Well, the old steam boiler gave up the ghost so time to revamp. I'd like to see if I'm on the right track here..... The house is in Concord, Mass, about 2,100 SF and part was built in 1892 and has cellulose in most of the walls, attics are R30. The other part was added 15 years ago and is conventional 2x4 studs w/ FG batt. The new part of the house (about 1/3 of total SF) has radiant heat in a basement slab, first floor slab and second floor in gypcrete. The old part of the house has (actually had) a one pipe steam system off an oil boiler. The radiant came off the tank side of an Amtrol indirect (used JUST for the radiant) that was heated with the steam boiler water - crazy, but it worked. The DHW is an electric water heater, separate from all the heating system. Occupancy is mostly my wife and I with kids and guests shuffling through. We plan to be here for the long haul and I am a devout greenie so I'm looking to do the right thing - within reason, and longish paybacks don't bother me. (We are completely shaded by huge trees that we don't own, so solar is not an option). Right now we have no heat and are using little electric heaters so the need is urgent. It appears I can now get gas service that was unavailable before (hence the oil) so I'm looking at revamping the whole system..... Here's what I'm contemplating - I'd welcome all comments.... The steam boiler, steam piping, radiators and Amtrol all go to the scrap yard for recycling. The radiant system (which works great) stays. I'd like to install a mod/con gas fired hydronic boiler (Buderus, Lochinvar, Triangle Tube - suggestions?) and replace the steam radiation with Euro-plate radiators (probably Runtals) piped with O2 barrier pex (some snaking is required). The boiler would also heat the radiant and DHW. After some research I'm thinking that the way to do all this is to modulate the boiler with an outdoor reset. The boiler water would be piped to the tank side of a reverse indirect water heater (like a Turbomax 23 or Ergomax) and the DHW would come off the coil of the Turbomax. The radiant would come off the tank side of the Turbomax and return to the Turbomax with temp controlled by a 2 way mixing valve controlled by outdoor reset. The plate radiators would be supplied directly off the boiler (which is modulated) and would return to the tank side of the Turbomax. The radiant panels would be only one zone with 8 radiators. The radiators in our two seldom used bedrooms would have thermostatic valves - the rest would just be balanced with stops at each radiator. Radiators will all need to be piped off a loop around the basement with a S&R off it to each radiator. The first floor is pretty open so heat tends to flow around and even out. Second floor is three bedrooms - two guest rooms with thermostatic valves to throttle down and close the door - third is the master with radiant heat. I 'd love to hear comments. To prime the pump here are some of my questions: With 8 radiators on two floors should I insist on a reverse return piping layout so the last radiator doesn't see cold water? How to I begin sizing all this stuff? My fear is actually oversizing the boiler and losing the efficiency I'm paying for when I buy it. Is the reverse indirect for DHW in the summer the right way to go or should there be a different plan for the summer? The electric water heater is only about 8 months old - would it make sense to pipe it in series downstream of the Turbomax to be an electrically "maintained" HW storage? Or should it just go on craigslist? Some of the piping for the runtals will have to be pulled and therefor won't get insulated - is that a big problem? (none will be in exterior walls) If you've read this far, many thanks. This board is amazing! Birdman

Close, but you can't use a turbomax like a buffer tank if you want to preserve the reset water temps for the radiators. You'd have a minimum temperature of at least 140+ to provide domestic in the turbomax, I think. I've heard some murmurings that maybe you can make DHW at 130 with the turbomax but even that is pretty high for a minimum reset temp.

I would suggest trying to size the radiators for something close to the radiant temp and doing this all single temp. That may or may not be possible depending on your radiant water temps and heat loads.

I can't imagine why you wouldn't just TRV every radiator that is in a distinct room. it's cheap and powerful zoning.

Step one is a good room by room heat load calculation. Step two is knowing what to do with that info. You're doing a lot here and it would most likely be best served by someone with some actual knowledge or you will potentially leave significant efficiency on the table.

If you and your wife don't use a lot of domestic, you could keep your existing water heater and just use a MUCh cheaper tank for a buffer if needed.

Well at least I got close..... Thanks for the quick response.
I understand about loosing the reset if I'm making DHW with the Turbomax. The fact that I posted this at 1:00 AM may be a factor in missing that. :-) .

