Burnham MPO-IQ versus Buderus G125BE Oil Boilers

[danboston]

Dana:

Thanks for the information on the insulation - very helpful.

The smallest zone is about 40 ft and that would give a mass of water within that zone at about 3.7 gallons. With that small zone therefore, I should think about adding more mass - i.e., a buffer?

Being a simple geologist, I'm still having a hard time wrapping my head around the "buffer" concept. Let me see if I have it straight: In your diagram showing the buffer with the "high flow resistance boiler", both the hot water output AND the cold return water enter the same buffer tank? If I get this straight, the buffer will provide a more steady source of hot water to the baseboard fins than if it was coming straight out of the boiler (which would be flashy at best) - is that accurate? It will also not only provide a more steady source, but because it is a larger source (mass) of hot water, it will be able to provide that to the fins for a longer duration? Hence, a more steady and comfortable temperature in the rooms that are accepting the heat AND the less time that the boiler is short-cycling to keep up with potential flashy temperature fluctuations in rooms accepting the heat in the absence of a buffer?

On the return side, the buffer will help to reduce the temperature of the return water before it goes back to the boiler and therefore increasing the efficiency of the boiler since the return water temperature has cooled even further and is at an optimum value? Furthermore, depending on how well the fins shed their heat, the buffer helps to prevent wide fluctuations in return water temperature back to the boiler? Thus minimizing the potential for "return water too warm" error messages showing up and subsequent atomatic boiler shut down in the new mod-con models?

If that is accurate, how does that work if you are sending both hot water directly from the boiler and cooler return water from the baseboard fins into one tank (the buffer)? Is there that much of a temperature separation in the buffer tank from top to bottom? Several contractors have told me that the water to be sent to the baseboard fins needs to be at around 180F. Does'nt the return water temp need to be at or below about 110 F for a gas-fired mod-con to operate efficiently in condensing mode? That would be a temperature difference of about 70 F in the buffer! (?). I thought I read in some of your other posts that the ideal temperature that can/should be sent to Al-fins is around 120-125 F? How do you balance that to keep temps from getting too low (for input to the fins) or high (for return to the boiler) in the buffer tank? What is the ideal temperature range of the hot water that should be going to the baseboard fins to provide adequate heat in the room, yet not too hot so that the return water ends up being too warm? Finally, what size buffer tank would be about right for my situation?

[Dana]

I may have over-stated the cost-effectiveness-R values somewhat. Another opinion has R7.5/R15/R30/R65 for slab/foundation/wall/roof- see the minimums for zone-5, Table 2 on page 10:

http://www.buildingscience.com/docum...-climate-zones

But still, the rigid iso was the expensive part, and you're already committed to adding the studwall- the ecoomics of adding batts still make sense.

Wtih the buffer tank piped as the point of hydraulic separation between primary/secondary loops the boiler flow rate is independent of the heating zones' flow rate(s). The boiler throttles back more and more as the return water temp rises, and at some point the tank temp is high enough the boiler's controls kill the flame, even as the flow to the radiation continues, even tually lower the average temp of the tank. When the boiler re-fires, much of the output water from the boiler short-circuits across to the heating zone loops, reducing the high rate of mixing that would otherwise occur in the buffer. But that's just one common method of doing it. Yours is a simple/small enough system that it might be fine to simply put the tank in series between the boiler output and the zone-manifold. The pumping & piping details can and WILL differ.

A real design involves doing the math on the flow requirements & pumping rates for the zones, the pumping rates required by the selected boiler and the relative head- pressures, etc. Unless you want to take the course in hydronic design or home-study it, you really need to find the right system designer/contractor. This is not and will never be a design-by-web-forum type of problem except in the simplest of cases (and even then, you often only get what you pay for.)

Any contractor who still thinks ANY fin-tube requires 180F water is stuck in the 1950s, or didn't take the course, and surely never read a fin-tube spec sheet.

