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High Output, Efficient Gas Furnaces for Oil-Gas Conversion

Mark_129
Mark_129 Member Posts: 17
I’m coordinating the replacement of 30 yr old oil-fired furnaces with new natural gas furnaces at my church. I’m calling licensed heating contractors to provide a complete quotation. Our basic requirements are to minimize the up-front installation cost while preserving long-term savings with a reasonably efficient unit:

(1) Use standard residential equipment for high reliability & low maintenance

(2) >90% efficiency (unless a very large cost adder over typical efficiency)

(3) Minimum 2 speed fan and minimum of 2 firing rates for efficiency of operation and quiet operation while the facility is being used

(4) Full output of 120,000-130,000 Btu/hr, but preferably more (see below)

(5) Ability to control remotely now (or in the near future since the pastor’s house is detached from the church building and we don’t want him to have to walk over and check on the heating system during the coldest days of winter. Note that a programmable thermostat would fix this but it’s hard to convince some people to trust these, therefore a simple remote monitoring approach would be nice)

(6) Easy to use thermostat that is programmable



The full output requirement of 120,000 – 130,000 Btu/hr is the biggest challenge that I see. We are well insulated, but unfortunately some unique architecture prevents us from insulating more and bringing down the heat load even further. I used a programmable thermostat during the last winter to get the heat load from the building, and the heat load is approximately 100,000 – 115,000 Btu/hr for our design day. The contractor will do their own heat load calculation to confirm/update this value, but because of the somewhat unique design I’ll believe the measured numbers more than I’ll believe a building model.



My main question: are there higher output furnaces that are still >90% efficiency? I would be more comfortable in the 130,000 – 150,000 Btu/hr output range, but the closest that I see are the following

(1) Carrier 58MCB 127,000 Btu/hr (one firing rate and 2 fan levels - the 2 fan speeds confuses me since the firing rate appears to be fixed. The next highest output Carrier is the 112,000 Btu/hr output 58MEC or 58MVC)

(2) York/Coleman TG9S130D20MP11

(3) Lennox G51/G61/G71

(4) International Comfort Products H9MPV

(5) Allied Air Enterprises GC95/G2D95



Any preferences among these options or any reliability issues to avoid? Are there any other options that would have higher output? I’ve looked at other commercial units for higher output, such as Modine. This would provide 150,000 Btu/hr, but at an efficiency of 80% with no apparent ability to modulate. This goes against requirements 1,2 & 3. We want to avoid twinning and using a boiler with hot water to air handlers since these options would be much more expensive.



What is a typical maximum firing rate compared to the building load? For example, in the above case I’m anywhere from 10% to 15% higher than the building heat load. I would like a little more margin.



The building is unused most of the week, and building occupancy times are well known in advance so there is always plenty of time let the building heat up to setpoint.



Other info: downflow system.



Any help in advance is appreciated. We’re in the Boston metro west area for anyone potentially interested in quoting!



Thank you AHRI for providing the best source of furnace data, including output heat and electricity consumption!

Comments

  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Things to think about.

    My Quaker meetinghouse (essentially a church) is an almost 200 year old building. Ceder shakes outside. Timber frame with the spaces between the timbers filled with bricks, mortared together. Inside walls plaster. It is heated with two forced air furnaces. These furnaces are barely enough to heat the building (125,000 BTU/hr input each). We have 7-day thermostats. One zone is more difficult to heat than the other (we know why). In the depth of winter, we must turn on the heat around 12:30AM in order to get the room up to 65F by 9AM. We allow the setback to drop to 42F when the building is unused. There are no water or drain pipes in that part of the building.



    The other part of the building does have water and drains, so we set back only to 60F there.
  • John Mills_5
    John Mills_5 Member Posts: 950
    Furnaces

    The 58MCB is a standard single stage furnace with conventional PSC motor.



    The York is a similar product though 95% efficient.



    I'm not as familiar with Lennox. I believe the G51 is single stage like the above. The G61 is 2 stage heat with or without variable speed blower. The G71 is modulating using a 2 stage thermostat. The G51 & G71 have been replaced by the new line, not sure if the G61s have yet or not.



    The ICP is like the G61V, 2 stage and variable speed. So is the Allied G2D95.



    The biggest 90+ models I've seen are around 130K input max. We used one of those Coleman models last fall to replace twin furnaces in a big house. It was larger than what our regular brands offered.



