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Question DetailsAsked on 5/6/2014

is high efficiency gas boiler worth the investment? what's the maintenance cost?

I am thinking to convert oil fired boiler and electric hot water tank into gas-fired boiler and gas hot water tank? some plumbers told me to go with high efficiency gas boiler and other plumbers told me to go with standard gas boiler. My existing heating system is gravity hot water radiator, which system makes sense for my situation? If you have converted into high efficiency gas boiler, do you like it? how long has it been? and any issues? thanks

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Couple of things to consider - you can also check out prior questions and answers on converting oil or electric to gas in the Home > HVAC in Browse Projects, at lower left :

1) figure your estimated fuel cost savings based on rated consumption of new furnace versus what you pay now for oil and estimated electric hot water heater usage - see Department of Energy or EnergyStar website for typical estimates - also on yellow tag on side of water heater, adjust their estimated cost proportionally based on your electricity cost. Commonly local utilities have website info to help you with this, as well as typical operating cost info for different size and efficiency units.

2) cost of removing oil tank and piping, and possible spilled oil contamination cleanup cost

3) Whether your larger gravity heating system can be used with a modern hydronic boiler, which depend on pump to circulate the fluid

4) A high-efficiency condensing boiler does not use a stack flue - it has to go by a short PVC pipe out the side of the house. The cost of doing this by itself is not significant, but it might mean the flue used by it will now be too large by code for a gas water heater, so it might have to be resleeved - and in difficult cases (in old chimney say) can run $500 or more additional cost - or you have to spend $500 or more additional to go with high-efficiency water heater too, with direct venting to outside wall.

5) A high-efficiency furnace certainly costs thousands more, so you have to weigh that up-front cost (with the reducting cost) and including the lost of use of that money (or interest payments, if being done with home improvement loan) against the differential fuel cost savings - how much you will save in energy over the years. EnergyStar and Greenstar and other sites have calculators where you can put your local energy cost, info on selected unit, etc in and estimate the number of years it will take to get a positive return on investment - typically comes out 10 or more years.

6) Clearly there are a range of results, but generally speaking, for the average american high-efficiency units do NOT pay off unless in an extreme weather or fuel cost area. High efficiency A/C units can pay off in desert or deep south areas for instance, and hgiher efficiency furnaces in northern tier and higher elevation mountain areas, for instance, as well as in very expensive fuel areas like California. This is partly because the average person only spends 5-10 years in a home and higher efficiency units commonly do not yield a significant recoupment of cost at resale time, and also because the payoff period is commonly on the order of 15-30 years, and in many cases the payoff period is longer than the estimated life of the furnace to it never becomes positive unless your furnace lasts signficantly longer than average. A very significant factor most people space on is the more efficient units cost way more up front, so you are out that money right now orhave to borrow it - and that is a cost it is hard to recoup with 20-30 years of fuel savings, unless you assume fuel costs are going to go way up in the future years.

7) The higher efficiency units do have more fans, sensors, switches, relays, and electronics hence their failure rate is higher both because the more items there are the more there are to fail and also because the computers/circuit board are far more prone to failure than the older electrical/relay controls, and they cost a LOT more to fix typically when they do fail. No longer the $125-250 typical repair cost for a simple gas-fired boiler repair - expect more on the order of $300-500 typical visit cost, and from what I have seen, every couple of years as opposed to every decade for a simple old- non-electronic unit. Also, your repair technician risk goes way up with high efficiency electronic controleld furnaces, heat pumps, etc - the risk you will pay for needless repairs, or for things the technician broke or fried, due to lack of knowledge or training. This is a significant risk, both with HVAC systems and with automobiles, where you unknowlingly pay hundreds or thousands more than needed because the technician cannot figure out the problem or breaks a board taking it out, so you end up paying for a new $400+ board that was not really the source of the problem.

8) My recommendation, unless in very high gas cost area (which brings into question the economics of the entire changeover scheme) or extreme winter climate, I recommend the ordinary 80-85% efficiency units in a reliable brand name for half or less the cost of a high-efficiency unit. Ditto to the water heater, which if you do the furnace change I would consider plumbing gas supply to while doing the furnace, but leave the water heater replacement till it needs changing out, unless it is already over say 10-12 years old.

