Hi everyone, I’m hoping there are some heating proffesionals out there who may be able to help me assess what my long term heating options are.
My home, here in the NE, is coming up on its first winter under my ownership. It is 1860’s vintage and is heated by gas fired, furnace driven, forced hot air. I have gas fired hot water as well. Both the furnace and hot water heater are of unkown age, but judging by their condition and design, I estimate they are greater than 5 yrs and less than 10 yrs.
I know that at some point in the future I’ll need to replace one or the other or both, and would like to put in a system that is clean, efficient on fuel, easy on mainenance and capable of a long happy life in the home.
My leanings from a maintenance and long life perspective are toward a boiler based system with a domestic hot water storage ‘zone’ and a heat exchanger ‘zone’ for the force hot air. My reasons being that the worst thing for a burner assembly is to be idle during the summer months in New England and that domestic hot water heaters are notorious for crapping out much sooner than expect. I’m also concerned about the possibility of mixing of exhaust gases into the air ducts if the firebox/manifold were to fail on a furnace.
What I don’t have familiarity with is forced hot air systems, so I can’t make an assessment on efficiency differences between a furnace driven system and a hydronic heat exchanger system. Can such comparisons be made and are their experiences out there that I can learn from? I also have no idea if there is an overwhelming cost difference that would mitigate my decision.
A preemptive thank you to all who take the time to read this.
HappyHomer
Replies
Like you, I started with an old, uninsulated home that was heated with forced air. I gutted our place because of the endemic infrastructure issues (new foundation, and up!). Thus, there was little resistance to go to RFH...
If you want to keep the ductwork, I second your suggestion of using a hydro-coil that is run off a small condensing boiler, which can also heat your hot water via an indirect hot water tank. You could also retrofit panel radiators in some areas to allow micro-zoning via TRVs.
As for getting hard and fast numbers, I cannot help you there. I will only cite others who have observed that the thermal efficiency of multi-stage furnaces decreases as their firing rate is decreased as well. Whereas boilers don't lose any efficiency... more likely they gain some as their firing rate is decreased due the much tighter HX passages found on a condensing boiler.
An intangible benefit is the ability to run your air system continuously while modulating the supply temperature of the water hitting the hydro-coil. Gone are the puffs of hot air and other tell-tales of forced air circulation as the boiler and the coil can modulate up and down seamlessly (modulating vs. staging the burner makes it possible).
One Wallie (Steve Ebels) did a school project where he replaced a giant furnace with two Vitolas running with TRV'd radiators. The savings in terms of electricity and fuel were ridiculous (40%+) You can read the whole thing here: http://www.heatinghelp.com/living_book_story.cfm?id=10
Lest you think this an outlier, I point to the 43.6% fuel reduction that Mike T. experienced when he switched from a 80% AFUE boiler to a Viessmann Vitola. He also uses panel rads and TRVs.
EDIT: The biggest thing to plan for is getting the place weathertight. Get the exterior walls insulated with an infiltration-proof insulator, remanufacture the original windows, retrofit Harvey Tru-Channel or eqv. storms, make the attic a part of the conditioned space (hot roof), etc. and you can save a lot of energy and expense later on.
Edited 9/30/2005 1:37 pm ET by Constantin
"My leanings from a maintenance and long life perspective are toward a boiler based system with a domestic hot water storage 'zone' and a heat exchanger 'zone' for the force hot air."
My experience with OLD systems verifies your "leanings". I have seen many hyrdronic/air handler based heating and cooling systems churning along 40 years after installation. I have never seen a forced air gas fired furnace in service after that amount of time.
"What I don't have familiarity with is forced hot air systems, so I can't make an assessment on efficiency differences between a furnace driven system and a hydronic heat exchanger system. Can such comparisons be made and are their experiences out there that I can learn from? I also have no idea if there is an overwhelming cost difference that would mitigate my decision."
First, the efficiencies of presently available equipment of both varieties are comparable. The best available gas furnaces have efficiencies in the 94% area. The best available boilers are also in the middle 90% range. The use of a hot water coil in the air handler will not change the efficiency appreciably, but the high efficiency boilers, in most cases, are more efficient at lower water temperatures, like what is common in the radiant floor heating applications, whereas a coil will typically use 180 degree supply water temperature, as will the indirect water heater. This, BTW, will be where you will see the greatest efficiency increase: hot water. The best residential gas-fired water heaters on the market have "recovery" efficiencies of 78 - 80%.
Relative costs of the equipment will of course vary from brand to brand and installed cost will depend on the contractor.
A top-of the line 2stage variable speed gas fired furnace - list price $3063, plus installation, gas water heater, 50 gallon, power vent (10 year warranty) $671, for comparison purposes say $5000 installed
A similar sized condesing boiler, hot water coil, indirect water heater and circ pumps, list prices approximate $ 6285, probably more along the lines of $9000 installed. Something to consider if you were to go this route, is that I would advise removal of the furnace anreplacing it with a new air handler that has the coil built-in. The cost would go up slightly, but not significantly. This is the option that I would recommend.
Keep in mind these are very rough estimates, but should give you some realtive, ball park idea. Get several quotes from reputable contractors in your area for real numbers.