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Unusual Approach: old & new boiler co-exist with hydro-AH plus gas-furnace Hybrid for forced-air
Tom C._3
Member Posts: 9
After thinking through all suggestions and key factors for our house’s HVAC upgrade, we have come up with <strong> a quite unusual, I believe, approach:
instead of replacing our old gas boiler/furnace system with a totally new gas boiler plus hydro Air Handler system, we consider to keep the existing old gas boiler, currently for 1100 sf baseboard heating, and on top of it add a new gas boiler with a new Indirect DHW, and twin those 2 boilers together to form an old-new 2-boiler system. Similarly, we consider to keep one of the 2 existing gas furnaces, currently for 4400 sf forced-air heating, while replacing the other furnace with a new Hydro-Air AH, and twin them together to form a gas-furnace-plus-hydro-AH hybrid system.</strong>
The reasons for such an unusual hybrid are multiple, and there are several benefits:
(1) Currently we have hard time to calculate Heat Loss accurately, partly because the wall insulation is not added in yet, and duct is not sealed and insulated yet etc (we also want to do some duct adjustment and baseboard addition down the road). So it is not easy to choose the right size of total boiler capacity right now. If we run this old-new 2-boiler system for 2 years, after 2 years when we complete all the house/duct insulation and baseboard works, it will be much easier to replace the old boiler with a new one with a right sized capacity based on heat loss at that time. In addition, the following 2 years of equipment running data will further help us to calculate total boiler size need much more accurately at that time.
(2) Similarly, by running 1-new-hydro-AH plus 1-old-gas-furnace hybrid system for 2 years, we will have much accurate means to size the second hydro-AH when replacing the 2nd old gas furnace at that time.
After 2 years, the system will become 2-new-boiler plus 2-new-hydro-AH system without old furnace and old boiler.
(3) As two boilers twin together, usually only the new boiler works until it becomes cold enough when the older boiler turn on, so more overall efficiency, I believe.
(4) Similarly, in normal days only the new hydro-AH works until it becomes cold enough when the old gas furnace turns on. More overall efficiency.
(5) If one boiler breaks down, there is at least another boiler working.
(6) with this old-new 2-boiler combination, the boiler is less likely to short cycle, even during the first 2 years when the 2nd old gas furnace hasn’t replaced by a hydro AH yet. Essentially the 2-boiler system increases the boiler’s overall turndown ratio.
As this is quite unusual approach, we wonder how it may work (or not work) and if there is any potential issue. Your opinion is mostly appreciated. In particular:
<strong>(a) the existing gas boiler is old-style On-Off unit: BURNHAM P-203-W , 62k BTU/ 44k BTU. Will it cause any problem when it twins with a new modern gas boiler with much larger capacity?
(b) What kind of new gas boiler we should choose so it will work with the old boiler well?
(c) We want to install outdoor reset ORC on the new-boiler to improve its efficiency, Will this ORC cause problem to the old boiler?
(d) The existing gas furnace is old-style On-Off unit: LENNOX G2005/6E-150-2, 150k BTU/120k BTU (currently 2 units twin together), will it cause any problem when it twins with a new Hydro AH?
(e) This old-new 2-boiler system also needs to provide hot water for 2 existing zones of baseboard heating for 1100sq, and a new Indirect DHW tank around 40G-60G. Will the old gas boiler cause any problem to those zones of water?
(f) As there are a number of things need to control/adjust in this hybrid system, what kind of central control system should we install ? </strong>
For your information, one of the contractors did our house’s Heat Loss cal, giving out a total heat loss of 85k BTU, where window loss is about 25%, and infiltration loss is about 20%, but it seems they take little basement loss into their calculation. I also tried a calculation myself at <a href="http://www.builditsolar.com/">www.builditsolar.com</a> by using Boston area’s Heating Degree Days of 6200 and Design Temperature of 0 F. I got a total heat loss number of 150K BTU, where infiltration loss is about 45% and window/door loss is about 23%. Thanks
instead of replacing our old gas boiler/furnace system with a totally new gas boiler plus hydro Air Handler system, we consider to keep the existing old gas boiler, currently for 1100 sf baseboard heating, and on top of it add a new gas boiler with a new Indirect DHW, and twin those 2 boilers together to form an old-new 2-boiler system. Similarly, we consider to keep one of the 2 existing gas furnaces, currently for 4400 sf forced-air heating, while replacing the other furnace with a new Hydro-Air AH, and twin them together to form a gas-furnace-plus-hydro-AH hybrid system.</strong>
The reasons for such an unusual hybrid are multiple, and there are several benefits:
(1) Currently we have hard time to calculate Heat Loss accurately, partly because the wall insulation is not added in yet, and duct is not sealed and insulated yet etc (we also want to do some duct adjustment and baseboard addition down the road). So it is not easy to choose the right size of total boiler capacity right now. If we run this old-new 2-boiler system for 2 years, after 2 years when we complete all the house/duct insulation and baseboard works, it will be much easier to replace the old boiler with a new one with a right sized capacity based on heat loss at that time. In addition, the following 2 years of equipment running data will further help us to calculate total boiler size need much more accurately at that time.
(2) Similarly, by running 1-new-hydro-AH plus 1-old-gas-furnace hybrid system for 2 years, we will have much accurate means to size the second hydro-AH when replacing the 2nd old gas furnace at that time.
After 2 years, the system will become 2-new-boiler plus 2-new-hydro-AH system without old furnace and old boiler.
(3) As two boilers twin together, usually only the new boiler works until it becomes cold enough when the older boiler turn on, so more overall efficiency, I believe.
(4) Similarly, in normal days only the new hydro-AH works until it becomes cold enough when the old gas furnace turns on. More overall efficiency.
(5) If one boiler breaks down, there is at least another boiler working.
(6) with this old-new 2-boiler combination, the boiler is less likely to short cycle, even during the first 2 years when the 2nd old gas furnace hasn’t replaced by a hydro AH yet. Essentially the 2-boiler system increases the boiler’s overall turndown ratio.
As this is quite unusual approach, we wonder how it may work (or not work) and if there is any potential issue. Your opinion is mostly appreciated. In particular:
<strong>(a) the existing gas boiler is old-style On-Off unit: BURNHAM P-203-W , 62k BTU/ 44k BTU. Will it cause any problem when it twins with a new modern gas boiler with much larger capacity?
(b) What kind of new gas boiler we should choose so it will work with the old boiler well?
(c) We want to install outdoor reset ORC on the new-boiler to improve its efficiency, Will this ORC cause problem to the old boiler?
(d) The existing gas furnace is old-style On-Off unit: LENNOX G2005/6E-150-2, 150k BTU/120k BTU (currently 2 units twin together), will it cause any problem when it twins with a new Hydro AH?
(e) This old-new 2-boiler system also needs to provide hot water for 2 existing zones of baseboard heating for 1100sq, and a new Indirect DHW tank around 40G-60G. Will the old gas boiler cause any problem to those zones of water?
(f) As there are a number of things need to control/adjust in this hybrid system, what kind of central control system should we install ? </strong>
For your information, one of the contractors did our house’s Heat Loss cal, giving out a total heat loss of 85k BTU, where window loss is about 25%, and infiltration loss is about 20%, but it seems they take little basement loss into their calculation. I also tried a calculation myself at <a href="http://www.builditsolar.com/">www.builditsolar.com</a> by using Boston area’s Heating Degree Days of 6200 and Design Temperature of 0 F. I got a total heat loss number of 150K BTU, where infiltration loss is about 45% and window/door loss is about 23%. Thanks
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