old gravity system

Jerry_15

Well I guess I really bit into it now. For more than a few years I have replaced boilers on existing systems by giving my supplier (at their request)a radiator count, complete with # of columns and sections, heighth depth, and type of system, at their request, to size the new boiler. The thinking, I believe, is that you have to be able to keep up with the maximum demand of the system in place, (plus a skosh), and reach op temperature or pressure within a certain time frame to prevent either short-cycling or constant firing. You guys in the cold country must have to up it a notch, but there's only so much pate you can get out of a goose, no matter how much you feed it. It's always worked so far, and I don't recall anyone wanting their money back. If I were designing a new system for a new building and had the luxury of choosing and placing the rads of course I'd calc it out, but on retro you work with what you got. I can hardly wait for all the dope slaps.

Stan_5

system balancing

I recently looked at a friends heating system because she said her natural gas bills were killing her, anywhere from $350 to $500 a month here in the Northeast. Her heating system is a hot water gravity system converted to a pumped system. The existing boiler was installed about 25 years ago. She has cast iron radiators with "Unique" radiator valves on them. There are (2) 2" supplies, one runs down one side of the basement and the other runs down the other side and each has their own return. The supplies are manifolded together at the boiler and so are the returns. I calculated that the radiators would supply approximately 77,000 btu's of heat at an average system water temperature of 170 degrees. I don't think the house is insulated so I did a heat loss calculation with no insulation and came up with 85,483 btu's, with minimal insulation it was 64,459 btu's. The existing boiler has an input of 200,000 btu's and a net output of 139,100 btu's. I figure the boiler definitely needs changing and we need to somehow verify if the house is insulated or not. I'd like to change the direct return piping to reverse return. I saw a piping schematic that suggests running a supply pipe and connecting to the ends of the existing supplies. The last pipe size is 1". It seems odd to feed a line starting with 1" and ending with 2". The homeowner keeps her thermostat at 61 degrees to try and keep her gas bills down. Any suggestions on how to resolve this dilema would be greatly appreciated.

Steamhead (in transit)

Size the new boiler to the heat loss

then check your circulator sizing. The Unique valves depend on gentle flow to allow the water to circulate thru the rads. If you overpump the system, the water will short-circuit in the bottom of the radiators. With proper flow you shouldn't need to repipe the mains.

Get the total EDR of the rads and the make/model of the existing circ, then go here for more info on circ sizing:

http://www.heatinghelp.com/newsletter.cfm?Id=125



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Brad White_54

I second Steamhead's advice

Your friend's situation is not unlike my Susan's: Twice-plus oversized boilers, old gravity heating system, huge gas bills despite keeping a cold house. Boiler replacement (to the heat loss as Steamhead says) is essential.

No need to re-pipe unless you want or need the head room. If you can at all leave the piping, I would suggest you do so, but flush and treat the piping first to remove any corrosion materials.

Reverse return is nice but no detectable benefit for all that effort. If you keep the piping, the head loss beyond the boiler is very, very low.

The old gravity pipes are large for the simple reason that the motive force, gravity is very gentle. Yes, the size differences between forced HW and gravity HW seem extreme, but there is a reason.

Be very frugal with pump energy. Multi-speed circultors can be helpful here.

With boiler replacement is the opportunity to do deep reset, use a modulating condensing boiler and so many other things.

clammy

honeywell unique

I had the pleasure of re doing a converted gravity with honey well unique valves Last fall and was told the HO 's where happy and there gas bills looked alot better then last season .I had assisted a buddy on the install he had speced a munchkin i believe a 140 but i had concenced him to put in a smaller 80 after i ran a quick heat lose and edr of the rads and the numbers where good .I had set the munchkin contrlloer for a max supply temp of 160 and we did install the vision one out door reset and installed a superstore indirect tank.I would certainly always consider a condensing modulating boiler for gravity conversion i believe they work well together i don't think i would start adding or changing much of the systems gravity piping like adding different convectors (toespace heaters ,removing rads and replecing with baseboard)you may end up with flow promblems .Here's a pics of the system i discribed .Good luck and peace clammy

Plumb Bob

Your friend does not need a new boiler just because the current one is oversized. Sure, she will save a little, but not enough to pay for replacement.

