM blower the CFM can be set low enough to always have warm air (105* +) at the outlet vents.

    In any event any time that heat pump would only be putting out 14K BTU then the hydronic coil would be in use too and it would be further warming the air that is heated by the 14K. Assuming it is a design condition period then the about of heat needed from the coil would only be 56K. (I'm sure that I will lock out the heat pump at some temp. Maybe 3* and then only use the boiler, but that should be for brief periods at the most.)

    So, there is no easy analysis of the operating cost of a mod/con boiler vs. an 86% efficient oil three pass one? And, there isn't a consensus as to whether using an oil boiler as a supplemental heat source is less efficient?
  • Joe Brix
    Joe Brix Member Posts: 626
    If you plan to use so little of the boiler

    > Scott,

    >

    > Yes, the blower will be an ECM variable

    > speed blower and I could have it set to maintain

    > whatever CFM I want in both the duty cycle and

    > not.

    >

    > Interesting idea to monitor duct temp.

    > I've never lived in a house that didn't have A/C

    > and ducts. There is always a lot of talk about

    > low discharge temps with a heat pump and how it

    > makes the home feel drafty and cold. I have

    > never experienced that situation and I have had

    > many heat pumps now in many houses. With the ECM

    > blower the CFM can be set low enough to always

    > have warm air (105* +) at the outlet vents.

    > In any event any time that heat pump would only

    > be putting out 14K BTU then the hydronic coil

    > would be in use too and it would be further

    > warming the air that is heated by the 14K.

    > Assuming it is a design condition period then the

    > about of heat needed from the coil would only be

    > 56K. (I'm sure that I will lock out the heat

    > pump at some temp. Maybe 3* and then only use

    > the boiler, but that should be for brief periods

    > at the most.)

    >

    > So, there is no easy analysis

    > of the operating cost of a mod/con boiler vs. an

    > 86% efficient oil three pass one? And, there

    > isn't a consensus as to whether using an oil

    > boiler as a supplemental heat source is less

    > efficient?



  • Joe Brix
    Joe Brix Member Posts: 626
    If you plan to use so little of the boiler

    I can't see where the added cost of a mod/con is worth it.
    11 cents electric doesn't sound very cheap to me unless all the monthly service costs are included. Wth an 8° design temp, I guess you're up in the mountains of VA. What's the COP rating of a compressor that's burried under 3 ft of snow? I'd rather heat with a boiler sitting on the INSIDE of my home with the chimmney 4 ft avove the roof line. Ice, snow and HP don't sit right together in my book. But hey, I'm old school.

  • jalcoplumb_7
    jalcoplumb_7 Member Posts: 62
    I am curious.

    Has anyone done a side by side comparison?

    Curious to know what a typical 80,000 btu home would cost to heat with a propane 93+ AFUE mod/con vs. a comparative oil fired boiler for a heating season.

    Both should have anual service cost figured as well.

    Any one have the time?
  • mac_5
    mac_5 Member Posts: 11


    Joe,

    Yes, my .11 cent electric rate is the all in rate.

    I'm not quite in the mountains, but somewhat close to the foothills of the Blueridge.

    We haven't seen three feet of snow in so many years I can't even remember it. In the off chance we did get that much snow and the heat pump was unable to function then I would shut down the heat pump and heat with the boiler (another reason to size for the whole load right).
  • Guy Woollard
    Guy Woollard Member Posts: 82
    variable speed

    An interesting thought about variable speed air handlers tied with a modulating boiler is that they work in tandem: When the blower speeds up, the delta T over the coil increases, therefore lowering the return water temp. The modulating feature of the boiler will react ON ITS OWN to that change. The only communication between the two is the water temps.
    As for putting an outdoor sensor in the ductwork- I would be more prone to putting it outside where it belongs and putting a secondary t-stat in the ducting to ensure stable air temp delivery. That way you get the benefit of the outdoor reset feature of the boiler.


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


    Again, I don't have the sizing numbers, but it was stated that the boiler would only supplement the heat pump. Given that, it would seem likely that the DHW load will be higher than the space heating load on the boiler in nearly all circumstances.

    Not suggesting to "oversize" the DHW--just suggesting that it will likely be the larger load and that it would seem rather foolish to have a boiler cycling to produce DHW. Thus the emphasis that the indirect should be able to absorb the FULL output of the boiler.

    Sized this way and with a mod-con you'll have the full modulation range of the boiler available to "boost" the supply temp to the air handler(s). If a non-modulating boiler were used, it would probably be cycling quite frequently when supplementing the space heating.