Many old school systems were DESIGNED to require 180F water to be able to be able to deliver the load AT DESIGN CONDITIONS (with a lot of padding built into the calculations- most could still work even then with 150-160F water on design-day), but 99% of the time it isn't -2F or 0F outside, and the heat load is a lot less. At ~35F the heat load is half the design load, and even in a minimalist system that required 180F water at 0F outdoors could keep the place cozy with 140F water.

Then consider:

A: The thing was probably overdesigned even for the leaky barely insulated 1950 version of the building with single-pane windows

B: You have fuel-use evidence that your true design-day heat load is closer to 30-35K and may be even less, a load easily met with 135F water in 145' of fin tube

C: You've cut the heat loss by probably 30-50% with your already-done insulation and air-sealing upgrades, and you're adding more insulation to the foundation.

Assume it was 50% overdesigned on day 1 in 1950 with 180F water (it was probably even more than that- 100, 200, even 300% oversizing isn't uncommon), and the design output for the 1950 house was in fact...

90K which is about the right output figure for 145' of fin-tube @ 180F (and suspiciously close to the 'heat loss" submitted by one of the contractors.)

...which means the real heat loss back in 1950 was really only....

60K

If you've cut down that 60K d heat loss 30% with insulation & better windows (it's probably more) it means you'd be currently looking at...

42K

... as your actual design condition heat load.

That's less than half the 180F output that it was originally designed for, and a BTU-rate the 145' of fin tube can deliver with sub-140F water. And that's on the COLDEST hours of the COLDEST days. Most of the time it can run even cooler boiler output. This is a VERY common scenario.

If it was even more oversized in the beginning (likely), or your improvements to date are better than 30% (also likely), you'd never need more than 125F-130F water to meet the heat load even during the coldest hours of the year.

Fin tube "sheds heat" primarily by convection, and it's output is very roughly (not perfectly) linear with the difference in temperature at the floor (~65F) and the water temp. At 180F that's an air-water delta-T of 115F. With 135F water that delta is reduced to ~70F, which is ~60% of the 115F delta, whtile the output is more like ~50% of the 180F numbers. (see the above linked short-spec for a particular fin-tube.) Because the fin-tube is relying on induced "stack effect" air flow for convection, that force gets to be really weak given the short height of baseboard, which is why it's tough to get reliable results with sub 120F output water in most fin-tube.

But even 130-135F water out will return 110F or cooler water if you set up the flows correctly, delivering low-mid 90s efficiency with a mod-con. From the description would appear you have sufficient baseboard on your existing zones to be able to run it there MOST of the time, maybe even ALL of the time, and a contractor with design skills can make that happen. (But surely not the guy who believes "Thou shalt deliver 180F water to baseboard" was engraved in stone by the hand of god.)

[danboston]

Dana:

I appreciate the explanation of the boiler temperature going to and returning from the Al-fins - that makes sense and I can easily convey that to a contractor going forward.

So it sounds lke the buffer tank is used in tandem with the boiler and that hot water from the boiler may or may not go directly to the buffer tank, depending upon the temperature of water in the buffer tank and the room temperature? Sounds a little complicated and maybe that explains why not one contractor has mentioned it so far....

I have decided between three different gas-fired mod-con boilers: Burnham Alpine 80, HTP Elite 80 and the Prestige Solo 60. These units all have the same lowest min (16 K btu/hr). My heat loss rate is between 25 to 35 K btu/hr. Am I better off going with the lowest max available (e.g., the Solo 60) or does it not really matter?

Alpine 80 output (min/max) = 16/80

HTP Elite 80 output (min/max) = 16/80

Solo 60 output (min/max) = 16/60


Also, if properly sized, do I really need a buffer tank? If I need a buffer tank, what is your opinion about the Boiler Buddy?

I appreciate your information it has been very, very helpful.

[Dana]

A Boiler Buddy is nice but, overkill for a system this small.

The need for buffering has noththing to do with the right-sizing of the boiler output to the design-day heat load, but rather the mass and output of the smallest zone relative to the lowest-modulated output of the boiler at the operating temp of the system.