I'm afraid sizing the radiators for the radiant temp would make the radiators huge and I'm kinda short on wall space. I did do a room by room heat loss and sized the radiators for a top temp of 170. I run the radiant in the dead of winter at about 105 to 110 and about 90 - 95 in the shoulder season max so that would bump the size of the panels up a lot.

On the T valves, I can't stand the look of them (we architects are like that sometimes - actually all the time) and since most of the house is open heat won't really stay in it's zone anyway - I figured that once I run the system I'll balance back any panel that seems to be pumping out to much heat relative to the others. We lived with the one pipe steam for 25 years and it didn't kill us - just closed the valve in the unused bedrooms and shut the door - it would be an improvement to have something in those rooms between full heat and no heat.

So am I to understand that if one wants to capture the real efficiency of a mod/con boiler you're restricted if you want to get indirect DHW? I would think modulating the boiler water temp between, say 180 at peak load, and 130 at the min reset would at least help the boiler see a cooler return most of the time. A mixing valve (or aquastat) could prevent overheating the DHW. (Is it typical to use checks or something to prevent the hot water from the indirect from siphoning back through the boiler?) Would it be crazy to use the electric water heater I already own to tack on the last 10 or 20 degrees after the indirect has done the bulk of the work? Maybe I'm over focused on preserving the efficiency of the mod/con, but seems a shame to buy the efficiency then as you said, leave it on the table, because the return to the boiler was too high or the cycles too short.

Thanks for the insights.

normally you only fire to high temp/high fire on the domestic demand, you don't turn all your demands into domestic demands.

170 is needlessly high target temps though. get creative

In my Worcester MA abode I have an Ergomax-centric system set to a constant 130F, and it makes hot water water just fine for our purposes, but maybe not enough for everyone's. As long as it's getting ~35KBTU or more with ~110F water entering the boiler, combined with the feedback of a drainwater heat exchanger on the shower we can run the shower pretty much 24/7 , even in winter. For tub fills it relies on the mass of the tank, and with a smaller Ergomax/Turbomax you'd need a higher setpoint, or more boiler-output to fill a tub fast, even with no space heating load.

It's plumbed at the hydraulic separator between the radiation & boiler. When a low-mass staple-up radiant zone first kicks off the return water seen by the boiler is ~100F for the first half, rising to 130F at the end of the burn (but not necessarily the call for heat). On maintenance burns with only the staple-ups running, the return water seen by the boiler is a bit under 120F. With a larger air handler coil zone (pulls ~40kbtu/hr) the return water at the boiler is just shy of 110F. With slabs and large radiators you'd be a bit cooler, higher efficiency with a mod-con. If you're running the radiator zones even at 150F it's likely you'd be giving up literally ALL condensing efficiency in the system. Even though my system would get marginally better efficiency with a mod-con, the numbers don't add up financially at my operating temps (I spent the difference on more retrofit insulation), so I'm running it with a cheap Takagi for now, and may run it off a cogenerator within 10 years. (The Marathon cogens have sufficient thermal output to keep up with even my peak DHW loads with the drainwater heat recovery in place, but I'd probably want an even bigger buffer than an Ergomax.)

But that mu system, and my PEAK temp is 130F. If you need 170F to meet design days needs a mod-con is WASTED in that configuration. Condensing doesn't start until the return water is under 125F, and doesn't get really GOOD until it's under 100F, a temp you'd almost never be seeing at the boiler drawing from the bottom of the tank except on cold-starts of the system.

With high-mass/low temp radiation like slabs or larger loop zones of radiators there's little additional benefit to centrally buffering it- with sufficient water mass on your radiator loops those also would not short-cycle the boiler. And with outdoor reset it would be run deep into the condensing zone MOST of the time. In your situation if you wanted to use a Turbomax plumbed in series with the radiation as pre-heat for something else that might work. Plumbed as a central buffer/hydraulic separator like mine at the temps YOU'RE talking it'd be an absolute condensing-mode killer- a waste of a good slab. Upsize your room radiation for lower temps and run outdoor reset if you're going to go mod-con, and run the indirect as separate zone- you'll get better than 80% efficiency out of it running it off a mod con- about the same annual true efficiency of an 0.82EF tankless (which is somewhat less than 80% in the real world.)