Her gas bills are high for the same reason everyone else's is high...high natural gas prices. Changing the boiler won't change that.

Jerry_15

I guess I'm a little bit simplistic in figuring this stuff out but I love sizing boilers for existing rad systems because I don't have to worry about heat loss calcs, etc., etc., etc. They will take what they will take, and putting a million btu boiler on it won't help. Size the rads, adjust for piping and pick-up loss, and be done with it. Workin too hard my friend, workin too hard. Insulation will help their bills, but has nothing to do with the boiler sizing. And yes, after 25 years they need a new boiler.

Jerry_15

PS don't worry about the change in pipe size as Brad noted.
I just ran a gravity system with 1" pex off a prestige with the 3 speed grundfos that came with it. Killer. Also DHW on the dedicated loop to a exchange tank. Short and sweet.

Jerry_15

Last PS I promise! Flow direction is critical with the Euro valves or they will chatter. Haven't used the Uniques, but be careful.

John Ketterman

> Size the

> rads, adjust for piping and pick-up loss, and be

> done with it. Workin too hard my friend, workin

> too hard. Insulation will help their bills, but

> has nothing to do with the boiler sizing.


As everyone else on this board (starting with Dan H) will tell you, this is complete nonsense. You do not size a hot water boiler by sizing the rads.

It's sad that so many people do this and then they call themselves professionals. They are screwing their customers.

Steamhead (in transit)

Uhhh, Jiminy

that oversized boiler is likely short-cycling. This allows the boiler and system to cool down too much between firings, which is quite wasteful. A properly-sized boiler will not generate this loss.

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John Ketterman

But it cools down within the heating envelope, so the heat is not entirely wasted.

Short-cycling is indeed less efficient, but it won't pay for the replacement of a working boiler.

Jerry_15

So you sized the boiler based on the rad demand? Help! I'm gettin murdered down below.

Brad White_54

Ayuh...

Sorry, Jerry. We were drawing straws to see to who would do an intervention...

JPG volunteered before the straws were drawn

Steam you got right- size the boiler by the radiators.

Hot water- size the boiler by the building calculated heat loss.


Against that, it is useful know what your (presumably existing) radiators are capable of on a design day at say 170 degree average water temperature (conventional HW sizing). The presumption here is that the house likely has been improved, insulated, new windows, etc. and you can now capture all of that with a "closer to reality" boiler size.

Once you know the "actual radiation versus heat loss" figures you can calculate what the real design-day HW temperature you will need. This temperature becomes your new high design temperature and you can set it back from there.

Chances are the radiation exceeds the heat loss. No harm in having too much radiation within reason, it allows lower HW operating temperatures and that means savings.

Dan wrote an essay (may be in one of his books) about a heating contractor measuring the 200 some odd feet of BBD in Dan's house, compared to the actual heat loss in the 40's MBH...

Dan calls sizing a HW boiler by the radiation the "Long Island Heat Loss".

Don't beat yourself up! Really. We all learn here regardless of how many years we are in this. Lord knows I still do, and stub my toes plenty.

Jerry_15

Thanks Brad - and you're right, I was referring to steam mostly. Most of the rads I see out here are copper fin tube, which are a no brainer, but on the cast iron, I don't even bother to run the numbers myself anymore. I measure rads, and give that along with basic info on the structure to my supplier, who figgers it out in 45 seconds for free. The last gravity system I did only needed 50,000 btu, but the Prestige starts at 110m and is fully modulating, and has the little 3 speeder built in. How much figgerin do I need to do? Also in the SF Bay area the weather is a bit more gentle. I don't change my oil anymore, or remove asbestos, or run around with a half mile of pipe that I can get delivered for free. I let a pro do it. Gives me more time with my family. I also don't put my sticker on top of the other guys, if you catch my drift.