Assume for instance your basement zone will have 40' of 3/4" fin tube and a 15-20 feet of distribution plumbing, which would have a water mass of about 10 lbs. Wtih 120F boiler output and 100F return water, you'd be putting 6Kinto the room, but would have 15K of boiler output at mid-mod, a 9K overrun. Assuming the boiler has an internal hysteresis of 10F in it's controls, it would take only 10lbs x 10F/9000= 0.011 hours(less than a minute) per burn cycle. Adding a 10 gallon tank in series would add 83lbs fo ra total of 93lbs, and you'd be looking at 93x10/9000=0.1 hours= 6 minutes of burn time- still not enough. At 30 gallons you're looking at ~260lf of water in the zone, and that would make for 260x10/9000=0.29= 17 minutes burn time at min-mod. But it's only half that if the hysteresis is only 5F.

Some small buffered Solo-60 installations I've seen were made to work with the zone returns all flowing through an unpowered HW tank that then feeds the boiler return. But whether that configuration works depends on if the head & pumping on each zone is low enough to guarantee the minimum flow on the boiler speced by the manufacturer. If it MUST be plumbed primary/secondary to meet the boiler flow spec, configuring the tank as the hydraulic seperator using Tees (as in the diagram from the other day) also works and is cheaper than a very-low-head Boiler Buddy or ErgoMax dedicated buffer.

Alpines & Solos have good reputations, but I've red no feedback on the Elite. As long as 0.86 x (maximum boiler output) or the DOE heating output number has at least some margin (even 10% is fine) over your actual heating design load they'll work, and with the min-mod numbers being the same they'll run at about the same efficiency. The experience & comfort level of the designer/contractor plus the local distribution support of the manufacturer generally trumps any of other differences between them. (Some installers rave about how easy it is to set up the Solo, but that's of little consequence once it's installed and tweaked-in.)

[danboston]

Dana:

OK, it's clear to me that for low-mass systems (similiar to the one I have) that a buffer will help with decreasing maintenance and increasing the life of the boiler by creating less frequent and longer burn cycles. That makes sense. What about on the return side? Does a buffer also help with lowering the temperature of the return water even further - ensuring that it is 110 F or less and optimum for making sure that the boiler remains in condensing mode? Or, if we can use 125 F - 130 F output water for input into the fins, it is very likely that the return water would be in the sub-110 F degree range anyway and a buffer is not generally needed for for lowering the return water temp?

Just to reiterate so you do not have to plow through my earlier threads, we currently have 145 feet of Al-fin baseboard heating on two zones for the ground floor (1,700 ft2) that is conveyed using 1.5-inch copper pipe. My current smallest zone is about 40 ft and that would give a volume of water within that zone at about 3.7 gallons (assuming 40 ft x 0.0918 gallon per foot). This would represent a mass of ~ 31 pounds (assuming 3.7 gal x 8.35 lb per gallon). Within the next two months, we are planning to remodel the rooms on the ground floor and will be taking down interior walls and opening up the floor plan. It is a 1950's ranch and each room is boxed off from the other. Rooms will be combined and will allow us to reconfigure the zones on the ground floor so that they are roughly equal (Zone 1 = 73 feet and Zone 2 = 72 feet). That would represent ~ 55 lbs within each zone.

Secondly, when we do install baseboard heating in the 1,500 ft2 well-insulated basement, maybe I should think about using cast iron radiators instead of fin tube? What if I went that route with 40- to 50- feet of cast iron radiators in the basement as Zone 3 off the boiler? Would I still need a buffer under that scenario?

Not sure what size piping is typically used for installing cast-iron radiators, but I could always use a large diameter pipe to add additional mass if the cast-iron radiators are insufficient?

Thoughts?

[Dana]

The buffer (in any plumbing configuration- either in-series or at the point of hydraulic seperation in a primary/secondary) doesn't appreciably affect the average return temp or the overall duty cycle, but HUGELY affects the number of cycles and efficiency losses related to the fixed losses of ignition & flue purges, and the wear & tear on the boiler.