9) Of course, lcoal utility or state, or any new federal energy rebate or credit program, has to be factored into your decision - there are places, like in my area, that have had programs that paid for almost the total cost of high-efficiency unit conversions - which can totally change your decision.

10) As for the issue of a single combined hot water boiler/holding tank to serve both heating and domestic hot water, a lot of people have them and like them. Personally, I do not due to the added cost, plus I like the option of having the water heater available to run hot water through the hydronic heating system in case of a furnace failure in winter and you cannot fix it immediately.

11) Hope this helped some - there are also a few articles in the AL articles section on this choice, and of course lots and lots on the web.

Answered 2 years ago by LCD


I have spent months doing a LOT of research into this same scenario for us. We got our place 5 years ago but were only able to install gas this year, due to a moratorium on cutting the street to access the gas line on the other side.

Let me address several points.

1. You need to do a good heat loss analysis, to know how to properly size a gas boiler.

- A good app can be gotten from slantfin.

- The "old way" (very conservative) was square footage * .35. Whatever this gives you is oversized but at least gives you a quick reference. .25 is probably a much better multiplier. But - do the job right and do a full heat loss analysis.

- An oversized boiler is not a good thing. It'll cycle on/off too often, which shortens its life. And it'll use too much energy = $ spent.

- So please take careful measurements and don't be too surprised when you really only need a 50K or 65K or 75K boiler. Most people have boilers that are way too large - especially if they are oil boilers. They really don't make "small oil boilers". They start pretty close to 100K btu.

2. While you are at it ... figure out what can be done to help your house envelope. We did a bunch of air sealing and insulation (our latest place is a victorian) and saved $700 this year in heating oil - and this was a brutal winter. You can get big gains from sealing every penetration to the attic (use fire rated foam), and adding more attic insulation.

3. Some oil boilers CAN be converted. But ... it may not make sense to do it, even if it's possible.

- Many oil boilers are oversized for their needs. A conversion gas burner can't be "sized right for the house" if it is "too small for the boiler". Here's an example. Let's say you have a 165K oil boiler. Let's say the real heat loss for your house is 70K. So, a 80K boiler is probably about perfect.

BUT ... you CANT install an 80K burner in a boiler designed for 165K. Sure it'll fit BUT it's just not big enough for the design and you can have nasty issues in your flue and boiler chamber, even leaking gaskets, etc.

You CAN downsize say 10-20%. But .. you can probably get an 85% efficiency brand new gas boiler (i.e. Burnham ESC series) for close to the cost of a conversion burner.

IF you are REALLY lucky, your oil boiler is only a 3-4 section. If YES and it is relatively new, then MAYBE it makes sense for a conversion boiler. BUT ... your chimney would also need to be stainless lined (state code dependent) to allow this.

Anyway ... what I'm really saying is PROBABLY a conversion gas burner for your oil boiler isn't going to make sense. But if your oil boiler is SMALL then MAYBE ...

4. High Efficiency Gas Boiler vs "Standard" Gas boiler. You CAN buy an 85% efficiency Gas Boiler for only about $2100-2400. These also qualify for the $300 rebate (ends very soon). Several companies make these. Burnham is one of these companies - their ESC model qualifies. Think of this as good old cast iron with smarter electronics upgrades. It'll probably last 25-30 years. You can even add an outdoor reset to these.

Ok so to me this is really a numbers game. A "high efficiency" wall mounted boiler will cost probably at least $1000 more than an 85% efficiency model. They use aluminum or stainless steel for their chamber. It'll also probably only last about 15 years. It will also be more touchy and require more skilled annual maintenance than a good solid cast iron boiler.

So .. any savings between 85% and say 95% with natural gas will be eaten up by annual maintenance and the high efficiency unit will probably be replaced in probably 15 years.

So ... it's just not worth it - in my opinion - to do a high efficiency modulating condensing boiler.

Note: you MAY need to add one or more pumps to your system. You mentioned it is a gravity system, which usually means it is a great big system with big pipes that originally was steam or water - and perhaps coal-fired, back in the day.

5. No matter what you decide. It is IMPORTANT that you have a good installer, and that he designs and builds "boiler protection" into the design - to ensure that cold return water is pre-heated to about 130 (or X degrees) before it gets back to the boiler (different boilers have different tolerances on return water temp).