    As for a control, sounds like something internet based.
  • Mark_129
    Mark_129 Member Posts: 17
    Thanks, JDB and John

    John - useful info on the Lennox furnaces. I'm assuming that the G71 has been replaced by the ML193UH135P60D. That seems to be the leading product because it's the highest output with 2 stages of heating/fan operation.



    Can anyone confirm how well the furnace modulation works? Does it require a special thermostat? My basic assumption is that these furnaces will modulate down in heat output and fan speed when they're within a degree or 2 of setpoint. When well below the setpoint, they should fire at a higher speed and blow at a higher speed. If the thermostat transmits an actual temperature it's really easy to do from a control standpoint.



    FYI - it's very frustrating looking for basic technical details for these furnaces. For example, heat output settings, fan settings, and how modulation actually works. All of the materials seem really dumbed down. I realize that us engineers are
  • Mark_129
    Mark_129 Member Posts: 17
    Response cut off...

    Just finishing the minor rant by saying that us engineers will be your biggest advocates if we understand basic details about how the system works. The marketing/technical materials for these products can be improved. Thanks again for any help you all can provide.
  • james k
    james k Member Posts: 12
    twin furnaces

    I installed one a few years ago by "twinning" two smaller furnaces. A two stage thermostat fired the first furnace and the second one kicked in if the temps. were low or if thermostat was raised more than a few degrees. Wanted to add a twinning control to alternate furnace firing, but wasn't in the budget. The benefit was that most of the year only the one furnace ran. Of course the btu output of both furnaces was the total heat loss, and one was only half. Most of the year except when it was really cold the one furnace handled the load just fine.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Technical details.

    For a great overview of the technical details of hot water heating systems, reading this book should be extremely helpful, although it does not cover forced air or steam.



    http://www.heatinghelp.com/products/Books/5/96/Modern-Hydronic-Heating-Third-Edition-br-by-John-Siegenthaler



    For the finer details, I read the installation manual for the boiler I have. I do not think the installing contractor read it all, although he got the near-boiler piping correct. I suspect the installation manuals for the forced hot air systems will be helpful.



    For my boiler, ordinary on-off thermostats are used. The outdoor reset is a tempreature sensitive resistor that goes into the controller for the boiler; the reset curve is set from this, and the controller adjusts the firing rate depending on the outdoor tempreature and the temperature of the supply water (and maybe ther return water too; that is not clear). I suppose something like that applies to forced hot air systems with outdoor reset.
  • James Day_2
    James Day_2 Member Posts: 191
    hot air furnaces

    I don't believe that hot air furnaces have outdoor reset. They fire at a fixed firing rate, unless you get a 2 stage furnace or a modulating furnace, which usually runs of some sort of time delay via the thermostat or the furnace itself.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I don't believe that hot air furnaces have outdoor reset.

    I looked some up on the Internet. There seem to be three types available. One firing rate, 2 firing rates, and modulating. One modulates all the way down to 15% in 1% steps. But none claim outdoor reset. They seem to use return air temperature, which I would call indoor reset. Many of them are condensing.



    Using indoor reset could help with houses whose heat load changed more from incoming solar radiation than from conduction through the walls. Outdoor reset would help in the other case, where the heat loss is due to the lower outside temperature. It seems to me, not based on experience, that outdoor reset allows the system to anticipate what is going to happen and be ready for change. This seems to be a good option for an in-slab hot water system. The indoor reset seems to be a good option for a heating system that responds more quickly, such as forced hot air.



    I am not sure what hapepns when an indoor reset system is running and the heat load is very low. If the furnace is firing at 15% and the blower is running, I wonder if the moving air would feel cool or cold, if it were at, say 80F.
  • Mark_129
    Mark_129 Member Posts: 17
    Boiler Comparison & Carrier Furnace

    The quotes are finally in and some questions:

    - Carrier Furnace: Looking at a 58MVC 3 stage furnace. Are there any concerns with the secondary HX? I've read about some issues with this in the past and hope they are resolved. Is the secondary HX SS? I assume not since it's coated. Do any furnaces still use SS on the secondary?

    - Modulation is important, as the church stays at about 50degrees most of the week. I'm looking for proportional control - as long as the setpoint is within a few degrees I want the unit to modulate for better efficiency and lower electric loads. Looking at the controls details for most furnaces - I don't get the impression that they do proportional control. I'm assuming the Carrier with Infiniti controls will do so. Any comments?