Cast iron or flat panel radiators add mass (and work linearly at lower temperatures than fin-tube), but you still have to do the math on the total mass & heat-emission at the temperature you're operating to know whether you'd need a buffer. But since you probabably still need a buffer on the other two zones to run at 120F withtout short-cycling, making it a commoned buffer in series with the return manifold and the boiler is probably the right thing to do ( and not a seperate buffer just for the basement zone.) Plumbed on the return manifold is slightly better than on the output side, since then it doesn't slow the response time of the system when under low- partial load- the radiation gets to it's peak temp sooner without having to raise the temp of the buffer first.

A quick refresher on the simplified math: At your low-load minimum of 120F your existing 40' zone with the 31lbs of water the fin tube would be delivering something on the order of 200BTU/foot x 40' =1600 BTU/hr into the rooms, but the boiler is putting out nearly 10x that at 15000 BTU/hr even at it's min-modulation, for a ~13,400 BTU/hr shortfall. Assuming a 10F hysteresis in the boiler you'd get 31lb x 10F/13,400= 0.023 hour= 1.4 minute (83 seconds) of burns- a seriously short cycle. If you raise the temp to 130F the output of the fin tube would rise to something like 250BTU/foot stretching it out a bit, but not enough to matter.

To deliver the full 15K in a 40' zone you need 375 BTU/ft, which would take 150F water, with a return temp over the condensing point. You'd need to buffer your existing zones to be able to operate in the condensing region without short-cycling the boiler into both lower efficiency & an early grave.

To stretch the burns to ~10minutes (.17 hours) with 13.4K of extra takes a system with 10F of hysteresis at the boiler takes 13400 x 0.17/10F= 228lbs of water, which is 228/8.34= 27 gallons. You already have 3.7 gallons in the zone, so a 20-25 gallon buffer will do. But if the hysteresis is closer half that (sometimes is) a 40gallon buffer would be better. (30-50 gallon electric HW tanks are CHEAP compared to a 30 gallon Boiler Buddy or a Ergomax BT26 or BT48 buffer, and won't add much pump head to a low-flow system like this.) It would take ridiculously large diameter & expensive distribution plumbing to achieve that much thermal mass.

[danboston]

Dana:

So, when I put heating in the basement, I should absolutely go with cast iron over fin-tube?

After weighing all my options, I have definitely decided to go with the Burnham Alpine 80. Burnham has a good reputation and the Alpine 80 modulating range looks good (16 to 80 K btu/hr) for my Heat Load (~35 K btu/hr). Also, I have found a couple of local contractors who seem competent and have installed quite a few Burnham boilers.

To get a better handle on the likely hysteresis of the Alpine, I looked up the Central Heating Setpoint discussion in the Alpine Installation & Operation (I&O) Manual (http://www.usboiler.burnham.com/prod...heaters/alpine)

According to the I&O Manual (page 94):

8. Boiler Temperature Regulation
a. Overview
Based on the call for heat type, the Sage2.1 controller
selects a firing rate target temperature setpoint.
Sensed temperature is compared to this Setpoint
to both adjust firing rate output and along with the
corresponding “Diff Above” and “Diff Below”
settings cycle the boiler.
b. Central Heat Setpoint
The Central Heat Setpoint is fixed at the contractor
selected value unless Outdoor Air Reset is enabled
or a “Setback” thermostat is connected. The value
of this setpoint is set up based on the type and
quantity of radiation installed.

Also on page 106 of the Alpine I&O manual:

The range of temperature values that the installer can input for the Central Heating Setpoint "DIFF ABOVE" is 2 F to 10 F (factory setting is 2 F).
The range of temperature values that the installer can input for the Central Heating Setpoint "DIFF BELOW" is 2 F to 30 F (factory setting is 10 F).

Does this help in the evaluation of buffer size?

It is interesting to note on Page 108 of the Alpine I&O Manual that the recommended Central Heating Setpoint temperature for Convection Baseboard Fin Tube Convective Heating elements is 160 F - 190 F.

Finally, which of the three diagrams that you provided in earlier threads of this post best represents what you describe as "a commoned buffer in series plumbed on the return manifold"?

Thanks, I appreciate all of your help!