Think of this as a loop near the boiler with a thermostatic valve to ensure that return water is mixed (when needed) with hot water to keep it at health levels.

If cold water is allowed to come into the boiler, then you'll have a boiler that develops leaky seals in probably 2-3 years. And you'll think that the boiler is at fault instead of your installation. This is a more common problem today - since boilers are more often right-sized than they used to be (or not massively oversized). Most old boilers were so powerful that they got by without this type of protection. Example - my current oil boiler is sized for 240,000 BTU. How crazy is that? I downsized the nozzle but even still it is 165,000 BTU!

6. You may wish to consider whether you wish to have a separate gas water heater OR to have an indirect water heater that is powered by the gas boiler.

Burnham makes nice indirect units that are stone lined. Think of this as one additional circuit that takes PRIORITY when it needs it. You do NOT need to change your boiler sizing if you add an indrect water heater.

A separate gas water heater needs it's own 4" vent. But ... if it goes, you just replace it. If your boiler has an issue you will still have hot water.

An indirect 40 or 50 will give you basically unlimited hot water, for most people's needs (unless you have a big hot tub? and someone else wants to take a shower with jets). It'll also last probably 25-30 years. But ... you will need to run your gas boiler all year round. It also won't need to be vented


OK - well I could go on and on but this should give you something to chew on. If I were in your shoes (and it so happens that I am), I'd just get a solid cast iron 85% gas boiler like the Burnham ESC3 or ESC4, get a GOOD plumber who knows how to design boiler protection, and consider if it's better to have a separate 40/50 water heater or an indirect water heater.

I hope this helps and wish you the best.

Answered 2 years ago by Jefferson


The answers provided by Jefferson and LCD are extremely well thought out and should be read throughly.

I would add my 2 cents that, 1.: Do NOT convert an old oil boiler to gas and 2.: You cannot convert an electric water heater to gas.

Depending on your square footage, you can purchase as small gas boiler for as little as $500 or spend as much as $1,500 to $2,000 for a high-efficiency model. You will ABSOLUTELY need to have your chimney professionally cleaned and/or lined with a stainless steel liner. High-efficiency models may use a PVC method of venting which will preclude the need for lining. You need to weigh the costs of each. DEFINITELY research HVAC contractors in your area and get at least 3 estimates.

It will most definitely be to your advantage to scrap that old oil boiler. Gas, whether propane or natural gas will be MUCH cheaper to run and maintain!! Oil-fired furnaces and boilers MUST be cleaned EVERY year to achieve efficiency and as soon as they are cleaned the effeciency starts to go down again. Gas is so clean burning, this is not necessary and they maintain their efficiency year after year unless affected by some outside force. e.g. rodent making a nest in heat exchanger or spider web in burner venturi.

To sum up....REPLACE don't convert. Gas will be cheaper to burn than oil.

Source: 32+ years in HVAC industry

Answered 2 years ago by LP1Guy


A high efficiency gas boiler (>90%) must have the flue gas condense in order to achieve that efficiency. The flue gas will only condense when the return water temperature is 120 degrees or less. An ordinary system has return water temperature much higher than this, because this is the temperature for which your radiators are sized (typically 180 supply, 160 return). If you have a outdoor air temperature reset it will condense when the supply temperature goes down in response to warmer weather.

Additionally, because the acidic flue gas is condensing, you cannot re-use your flue for a high efficiency model. That needs to be replaced with very expensive stainless steel or CPVC.

The bottom line is unless you have your system redesigned by a competent professional, you would be wasting your money. I recommend a standard efficiency cast iron or water-tube boiler.

Source: Commercial HVAC engineer

Answered 2 years ago by watermain


I basically agree with watermain. The most you can get with the condensing boiler during the cold winter months is about 85% which is slightly higher than the typical 82% one can expect from a cast iron boiler. So the savings will be minimal.

However during the mild months like fall and spring where you can drop the boiler supply temperature so that the return temperature is below 140F then the boiler can be in condensing mode and will return a higher efficiency. If you can install a reset controller that will adjust boiler water supply temperature based on outside temperature that would help in maximizing the boiler's efficiency. Whether the savings is enough to justify the added investment is something worth considering and I personally don't have that answer.

Answered 7 months ago by JoeG

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