    Good thought about the real possibility of "cold air" during a DHW call when the boiler is also needed to supplement the space heating. The easiest way around this would be to shut down the heat pump system during a DHW call--a more elegant way would be to shut it down only if the boiler is also needed for supplemental space heating.
  • mac_5
    mac_5 Member Posts: 11


    > Again, I don't have the sizing numbers, but it

    > was stated that the boiler would only supplement

    > the heat pump. Given that, it would seem likely

    > that the DHW load will be higher than the space

    > heating load on the boiler in nearly all

    > circumstances.

    >

    > Not suggesting to "oversize"

    > the DHW--just suggesting that it will likely be

    > the larger load and that it would seem rather

    > foolish to have a boiler cycling to produce DHW.

    > Thus the emphasis that the indirect should be

    > able to absorb the FULL output of the

    > boiler.

    >

    > Sized this way and with a mod-con

    > you'll have the full modulation range of the

    > boiler available to "boost" the supply temp to

    > the air handler(s). If a non-modulating boiler

    > were used, it would probably be cycling quite

    > frequently when supplementing the space

    > heating.

    >

    > Good thought about the real

    > possibility of "cold air" during a DHW call when

    > the boiler is also needed to supplement the space

    > heating. The easiest way around this would be to

    > shut down the heat pump system during a DHW

    > call--a more elegant way would be to shut it down

    > only if the boiler is also needed for

    > supplemental space heating.



    Mike T,

    I missed several postings as I am new to the this board and the way the post follow the prior postings.

    You may not have seen an earlier response where I stated that I have done a heat loss calc at 70K BTU. The boiler will be sized for the full load in case the heat pump is down or temps are below a lock out point of about 3*. In normal operation though I assume the boiler would only be providing 40-50K BTU heat as supplement to the heat pump.

    I don't know for sure but I have assumed the t-stat will call for all stages of heating. Two stages of heat pump compressor and one or two stages of supplemental heat from the boiler. I also assume that there would be a relay, or several, and if there was a call for supplemental heat the the relay would send a signal to the boiler and kick up the fan speed. But, that signal may first need to go through a relay that is controlled by the call for DHW. Thus, if there is a call for DHW then the call for heat is held off and the fan would not kick up.

    I appreciate your comments on the mod/con. I guess what I am really after here is whether the heat load that would be carried by the boiler would cheaper with a mod/con or an oil fired unit.

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


    Thanks for the numbers and additional info Mac--they helped significantly.

    While LP is more expensive on a btu basis, I suspect that a mod-con would prove less expensive to run.

    70 mbh finds even the smallest mod-cons from most lines somewhat oversized. I'm not positive, but I seriously doubt that a two-stage oil boiler would be available in this BTU range.

    Since the boiler ordinarily won't be needed for space heating--and even when it is only as a supplement--a non-modulating boiler sized for the entire load would be hideously oversized almost every time it has to supplement the heat pump. This will result in frequent and energy robbing burner cycling.

    Installed and controlled correctly however, a modulating boiler will in most cases be able to provide just the amount of additional heat required without cycling. Since the boiler will be "topping off" the supply temp, all the piping methods I'm thinking of wind up with the mod-con return being essentially at the temperature of the supply from the heat pump. This will result in less efficiency gain from condensation (compared to if the mod-con saw the return temperature from the emitters), but the ability to modulate will be a great plus.

    Actually, I can see a very simple piping scheme that uses the inherent control ability of mod-cons. Mod-cons generally have far less "standby" heat loss up the flue when the burner is not firing and even with water if circulating through the heat exchanger.

    With its very low head loss, the Triangle Prestige may well be able to be hooked up in series (after) the heat pump in the primary loop. As long as you keep primary flow within the acceptable range for the boiler, merely using the built-in outdoor reset ability of the mod-con will allow it to automatically and proportionately boost the supply temp when the heat pump alone is insufficient. No additional controls required. This would certainly have to be confirmed with the boiler's manufacturer, but I'm not seeing any problem as long as system flow rates are within the acceptable range for the boiler.

    If budget will at all allow, I would still use a high-quality indirect like the Viessmann. Even the smallest model is capable of instantly absorbing the full output of a boiler in this size range in most circumstances. This will minimize the time required to refresh the indirect. Since nearly all mod-cons operate with full DHW priority (no contribution to space heating when supplying DHW), refreshes that are as quick as possible will minimize the time the boiler can't supplement the heat pump. If the situation arises with objectionably cool air in cold weather when supplemental heat from the boiler is required during a DHW call, it would be a relative simple matter to add control to shut down the heat pump as well during a DHW call.