[Dana]

All prior schematic were variations on buffer-as-hydraulic seperator, none were a simple series-return diagram a partial schematic would look like this:




|boiler return input|<============<[buffer tank]<====<|zone1 return|<|zone 2 return|<|zone 3 return|

indirect return Tees in here-^

The returns from the zones all come together and feed into the cold input of your buffer tank or elec. HW heater, and the output of the buffer then feeds the return input to the boiler. If running an indirect HW heater as a seperate zone it needs to Tee in between the buffer tank and the boiler's radiation return.

Cast iron or high mass radiators aren't essential, but generally provide more comfort than fin-tube, since you can feel the direct radiation on exposed skin, whereas fin-tube is primarily just heating the air. Since you'll need a buffer to deal with your existing zones to run at low-temp, using the same buffer for all zones means you don't have to use some monster-sized radiators in the basement just to raise the thermal mass of that zone.

The fact that the hysteresis on the Alpine is programmable up to 10F means that you can indeed use a 20-25 gallon buffer rather than a 40-50 gallon buffer without short cycling on your 40' baseboard zone, as per the last paragraph of my prior post. If you run at least 40' of baseboard in your basement it'll be pretty similar to the setup for the other zones as far as buffer sizing. If left at the hysteresis programmed to the 2F default you'd still run into sub-5 minute burns, which would be bad for the boiler maintenance & efficiency. At 5F or less you'd need 40gallons or more to really stretch it out with that type of radiation.

Whatever you put for radiation in the basement zone, make sure it's output @ 120F water temps is at least that of your other zones, but in the same ballpark. Read the specs carefully. (If you're up for it, radiant ceiling is pretty nice, but that's a whole 'nuther design issue.)

[danboston]

Dana:

That helps a lot. However, if you have an example of a simple-series return schematic that would be really great since I am having a hard time picturing that.

Yes, we plan to add a 50-gallon indirect water heater (Burnham Alliance stone-lined). One of the Burnham contractors said that the water in New England is not good and that a stainless steel IWH would not last as long as a stone-lined one. Do you agree with that?

On a side note I was able to download the free Slant/Fin Hydronic Explorer Heat Loss program. It is the same program that a previous contractor used to calculate a heat loss of 90 K btu/hr for my house. In addition to using an outdoor design temp of -10 F for my area (about 20 miles north of Boston), there were also some omissions they made for insulation. I plugged in an outdoor design temp of 0 F and added the appropriate insulation factors and came up with about 56 K btu/hr. That is still more than 1.5- times greater than the rate that I calculated using the NORA Fuel Savings calculator (~30- 35 K btu/hr), but I guess as long as it is less than the net I-B-R for the Alpine 80 (which is 63 K btu/hr) it should make the contractor feel good about the sizing. Do you know what the ASHRAE Outdoor Design Bin Temp is for: Boston , MA and Concord, NH ? If not, do you know where I can find those?

[Dana]

The ASHRAE 97.5% design temp for Lawrence MA next door is 0F, the 99% number will be something like -3F or -5F. If the boiler's output is even 15% oversized for the heat load at the 97.5% number it'll keep up at -10F. For a listing of some ASHRAE 97.5% numbers see:

http://www.crownboiler.com/educate/heatloss.asp

For some 99% numbers see:


http://www.chromalox.com/resource-center/design-guide-pages/dg-comfort-heat.aspx

Note, Boston's numbers are +9 F and +6F, due to the moderating effect of being on the coast. Figure on Lawrence/Andover's 99% number will be about -3F or -4F, not -10F.

Whenever I've used the Slantfin tool it's come out 35-40% ahead of measured reality. Other Manual-J or IBR type tools tend to hit in the 15-25% overage. If you use fuel use and degree-day data carefully to calculate the K-factor and parameterize the boiler carefully the NORA tool tends to hit within 10%, but can sometimes hit 10% low. The NORA tool tends to use some 99.9% number for design or somethingfor the cities on their pull-down menus. If the Slantfin tool is telling you your 0F load is 56K, measured reality will be no more than 45K. If the NORA tool is telling you 32K, it could be as high as 36K, but it's probably 32K. Unlike heat loss calculators, the NORA tool is using the boiler to MEASURE the heat load.

