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looking for some pointers on retrofitting an older house and sizing a boiler

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by semlin, Nov 23, 2009.

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  1. semlin

    semlin New Member

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    hi, i have been reading this site for the past couple of days. this is a great forum. this is sort of an introduction/please point me the right way thread. i hope it is in the right place. thanks for any advice...

    i have an old house with the original radiator system. i would like to add a wood boiler in parallel to my natural gas boiler, and i guess from reading i should have a storage tank system. i have found a lot here about various add ons like control valves and storage tanks and extra pumps and controllers but i would like to find more on how to do a cost benefit analysis, or how you size stuff like the boiler.

    i would like to read about any similar projects here. the heated portion of my house is 4200 sf on two stories with high ceilings plus a 2000 sf full height unfinished below grade basement that is incidentally heated to about 5 c less than the house by the main 3" radiator supply and return pipes that circulate between the floor joists (there are two zones that split off near the boiler and each services both floors on about half the house). apart from the windows, the house seems to have good insulation for its age (85) although i have no idea how to rate it since it is powdered gypsum. it has a 200,000btu slant/fin galaxy cast iron boiler that is rated about 80% efficiency but is 20 years old. the boiler heats the house pretty quickly and the radiators work like a hot darn, but i have been told by a local boiler serviceman that the heat exchangers are probably scaled up (we have pretty hard water here) and that i should be replacing it. i am thinking of turning it into a backup.

    the house has a lot of large original single pained windows that are in good condition and impossible to replace. i looked at adding wooden storm windows and i think i could do a wood boiler cheaper, and chopping wood sounds more appetizing than fitting and packing away that many big storm windows every year. so here i am.

    last winter consumption was about 263.5 gj of gas for the radiators, plus some electricity (hot water and the kitchen area about 250 sf has baseboards as it originally was heated with a wood stove). our thermostat tracked usage and the furnace was usually on 8-10 hours a day mid-nov-early march so by my math we were using at least the rated capacity of the boiler, but i have no idea at what efficiency. the boiler heats to 160 and returns around 135-140 so i wonder if i am losing something there.

    i live in a zone where we get about 6500 heating (f) degree days a year. pretty mild winters down around -5 c usually with a couple of sustained cold snaps at -20 c or so.

    lots of room in the basement, and the chimney for the boiler was rebuilt 20 years ago and one side has never been used.

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  2. Nofossil

    Nofossil Moderator Emeritus

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    Welcome to the forum. I think you're in the right place. We have several people including one of our moderators who heat older houses using cast iron radiators. Payback is a bit subjective because it depends so much on the cost of firewood and whether or not you consider your time and effort as part of the equation.

    In my case, I was spending a bit of $2000 a year on fuel oil, and I've replaced virtually all of that with wood that I cut myself. At the simplest level, I'm saving about $2000 per year. However it would be fair to consider wear and rear on my tractor and chainsaws, the value of the time that I spend cutting, splitting, and stacking wood. On the other hand, if I weren't doing that I'd probably just be annoying people on forums.....
  3. benjamin

    benjamin Minister of Fire

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    I can't help you with the boiler, but I will say that first and foremost, you want to seal up all of the air leaks, especially in the attic and at the sill plate. The chimney stack effect is what causes most of the heat loss in an old two story house like this, the warm air inside creates a pressure difference causing the cold air to suck in to the basement and first floor, flow through the house and leave through the attic.

    I have an old 3,000' house in WI with a similar HDD climate, though it hit -30F last winter, similarly sized radiators. I've been told that the gas bills had topped $1,000 a month with no insulation and who knows what for a boiler. With cellulose insulation, air sealing and an 85% forced draft? boiler, the worst bill was $300 last winter, heat and hot water. If I told you what the boiler temp is I'd be indicted for first degree reckless boiler murder. I'd like to put in a modulating condensing boiler eventually.

    Most on this site will disagree, but I'd tend to use the house as the mass and not go to the trouble and expense of adding tanks.

    With that many square feet the house should take quite a while to heat up and cool down, assuming of course that the structure is relatively compact, not sprawling, and the air leaks are kept to a minimum.
  4. Como

    Como Minister of Fire

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    I have a building that is somewhat older than that.

    My weather conditions are however more extreme.

    BC I think is pretty wet, and as you have said not that cold.

    Best buck for money:

    Insulation/Infiltration - exactly how far you need to go will depend on your current levels.

    You will be losing a lot of heat out of those windows, plus draft etc.

    I would look to put in a new NG Boiler, it is unusual for wood to be worth the hassle if you have access to NG.