    As with any system with DHW priority (especially hydro-air), you want to avoid the simultaneous occurrence of setback recovery and high DHW demand (like morning showers). Use "smart" thermostats to ensure that space temperature is fully recovered before any peak DHW demand.
  • mac_5
    mac_5 Member Posts: 11


    Mike T,

    Wow, great write up. Any chance we could get some numbers put to this.

    Yes, you are correct, I haven't found any two stage oil burners in this size range so it would fire at a single rate.

    To keep an oil customer my oil company is offering a very attractive deal if I use an oil fired boiler. I haven't investigated it much, but if a mod/con is significantly more for the purchase then it is going to have to save a lot in the operating expenses to offset the price difference. I just did a quick look at the TT Prestige and the smallest has an IRB rating of 79K - so not a lot over rated. It is rated at 93% efficient, but isn't the efficiency at a much lower BTU like 40K much less?

    I know short cycling isn't good for the equipment, but are we talking about so much short cycling that I am going to have problems with an oil fired boiler? Certainly, during the summer it will have only the DHW calls.

    I appreciate your help.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    An I=B=R rating of around 80mbh for the smallest mod-con in a given line is quite common. Heat Transfer Products (Munchkin and Contender) are available in smaller outputs but the smallest (50 mbh) Munchkin certainly and I think the Contender both will put out about 80 mbh for DHW productions.

    All else equal efficiency of mod-cons actually increases slightly as output decreases.

    No, I doubt that short-cycling of an oil boiler would be severe enough to cause any equipment problems, but it will most certainly decrease efficiency when about the only time full (non-cycling) output is required is for DHW production.

    You'll almost certainly use significantly less energy with a mod-con, but since they do cost more than simple boilers and LP is more expensive on a btu basis and complicated by the fact that the mod-con won't be operating during low load conditions (when both condendensation and modulation savings are maximized) overall costs are a bit of a wild card and likely will wind up nearly on par.
  • chapchap70_2
    chapchap70_2 Member Posts: 147
    Something else to consider

    Mac,

    Have you considered the System 2000 boiler. The EK-1 has a water content of 2.5 gallons and it purges the heat out of the boiler to the last zone that called. Then it sits cold when no load is required. It would seem this would be a good backup if you want to use it that way. Since the boiler does not retain residual heat, there is no need for outdoor reset. Why maintain even a low boiler water temperature and have a 85 watt circulator(s) running all the time for backup purposes? The boiler could sit cold for days until it is needed or run just when there is a call for hot water. When the heat pump can't keep up, it takes less than 90 seconds for the boiler to heat up and deliver heat to the heating zones.

    Since a good indirect or storage tank loses less than 1/2 degree per hour, the burner would rarely run to maintain the domestic hot water temperature. I believe it takes less than 10 gallons of oil to maintain the temperature of a 30 gallon indirect for one year. This boiler can also be fired with LP or Natural Gas by switching the burner.
  • scott markle_2
    scott markle_2 Member Posts: 611
    communication by water temp

    Guy, your point about water temps. is insightful.

    "When the blower speeds up, the delta T over the coil increases,"

    The fact that boiler modulates up for this constitutes a form of feedback... Cool! or Hot as the case may be.

    I noticed this effect years ago in simple tank dhw radiant setup.The polaris I was using had a dial adjustment for temperature. I ran constant circ at around 80 to 110 tweak intuitively, running intentionally deficient to give a reason to burn wood.

    When I renovated and took the wood stove out and stopped tweaking the water temps.(left it at 90) I noticed that room temps were more stable than I expected.

    I also noticed that the polaris Which short cycled miserably (and had my anoyed attention) would fire considerably more as it got colder. This was a form of feedback with no thermostat at all.

    By the way I'm not suggesting that the outdoor sensor be placed in the duct. My thought is that instead of modulating to maintain target supply WATER temp. modulate to maintain duct temp instead. Target duct temp can be based on ODR but within the narrower perimeters of acceptable air temps. Also target duct temp reflects the contribution of the heat pump,water supply temps do not.

    More of an academic exercise more than practical advice.

    Carrier makes a nice condensing furnace that is designed to integrate with a heat pump, Not sure if heat pump runs with burner or if it's a switch over arrangement.
  • jalcoplumb_7
    jalcoplumb_7 Member Posts: 62
    Domestic hot water.