I suspect the Slantfin tool assumes an unrealistically high infiltration rate compared to that of tighter homes built in New England generally have, and pads out the result a bit from there. Better tools allow you to play around with air-leakage factors a bit more.

I have no opinion of stainless vs. stone lined. A generalized characterization of New England water is pretty bogus though- town water differs widely depending on source and treatment, but most towns don't have water hardness issues the way they do in the midwest. Call your town water department or if online, look up the water quality reports and see the average grains of hardness. Worcester uses primarily surface water from reservoirs, and I ran a tankless HW heater (notoriously sensitive for liming) for 15 years without ever having to de-scale the sucker, and retired it fully functional when I replaced it with a reverse-indirect buffering the heating system (necessary due to micro-zoning.)

To clarify in my previous post, "Whatever you put for radiation in the basement zone, make sure it's output @ 120F water temps is at least that of your other zones, but in the same ballpark." , I really meant to say "...at least that of your other zones relative to the calculated heat load for that zone..." The heat load of the basement will be much lower than the upper floors due to less above-grade exposure and less glazed area.

A series connection means that it's one pipe in , one pipe out, no Tees. Think of the tank as a fat spot in the pipe between where the heating returns come together and the boiler. All heating returns dump into the tank, not the boiler, only the tank's output feeds the boiler.

But IIRC from a recent thread on the boiler forum on this site the Alpine's min-flow requirements are such that it REQUIRES a primary/secondary configuration, so using the buffer (rather than closely spaced tees) as the hydraulic seperator is the right way to go, if that's you're boiler. (The Solo 60 could probably be done in series though.) See TK03's comments about P/S requirements for the Alpines here:

http://www.terrylove.com/forums/show...Tweaking/page4

You may want to read that entire thread- it's essentially the same design and setup issues that you'll have to address.

The LAST thing you'd want to do is up-size it to the Alpine 105, since it's min-mod output is probably going to be over half of your design-day heat load. The 73K DOE heating output of the -80 is well over even the Slantfin calc. The IBR number shouldn't be used here, since that's essentially a ~15% allowance for pipe & boiler losses when the boiler & distribution piping are outside of conditioned space, such as an unconditioned attics or crawl spaces, etc. Even if the boiler is installed in your basement, it's a conditioned basement inside the insulation, not a ventilated/uninsulated crawlspace. You very likely have 80-100% more burner than you need on the high-end even using the -80, but the fuel efficiency is dependent on how low you can go on the minimum. If your true 0F heat load is 35K, the -80 would nearly continuously (mostly in condensing mode) whenever the the outdoor temps drop below 40-45F once you have the flows & reset curves tweaked in. If you upsized the boiler to a higher min-mod, you'll experience more cycling losses when it's 30F or above (unless you added even more buffer), which is MOST of the heating season.

[danboston]

The hardness of Town water has been consistent for years at 2 grains per gallon or 34 milligrams per liter. Is this enough to cause scaling problems?

As provided in the Chromalox web-site, the 99.9% ODT values for the following cities are: Boston [6 F], Worcester [0 F], Concord, NH [-8 F].
I re-ran the Slant/Fin Hydronic Explorer Heat Loss using -4 F for Lawrence as the ODT (average of the 99.9% ODT for Worcester [0 F] and Concord, NH [-8 F]) and that gives me 63.5 btu/hr. As I mentioned, based on oil consumption, the NORA calculator gives me about 35 K btu/hr.

After reading TKO3's thread, do variable speed pumps help with reducing short cycling by increasing the delta T?

As far as the size of the buffer, I just double-checked the pipe diameter associated with the smallest (40-ft) zone and it actually is only 3/4-inch copper pipe (the other zone is conveyed with 1.5-inch pipe). Apparently this smaller zone was added as part of an addition that was put on before we bought the house and they used smaller pipe for that zone. Therefore, we have a little less than 1 gallon in that zone. So, is a 20-25 gallon buffer still appropriate? Also, looking at the room-by-room Heat Loss numbers that I calculated using Hydronic Explorer, I added up the Heat Loss for the smallest zone on the ground floor and it is roughly equal (slightly less) than the predicted heat loss for the entire basement (insulated).