I took the central buffer approach primarily because my design temps were higher than condensing, and the heat loss characteristics of the various rooms were SO radically different that balancing it wasn't impossible, so I cut it into 5 zones, 2 of which are quite small (and quite different from one another, both in desired room temp and heat-loss characteristics.) I counted on not beating 80% by much for the as-used AFUE with a low-mass ~82% combustion-efficient burner like a tankless, but the math said it would average only in the high 80s with a mod-con, not 90%. It absolutely doesn't short cycle though, so the system efficiency pretty much hit's the numbers it would in an EF test, as long a the return water temps stay below 130F (which they do, by design.)

If your design day heat load is over 75K (mine's less than half that), a hard-corps greenie like you should check into running with a Freewatt cogenerator. The kilowatt-Honda (that thet married to a ~200K mod-con) would pretty much heat the place during the shoulder seasons and most/all of your summer hot water, and you'd get decent condensing operation out of the mod-con. It would have short-cycled itself into oblivion at my place- the lowest modulated output is something like 60MBH, but my biz partner (in a ~6000s.f. 1840s antique in Newton) can't say enough about the system. Both his fuel consumption & power bill have been cut in half from his previous high-mass 1950-ish boiler. See:

http://www.freewatt.com/hydronic.pdf

http://www.freewatt.com/products.asp?id=172&name=Hyd

(scoot me private message if you want the name & number of the sales rep I've dealt with.) It's not cheap, but the payback is decent- even at Concord town power's relatively low rates.

OK - here's how I see this working. My building envelope load is about 60MBH (including infiltration). About 1/3 of that is radiant heat in a good thermal mass and about 2/3 is to be heated by panel radiators. And there will be an indirect water heater.

I thinking about a boiler with a peak output of about 75 MBH. The indirect will have priority and on a call from the indirect the boiler will fire on high until the indirect is satisfied, probably using about 40 to 50 MBH of the 75. If there is a heat call at the same time the heating will get the remaining 25 to 35 MBH.

Once the DHW is satisfied, which should be most of the time, the boiler will modulate to outside air temp and run fairly long cycles at fairly low temps.

All the above is the goal and or the theory. In reality I'm having trouble getting enough radiation panel in to deliver the heat I need at the low temps.

Here's one thing that may help. The old steam boiler and piping gave up a lot of heat to the basement (about 1/3 of the basement has a radiant slab, 1/3 is an old cellar of 7' and the last 1/3 is a crawlspace but completely open to the old cellar - therefore "conditioned") I figured the basement heat loss into the total envelope but did not figure on putting radiation in the cellar - I suppose if I did that it would help drop the return temp to keep my little boiler condensing, right?

In theory I think the modulation to outside temp is adjustable so that the boiler water temp can and should be set low enough so the boiler needs to fire for long cycles to put enough low temp water into the system to meet the demand. If modulated down too far eventually the water temp would be so low that the boiler would fire constantly (albeit at low input) and still not keep up. So there must be a "sweet spot" where the boiler is firing about the optimal cycle (80%? 90%) at the lowest temp possible and one hopes that is below the condensing range (110 or so?) most of the time. The the DHW call comes and all condensing bets are off, boiler high fires for 20 minutes, or whatever, at the less efficient, non condensing rate. Have I got that right?

Dana, did you get my side message?

Again, many thanks.

you never gain efficiency by increasing your heat loss, which is what you are doing if you heat an area you don't really want to heat in order to lower return temps. either isolate and insulate or heat the area properly.

130 is condensing break but you do still increase efficiency going cooler than that. I typically see 120 SWT as a practical limit for baseboard and 110 for radiators. but the rads do get big. 120-140 is more typical for "economical condensation" under all conditions.

modulation rate is simply determined by the different between return and target supply temp, and flow rate. you're overthinking a bit. 20 minutes of off time or less for a DHW call won't let the water in the pipes cool off THAT much, and happens infrequently enough that you shouldn't even worry about it.

Understood. I was also thinking that the DHW "stealing" most of the boiler capacity for very short periods was no big deal but I probably didn't express that too well.

I do heat the basement, although cooler than the rest of the house, but it was done in the past partly with the radiant and partly with heat coming off the boiler and piping. I suppose now I'll have to be more deliberate about it and figure a way to control it as well.