    - Boiler: Lochinvar vs Triangle Tube: The TT has a 10 year limited vs a 7 year limited for the Lochinvar (please correct me if I'm wrong as I'm reading the fine print this weekend). Lochinvar seems to have more control flexibility vs the TT. TT has lower loss HX. My reading on this site says both are in just about anyone's top 3 or 4. Any other factors to consider? The potential installer is equally comfortable with both but is higher volume on Lochinvar.



    Thanks for any input that you can share.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    scorched air for the church

    i would have a look at installing some form of hydronic heating system here, as the fuel costs will be less.

    with a furnace, you will be heating the whole building envelope interior; whereas with a hot water system, the radiators will heat the people.

    the existing furnaces can be retained for the air-conditioning.

    if certain parts of the building are used/not used on different schedules that is easier to arrange as well.

    new condensing appliances will need particular attention to the flue gas exhaust piping and combustion make-up air. a boiler has more flexibility in its location for this.

    why not also re-post your question in the section dealing with forced air here?

    you may well be able to achieve greater comfort, and efficiency for the same cost of installation.--nbc
  • Mark_129
    Mark_129 Member Posts: 17
    Thanks, but we're keeping the ducts!

    Oil to gas conversion doesn't make financial sense unless your heating equipment is at end of life, even with the difference in gas/oil prices today, and our equipment is end of life. Switching to hydronic would help somewhat at a system level since we would lose less heat to the ductwork in the foundation, but at >10k it is cost prohibitive. If you have any comments on my questions above please let us know. Thanks.
  • Steamhead
    Steamhead Member Posts: 16,796
    Pay now or pay later

    if you cheap out now you will pay a lot more for fuel later. But if that's what you want......
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • Mark_129
    Mark_129 Member Posts: 17
    Economics

    Our fuel bills after the conversion and insulation will be on the order of $2000/yr. We might save 5-10% going to the hydronic route, but probably with a loss of comfort because of the large volume of the buildings and the increased stratification that would occur. Since we're converting the parsonage rectory to a boiler/baseboard and the church I have direct cost comparisons. It's basically 4x as expensive on a Btu/hr basis to convert the boiler to gas vs converting the furnaces to gas (not including the baseboard and other copper piping that would need to be put in to convert the church to baseboard). This would put the payback in oh, 3-4 decade range. But again, if you have any comments on  my questions above would be glad to hear them.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    lifespan of furnaces

    i doubt that you will see 30 years of service with a new furnace such as you have had with the old. i assume that the furnaces will also blow a/c around as well, in the summer. you may only get a third of that. boilers have a greater lifespan, if they have been well installed. the radiant form of heating they provide negates the stratification issue.

    since you already have a boiler nearby, could it be piped over to do the church as well?--nbc
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Hydronics

    Did a church before last heating season. Panel Rads and condensing boiler. Boiler condenses all year long and the rads provide both radiant and convective heat. Very little stratification if any. No loss of comfort level. System running on constant circulation with indoor feedback from Vitotrol. Boiler is a Vitodens 200. The vitotrol allows for schedule programming based on the needs of the church so set point can be easily changed to meet them. System was designed to run 130 degree water temp at design of 0. I would say the savings over the forced hot air is greater then 10%. I'd put my money on 20-25%.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    time for some assistance in this choice!

    as you have admitted, "Just finishing the minor rant by saying that us engineers will be your biggest advocates if we understand basic details about how the system works. The marketing/technical materials for these products can be improved. Thanks again for any help you all can provide"

    would it not be best to find someone with the proper experience and knowledge to help you with this project?

    your church is at the crossroads of either keeping the same old system [albeit with new equipment], or making a change in favor of efficiency.

    so far, you have been a good shopper, but what about getting help in becoming a more informed chooser?--nbc
  • AFred
    AFred Member Posts: 81
    Tin

    Talk about gettin the hard sell.

    I think they'll be stickin with the tin.

    Don't get me wrong, boilers are the way to go, but I don't see the payoff. I don't forsee ANY condensing boiler lasting 30 years.

    -Andy
  • CMadatMe
    CMadatMe Member Posts: 3,086
    I Don't See

    Any boiler lasting more then 12-15 yrs as technology keeps changing. Technology will eventual make the basic non computer controlled boiler obsolete in the next 10yrs if not sooner.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
This discussion has been closed.