    But grants and free wood could well change the equation.
  5. semlin

    semlin New Member

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    thanks. our worst month ever was over $1,000 and $700 is normal and our gas here is cheap. i could definitely do more to seal up the sill plate, but we have a cover we use to seal the fireplace chimney when not in use and the roof design works pretty well (it is a flat roof with an insulated dead space between the ceiling and roof and produces almost zero snow melt). the big problem is that i have over 500 sf of single pane windows i can't remedy except at huge expense, so that's why i am looking hard at cheaper fuel!

    in mid winter with the furnace off we will drop from 19 c to 15 c on the main floor in about 3-4 hours.
  6. benjamin

    benjamin Minister of Fire

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    I have 50 some windows, most of them 28-38" x 64" double hungs. I've replaced most, but only because they needed a lot of work and it was just easier to slap in a replacement. The bigger gain was filling up the hollow spaces where the weights were at the same time that I replaced them. That was a huge area with just two layers of trim boards between the inside and outside.

    I'd bet that the heat loss is mostly going around the windows and not through the single pane of glass, especially with your relatively mild climate. I removed three chimney flues down to the second story ceiling level and sealed them up there.

    I agree that wood is much more attractive if your other options are more expensive than natural gas. maybe a modulating condensing boiler would work better financially. For it to function as a condensing boiler with radiators the returning water temp has to be low enough, I believe you can achieve that efficiency fairly easily, at least compared to installing a wood boiler, storage, backup... A lot depends on how much you like to cut wood etc.

    For what it's worth my 90,00 btu boiler ran about five minutes, every half hour on the morning when it dropped to -29f early this year. The boiler OUTPUT temp was low enough to work as a condensing boiler.

    If you could reduce your heat load and go with a modern gas boiler, the payoff for a wood boiler would start to look like solar, ie somewhere in a land far far away.

    On the other hand, it's more straightforward to just replace the boiler and cut the wood, especially if that's what suits your fancy.
  7. in hot water

    in hot water New Member

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    a heatload calc and a blower door test is the best way to size new equipment. The blower door will nail down the infiltration number to use in the heatload calc, and I suspect it might be fairly high.

    If funds allow blower door it to locate and show you the leak paths. Insulate and tighten up to the highest $$ amount you can budget. then re-test with the blower door and size the equipment to the updated loads.

    Anything else is just a guess.

    But with todays modern modulating boilers EXACT sizing is not a critical. the boiler will adjust it's output based on the ever changing load, and do it while maintaining highest efficiencies. It does need to be sized to the largest expected load, which is where the load calc helps.

    hr
  8. heaterman

    heaterman Minister of Fire

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    Hmmmmm .............
    3" mains make me think you either had, or maybe still have a gravity circulation system. Is there a circulator in the piping by the boiler?
    If it is still gravity circulation and you want to keep it that way, you will want to undertake any piping changes or additions only after some careful consideration.
  9. semlin

    semlin New Member

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    it has an electric pump circulation (armstrong s-25) and is closed. the piping is 85 years old and it had a huge cast iron wood boiler until 20 years ago. i don't know if it was originally gravity or open. i don't even know how gravity works. how might i tell?
  10. heaterman

    heaterman Minister of Fire

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    A "gravity" or more accurately put, a thermal convection system, uses the weight difference between hot and cold water to induce flow without the air of a circulator. All gravity systems by their very nature demand piping with virtually no head or flow resistance. Hence the large piping in your system. I would bet that if you could look inside your walls you would find that you have a main riser that goes from the basement all the way to the top of the house. I would also bet that given the size of the main your system used to be gravity.
  11. Gooserider

    Gooserider Mod Emeritus

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    Correct me if I'm wrong, but the probable former gravity system also sounds like somewhat of a potential energy saver - they aren't cost effective by todays technology as that fat pipe is expensive, but as I recall what I've read on gravity setups, because they had fairly low flow rates, they tended to have lots of emitter surface by todays standards, and between not needing a lot of flow, and having plenty of emitters, they shouldn't need a very big circulator, and may well be able to run down to nice low water temps...

    IOW, it would probably be smart to keep the existing distribution system unless there is a really good reason to change it... Just plug the new boiler into it...