Doesn't the "modulation" mean that as the outside temp rises the target temp lowers, then the burn is controlled to maintain the target temp leaving the boiler? The "adjustable" part I was thinking of was adjusting the relationship between outside temp and target temp.

I think is was Yogi who said "If you don't think too good, don't think to much." Maybe I should heed that!! Thanks.

our industry is not very good at differentiating terms. the word "manifold" for example applies in at least two portions of a system. and just try and figure out which is primary and which is secondary in primary/secondary piping

the TARGET TEMP does modulate, yes, as you note, but we call that "Outdoor reset" as a shorthand to describe a re-set of target water temperature based on outdoor temperature.

Modulation can occur without outdoor reset, or with it as well, the "mod" in Mod/con boiler refers to the gas valve modulation. they can burn a little or a lot of fuel. they determine how much to burn based on how much temperature they have to make up through the unit to achieve their current target temperature.

If you are going to heat an area anyway and it can run on the return temps from the other zones, great. you're increasing your radiator size in that case but it will generate lower return temps and lower your flow rates as well. it's tricky though and you have to be sure you're going to have flow from the other zones suitable for the one you are looking at... which is a tricky proposition in most cases.

So in theory anyway I guess I'm not that far off. The outdoor reset reduces the setpoint of the target temp as the outside temp increases and the boiler modulates the burner to achieve and maintain that reset target temp. Am I right in thinking that a reasonable way to try to keep the boiler condensing as much as possible is to keep the target temperature low so as to keep the return temp low? So given a constant flow, the lower the target temp the longer the cycles and the better the odds of returning water at a temp cool enough to condense the gases. This would be the case in the shoulder seasons but the outside reset would force the target temp up in the cold season and most likely the condensing efficiency would be lost during that time (unless one had enough radiation to keep the target temp low - hence the elegance of radiant floors) I'm assuming this would get worse the more the boiler was oversized, which is why I'm trying to keep the boiler small. I'm some what wary of one of the installers I spoke with when he thought I should take the house load and the DHW load, add the two together and that should the boiler size. This seemed wrong to me as the two loads are, for the most part, not concurrent - and over sizing seems to negate the advantages of condensing efficiency.

Kinda makes me wonder if there aren't hundreds of thousands of condensing boilers out there that rarely condense!

I would think a reverse return layout would solve the issue of adding radiation the cellar. Only 3 of nine upstairs radiators will have thermostatic valves so the rest will bypass. I'd put T-valves on the radiators in the cellar and set them lower.

There are definitely hundreds of thousands of mod-con boilers operated in such a way that they rarely condense, and modulate very little- it's all about designing the system correctly to maximize both.

My Rube Goldberg contraption of a system modulates quite will without outdoor reset, despite a fixed output temp of the boiler and fixed flow rate on the loop between it the ErgoMax, based only on the varying return water temps, which vary with the loads being drawn from the tank. After a few tweaks it's been set it up to finish a burn at the ~15K minimum modulation of the Takagi, but runs ~35KBTU/hr output with 110F return water, maximizing burn length, and delivering enough hot water to shower comfortably. Even with 100F return water to the boiler during a staple-up start the DHW output stays above showering temp. If I needed a lot more DHW for some reason I could easily bump up the output of the Takagi (by a factor of about 3x), simply bumping the fixed output temp on the controller panel and cranking up the 3-speed pump, but it wouldn't modulate as nicely under low heat-load conditions, leading to higher cycling losses. Having the drainwater heat recovery HX on the shower definitely makes a difference on how low I can set it, which governs the amount of modulation I can garner.

Any possibility of installing radiant ceilings, to get the operating temps down? Or a plated-staple-up for radiant floors, at least on the first floor?

Are the walls of the basement insulated?

How about the basement floor in the older part?

When dealing with existing homes getting the total heat load down with more weatherizing can be cheaper than going highest-efficiency on the heating system, especially if getting the operating temps down into the condensing zone is going to prove difficult/expensive. A perfectly "right sized" 83-86% AFUE cast iron beastie isn't always the wrong thing to do, especially when combined with outdoor reset (or economizer controls like an Intellicon HW+ or similar) and some buffering thermal mass for any microzoning.

Contractors that do not offer a computer generated heat load-up front- can be eliminated.

Condensing boilers almost never need to be up-sized for domestic hot water.