So you think the Alpine 80 with a 25-30-gallon insulated buffer and 50-gallon IDW heater is appropriate? The low end of the Alpine 80 is 16K btu/hr and the high end (DOE value) is 73 K btu/hr? Do you think that is still too much boiler and maybe a smaller combi-unit might be the better way to go? Do combi units provide a lower output (< 16 K btu/hr) than the Alpine 80?

[jadnashua]

Combi units (those with a built-in HW coil) aren't the best idea when considering efficiency - the output is limited, and you have to maintain the boiler hot 24/7. An indirect, especially on a mod-con, means on a warm summer day, it may only fire once to bring the WH up to temp after the morning rush.

On a boiler plumbed with p/s, except for a few situations (heating the indirect may be one), the heated output goes in one loop back to the input. On that loop, there are some T's where there is a takeoff for your heating zones. It inserts the cooler return water on another T. So that there is actually flow in the secondary loop, the placement of the T's in the primary loop is somewhat critical. There would be one circulator pump for the primary and other(s) for the secondary loop(s). On mine, it pulls off the primary loop for the indirect, but that is not always done that way.

[danboston]

The water piping diagram for the Alpine is shown here (see attachment):

Alp WP 100Res.pdf

Where would the buffer go?

I looked at the HTP VERSA, which has a modulating burner that appears to range from 100 K btu/hr to a low of ___? Not sure what the low end is although they say it has a 5 to1 turndown ratio on the main combustion unit and a 10 to 1 turn down ratio for space heating - does that mean it will go as low as 10 btu/hr? It does appear to have built-in coil. So this does not act like a Indirect hot water heater and the storage temp of the combi must be maintained hot 24/7? Is'nt that how an indirect water tank works - it must be kept hot all the time?

See http://www.htproducts.com/versa-hydro.html

I am also considering having a solar hot water panel installed in my backyard. Massachusetts is offering a significant ($3,500) rebate program in addition to some Federal tax credits. I assume that would save the boiler from generating any hot water at least 9 months of the year. Of course I would need a larger IDWH (80 gall) and it would need to have two coils (one bottom coil for the solar heated fluid and the top one for the boiler). The other issue with the panel is location - it would be about about 80 feet from the house and another 50 feet to the tank in a conditioned space. The vendor stated that the piping would be installed in a trench 1.5' below grade and would be well-insulated. Not sure what the heat loss would be over that distance but have a feeling it would be pretty bad in the winter-time?

[jadnashua]

That's a hybrid, similar (I think) to an older designed Trianco HeatMaker (Which I owned, but eventually threw out). Not in a position to evaluate it at the moment.

[Dana]

The hardness of Town water has been consistent for years at 2 grains per gallon or 34 milligrams per liter. Is this enough to cause scaling problems?

As provided in the Chromalox web-site, the 99.9% ODT values for the following cities are: Boston [6 F], Worcester [0 F], Concord, NH [-8 F].
I re-ran the Slant/Fin Hydronic Explorer Heat Loss using -4 F for Lawrence as the ODT (average of the 99.9% ODT for Worcester [0 F] and Concord, NH [-8 F]) and that gives me 63.5 btu/hr. As I mentioned, based on oil consumption, the NORA calculator gives me about 35 K btu/hr.

After reading TKO3's thread, do variable speed pumps help with reducing short cycling by increasing the delta T?

As far as the size of the buffer, I just double-checked the pipe diameter associated with the smallest (40-ft) zone and it actually is only 3/4-inch copper pipe (the other zone is conveyed with 1.5-inch pipe). Apparently this smaller zone was added as part of an addition that was put on before we bought the house and they used smaller pipe for that zone. Therefore, we have a little less than 1 gallon in that zone. So, is a 20-25 gallon buffer still appropriate? Also, looking at the room-by-room Heat Loss numbers that I calculated using Hydronic Explorer, I added up the Heat Loss for the smallest zone on the ground floor and it is roughly equal (slightly less) than the predicted heat loss for the entire basement (insulated).