    Gooserider
  12. heaterman

    heaterman Minister of Fire

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    Just plug the new boiler into it…

    That's the catch in many cases. The old system and piping was designed for a boiler that took some time to heat up. Lot's of mass in those pipes, lot's of mass in the boiler, lot's of mass in the emitters usually also. When they were connected to a beast such as the old wood boiler you had a tremendous flywheel effect that could be a little hard to manage in shoulder seasons. When all that tonnage got heated up it took a while to cool down also. Once the weather settled in though, these old system worked just like a modern system with outdoor reset. They provided a gentle and constant flow of heat that matched the heat loss of the home exquisitely in a lot of cases. The system water temp varied along with the load to increasing and decreasing "on cycle" times as the weather changed.
    The issue with a modern boiler is often installation related, the main faults being over pumping and incorrect control strategy. In a lot of cases a person has to resort to a mix valve to protect a modern cast iron boiler from condensing due to the volume of cool water and iron that has to be heated up. It can be done and will work very well but the installer has to have an understanding of what he's working with. A condensing/modulating boiler is a near perfect match for an old gravity system provided the piping is designed for separate needs of the boiler and the system.

    As to integrating a wood boiler into a system like that, I would probably approach it with the intent of mimicking the operating characteristics of the old wood boiler. In other words, storage with a fairly accurate firing rate, or a large storage that would gravity flow by itself when opened to the system via a large zone valve. I'd love to do one that way sometime but there are very very few left around here.
  13. Gooserider

    Gooserider Mod Emeritus

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    Yeah, I was kind of suspecting I wasn't saying the right thing in terms of "just plugging it in"... A lot of the stuff you say makes sense, and that's why I'm thinking it would make a lot of sense not to mess with the existing system all that much, which was my main point...

    My approach to system design, which probably comes in large part from my electronics / computer background, is to look at the setup as a collection of functional blocks - each block having it's own set of requirements for what goes in and out of it, but being basically a "black box" to all the other blocks. The tough part is figuring out how to connect the blocks in such a way that each one gives and gets what it needs... I worry about what's inside each block only as I need to do the details on it, including making it match the other blocks in it's input and outputs... In this case, I thought that the "emitter block" sounded like it was in good shape and had a lot of desirable properties, so I wanted to not touch it, and just connect the "boiler / storage block" to it... Obviously to me, but that I didn't say, is that one would have to do some "impedance matching" in the boiler block to make it play well with the emitter block, which it sounds like your ideas would do quite well...

    Gooserider
  14. Tennman

    Tennman Minister of Fire

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    You've pretty much described the situation I was in with our 150 yr old house. Difference is I'm in Tennessee. You're welcome to PM me. I purchased a 200K boiler and am just heating the downstairs. Much of my old home does not have storm windows because I love to look out thru those old wrinkly glass panes. Your location is far more severe than mine but I'll be glad to pass on what I learned mostly at this site.
  15. semlin

    semlin New Member

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    somehow i missed these posts. thanks for this information. the circulation pump on it now is 1/12 hp so what you say makes sense.
  16. semlin

    semlin New Member

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    thanks for the advice. anywhere i can read up on gravity flow systems?

    is there any chance it still has a gravity flow right now? i just ordered an intelicon hw+ in the hopes that it would continue circulating after the boiler stops since i figured i was wasting a lot of heat if the system was only circulating when the pump was on. maybe i am mistaken.
  17. Gooserider

    Gooserider Mod Emeritus

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    This was some good advice... Note that I mentioned in a later thread that "just plug it in" was probably not the best way to phrase the concept - there is a certain amount of "impedance matching" needed when connecting to an existing system in order to make sure that what the new boiler is putting out matches what the rest of the system expects for an input....
    You might find a bit here if you use the search, and if that doesn't work, I'd do a good bit of looking on HeatingHelp.com.
    From your description (all we have to go on...) it sounded like it was a gravity system at one point, but by definition, if you have a circulator, even a small one, it isn't a gravity system any longer... What you need to figure out is what changed? It is entirely possible that if the original boiler has been replaced, that the distribution system is untouched, and is still essentially a gravity system, but that the circ you mentioned was the "impedance matching" for the replacement boiler...

    Gooserider
  18. semlin

    semlin New Member

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    this is a great article on old gravity systems. i have a converted conventional (not an overhead) former dual feed system that follows almost everything mentioned here. i can't find a main riser and original expansion tank, but there are blocked risers on the main feeds right near the boiler and there is old strapping right over the boiler in the joists that might have held a tank there. also, i have no flow control valve above the circulation pump so it appears i would still have residual gravity flow when the circulator is off, which might mean the intellicon was a waste of money.

    http://www.oldhouseweb.com/how-to-advice/gravity-hot-water-heating.shtml
  19. heaterman

    heaterman Minister of Fire

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    Written by a guy who is very very adept at describing the nuances of these old systems. Take everything there to the bank. It's a very enlightening experience to sit down and yak with Dan or Rich Trethewey about stuff that's 80-100 years old. The "dead men" knew their systems in a much more technical way than most modern technicians do.
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