    If you are using an indirect then I would opt for the mod/con. You will be firing the boiler to make hot water every day.

    With the mod/con turn down you will be able to match your heat load better. The Munchkin T-50 and T-80 both fire at a higher output for the DHW load when tied to a Vision 1 sensor.

    If you have a 70,000btu load and are using 40,000 btu's then the oil fired will be wasting 30,000 btu's. It will most likely short cycle. The mod/con will match the load better.

    You can run a mod/con at a lower temp and not have to worry about flue gas condensate as with a cast iron boiler. Most cast iron boilers don't like to see return water temps lower than 140 or they will condense and rot out the heat exchanger.

    “To keep an oil customer my oil company is offering a very attractive deal if I use an oil fired boiler.” Think about that. They will recoup what you saved in no time and then some.

    If you feel your electric rate is cheap then have you looked into an electric heat source? You get out 100% of what you put in.
  • PJO_5
    PJO_5 Member Posts: 199
    I am doing something similar...

    Am in the latter stages of a replacement project...I had mentioned it on The Wall a couple of months ago.

    Old system: 30 year old oil-fired W/M 4 section boiler. Lowest feed I could do w/o hurting it too much was 0.95 gph. The tankless coil had rotted away 15 years ago and a 50 gallon gas HWH was there in it's place. This fed two of three apartments (third was electric baseboard).

    The system was working okay for an older unit that didn't have much TLC, but as we were renovating 2/3 of the building it was time to fix the heat/HW situation as well. It burned between 1,600 gallons last year, and 1,850 in the prior year (1.10 nozzle and boiler was very dirty when we bought the place). The same tenants lived there both years.

    Initially, I was going to simply replace the boiler with a Crown Freeport CT 3 section w/ 0.75 gph nozzle, and the output was a perfect match for the heatloss calc of 85,000 btuh (80k IBR rating, but I insulated everything I could - including all the baseboard piping). Coupled with a TT Smart 50 indirect, it would have been ideal w/ the extra boiler water through the indirect as a buffer...except we were still paying the heat and hot water.

    For about 15% additional cost, I decided to install two MC-80 Contenders w/ 30 gallon Smart Indirects. I was going to go w/a MC-50 for the smaller apartment (35kbtuh), but the two units modulate down to 18k and 19k...so it didn't matter much except for HW demand. I wanted to do a simple piping layout similar to what's described in the next paragraph, but the contractor was worried about voiding the warranties...so we followed the HTP pipe design exactly.

    I had done a Primary/Secondary set-up w/ a M-80 at my friend's house last year...the Munchie was right in the primary loop w/a Smart30, and the two heating zones (one basbeboard and one radiant) where the secondary loops. It had three pumps total, and the call for HW simply ran the primary pump - works like a champ!

    The job will be finished this week, and not only will I have a decent comparison, I won't be paying the gas bill for one apartment. We are using the bigger one in the summer, and will rent it through the winter, so it will have a heat demand probably similar to the prior tenants...although when the heat is free it's usually wasted more ;-)

    I will be comparing a 30 year old oil boiler - 76% AFUE or so on last check - to a couple of new Contenders, so it won't be totally fair. Plus, the price of oil in that area - NJ shore - is about $0.20 per gallon higher then elsewhere, and we have installed new outside doors and storm doors (maybe 5% change in heatloss). My guess is that the cost will be slightly less for the gas, but mostly due to the above factors.

    When the project is done I will post it here for all to see and comment. It is the best of the three systems I have specified and designed IMHO. I am also going to track the usage costs as much as I can for one year.

    Take care, PJO
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    How about a condensing oil boiler?

    the Monitor FCX has a rating of 76 MBH which is pretty close to your needs. Our friend Ken has one and loves it- he's in upstate Vermont.

    http://www.monitorproducts.com/mzboiler/products/fcx.html

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • mac_5
    mac_5 Member Posts: 11


    > the Monitor FCX has a rating of 76 MBH which is

    > pretty close to your needs. Our friend Ken has

    > one and loves it- he's in upstate Vermont.

    > http://www.monitorproducts.com/mzboiler/products/f

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    Steamhead,

    Thanks I will check it out. Everything I have heard to date about condensing oil burners was that the technology didn't seem to be quite ready for the mass market. Maybe that was for an oil fired furnace. Will we hear from Ken about his?

    Condensing is nice but it seems like the direction this discussion has been going is to the benefits of the modulating units.

    Thanks again.
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