So you think the Alpine 80 with a 25-30-gallon insulated buffer and 50-gallon IDW heater is appropriate? The low end of the Alpine 80 is 16K btu/hr and the high end (DOE value) is 73 K btu/hr? Do you think that is still too much boiler and maybe a smaller combi-unit might be the better way to go? Do combi units provide a lower output (< 16 K btu/hr) than the Alpine 80?

Maybe I was too wordy in my response on that thread but simply put:

No, variable speed pumps do not and cannot reduce short cycling. It doesn't change the minimum modulated output of the boiler, or the rate at which the baseboards emit the heat into the room.

The lower the water temp, the lower the heating rate of the baseboard. When the boiler's output is many times that of the heat emitted the water temp rises rapidly and the burner kicks off.

To eliminate short cycling at condensing temps you either need more baseboard, or add mass to the heating zone, or increase the hysteresis of the boiler temp to use the thermal mass of the water in that loop.

From a practical point of view with 50 ' of baseboard at 120F you're looking at 1000BTU/hour, or about 7% of the boiler's ~15K output. Putting in 750' of baseboard won't cut it.

Raising the hysteresis of the boiler for 10minute min-burns on a sub-5 gallon loop takes it well out of the condensing zone for most of the burn.

Adding thermal mass to buffer the heat is the only real solution.

An Alpine 80 + indirect + buffer tank could be an appropriate solution. In that scenario, place the buffer at the point of hydraulic seperation to the heating zones, but not the indirect (I think you can eliminate P3 if you plumb it tight & right):




The HTP Versa is also an appropriate solution, and will run as efficiently as the Alpine 80 if usng fin-tube as the heat emitters. With radiant floors and/or panel radiatorsit's possible for a good designer to squeak 3-4% more efficiency out of an Alpine 80, since you could then run the heating zones at temps much lower than domestic hot water for high combustion efficiency. The Versa is inherently self-buffered, and making the system design & implementation comparatively simpler- you'd have to work at it to really screw it up (but I'm sure there are installers up to the task. :-) ) Unlike Trianco or Bradford-White combis, the Versa is fully modulating & condensing, with a 5/1 turndown ratio to (same as the Alpine 80's high/low ratio)- they're not really in the same class. Even though the smaller-burner Versa only cranks down to ~25K of outuput compared to ~15K for the Alpine 80, the 55 gallons of water in the smallest Versa is more than 2x the minimum buffer we were talking for the Alpine-80 solution with 10F of hysteresis, for less than 2x the min-mod burner output. There's literally no way to short-cycle it, and it'll average better than 90% efficiency at your anticipated fuel-use derived heat load using your given lengths of baseboard, since you won't need to it up above typical DHW storage temps- it'll ALWAYS be firing in condensing mode.

The heat loss out of a trenched-in solar loop isn't as bad as you might think. Heat loss is all about the total surface area, delta-T and R values. The surface area of the pipe is around 1/3 of a square foot per running foot, so at 80 feet out, 80 feet back you're looking at about 50 square feet. Assuming an average water temp of 90F and an wintertime soil temp average of 40F at that depth, and R10 of insulation (2" of closed cell foam) that's 50F x 50sq-ft/R10=250BTU/hr. That's the order of magnitude, and likely overstated by 34x, since the temps are only significantly elevated during mid-day sunny hours, which are less than 1/4 of all winter-hours, so the average delta-T may in fact be far less than 60F. Any losses to the conditioned space portion comes directly off the heating load, and can be ignored. Insulating underground flat-panel solar plumbing to a reasonably high-R can be done by sheathing the pipe in an R4 high-temp pipe insulation then strapping it to a strip of 2" of XPS and spraying 1.5-2" of closed cell polyurethane foam over that. If using evacuated tubes you may run into higher temps than XPS or SPF are rated for- let the solar designer spec the insulation.

The heat loss in summer would actually be higher, since the soil temps wouldn't be more than 20F higher, but the average water temps would be substantially higher due to lower panel losses and a greater fraction of sunny hours.