Connecting into existing boiler

[Bob Rohr]

Perhaps the most common question around here is is how to best pipe multiple boilers together. And there are dozens of ways to accomplish the marriage.

Ideally you want some buffer capacity for the wood or pellet boiler. But you don't necessarily want to heat the buffer with the fossil fueled boiler.

You want to generate DHW from one or both, but not at the expense of flowing through both in series.

Also you would want to use, or upgrade heat emitters that could run down to the lowest possible temperature, that leverages your buffer capacity. A mixing device on outdoor reset would be ideal for controlling distribution temperature most efficiently, but certainly direct to higher temperature emitters would work, count on more boiler cycling, however.

Good, better best emitters hydronic baseboard, air handlers with generous coil surface, panel rads, radiant surfaces, perhaps the lowest supply requirement. Remember radiant works great in walls and ceilings, and can be retro fitted easier than a radiant slab.

This drawing, thanks to John Siegenthaler, a hydronic designer and former Mohawk Community professor, with Appropriate Design in upstate NY, is a bit of a Euro approach. Johnnis working with NYSERDA to help installers get the best piping and help eliminate bad installations that dissuade homeowners. Not all plumbers have the experience to get the finer details correct.

Euro designs put the buffer at the end of the headers. This allows heat to get to the distribution quicker, it also lessens breaking up the stratification levels if you flowed in one side of the tank and out the other.

The fossil fueled boiler only "sees" a small portion of the buffer, for quick start up when the fire has died out. Adjust that dip tube or side port accordingly.

DHW is produced with a plate HX off the buffer tank. A small tankless, maybe even electric, would provide DHW if you chose not to fire either boiler. My system, piped with a plate HX produces plenty of DHW even with tank temperatures as low as 120F

Solar thermal could tie into this easily also.

Boiler return protection is either with a 3 way thermostatic for either or both, or with VS pumping for those that dislike mix valves

Taco released the upgrade to the BumbleBee last week, now called a Viridian model. I'm told it can now do reverse delta T, so it would be a shoe in for boiler return protection as shown here.

All other circs could be ECM for additional power savings, TRVs are excellent for non-electric proportional heat control at the emitters.

Control logic is spelled out and could be simple off the shelf aquastats, or a dual solar controller.

Air elimination is correct, expansion tank location correct, fill valve tie in, check valve locations, etc should all be correct. Boilers are shown stacked just to keep the piping easy to follow with the least amount of crossing. Boilers could be side by side, anywhere really.

Thoughts?

 

[ewdudley]

(First of all, his storage tank drawings often show a lot of wasted storage capacity above the highest ports and below the lowest ports. A correct implementation would have ports as close as possible to the top and bottom of storage, or would incorporate riser legs and/or dipper legs inside the tanks to capture that lost capacity.)

I believe his more recent designs feature a smaller elevated buffer that is fed by gravity from the main buffer. The wood boiler and optionally the solar inputs feed the the main buffer, and the hottest water rises to the smaller (e.g., 10-50 gallon) elevated buffer, and as importantly the coolest water falls back to storage. The fossil boiler, DHW load, and all the other loads then connect to -- to coin a term -- the nexus buffer. This arrangement is close to ideal since it promotes main buffer stratification in a nearly foolproof manner and it simplifies integration of the auxiliary fossil boiler and all the loads.

In cases where elevation of the nexus tank is not possible it is a simple matter to use a shuttle pump to supply the nexus tank from main storage. Sensors and control logic are then needed to maintain sufficiently hot temperatures at the top of the nexus tank as well as optimally low temperatures at the bottom of the nexus tank.

Also when connecting a wood boiler and storage to an existing system the wood boiler and storage will be located away from the existing fossil boiler and load connections, so it would make sense in these cases to install a central nexus buffer tank with shuttle pumping from the wood boiler storage tank.

A further improvement would be to incorporate a 'reverse indirect' or 'tankless coil' into the nexus tank for DHW, which does away with the whole DWH external heat exchanger and pump arrangement.

 

[ewdudley]

The fossil fueled boiler only "sees" a small portion of the buffer, for quick start up when the fire has died out. Adjust that dip tube or side port accordingly.

Very nice. This in general eliminates the need for return temperature protection on the fossil boiler when connected to a large storage tank.

 

[ewdudley]

Taco released the upgrade to the BumbleBee last week, now called a Viridian model. I'm told it can now do reverse delta T, so it would be a shoe in for boiler return protection as shown here.

What an embarrassment. This took them as long as the ridiculous delays in bringing HEC-2 itself to market after it was announced to be available 'real soon now' -- for years. And not really relevant to residential applications, the smallest Viridian weighs in at nearly 3/4 HP.

 

[Bob Rohr]

What an embarrassment. This took them as long as the ridiculous delays in bringing HEC-2 itself to market after it was announced to be available 'real soon now' -- for years. And not really relevant to residential applications, the smallest Viridian weighs in at nearly 3/4 HP.

Don't know what they were thinking with that Bee pump, got ahead of themselves. here is the new version of the BEE. Note however the way they show boiler protection in their schematic WILL NOT assure protection. Even with the bypass circ running full speed a cold slab or iron rads, or buffer tank could still pull down the return temperature for extended periods.

The ONLY way to provide 100% boiler return protection is to sense and respond to the temperature at the boiler return connection.

(broken link removed to http://www.emersonswan.com/ckfinder/userfiles/files/Viridian_VT2218_100-114.pdf)

 

[heaterman]

The key to simplicity of the installation is the tank. Nexus it is!
I'd love be find a buffer style tank that has a couple coils in it capable of transferring around 100,000 btu with appropriately located in/out ports and at least 3 sensor wells.
..........that doesn't cost half the price of the boiler it's connected to.
Have seen them in Europe and from a few custom manufacturers here but oooolala what a price they want for them.

 

[Bob Rohr]

I think I figured out a way to install coils in a blank insulated tank. We have this ThermoCon tank which has 7- 2" ports around 3 sides. I have a bunch of corrugated stainless tube and I think I can push 50 feet or so inside and used the various side ports. I'll use the CSST wall penetration fitting to bring the tube out of the tank.

This would allow you to use the SS coil for boiler or DHW production. Possibly install one top and bottom if I can figure out how to suspend the upper one.

We also have the SolarCon tank with two large diameter coils, upper and lower. They are 1-1/2" coils so they can easily handle 25-30 gpm. Some installers series the two coils for 66' of HX coil from top to bottom. But it doesn't have all the nice side ports as the ThermoCon.

These are US made tanks, thick steel with a 4 mil AO Smith manufactured, glass lining. Blue glass coating like all those AO Smith silos you see.

 

[velvetfoot]

I have a Froling Energy Tank and it has a corrugated 1.5" stainless dhw coil.
I don't know how you'd get away with a 120 buffer temp with the time a pellet boiler needs to heat up; the 100 gallons or so doesn't go that far-it's not like a 1000 gal. tank.
Pellet boilers are not like batch burning wood boilers; they modulate down to quite low output, and if there's a decent load and the boiler isn't oversized, I can see it modulating to meet load making the need for a buffer tank unnecessary, plus it doesn't have to start and stop and heat up the buffer tank to a higher temperature.
Why wouldn't the air eliminator include the oil boiler as well as the pellet boiler?
TRVs galore? Can they be individually controlled by network?
What's the scenario for running the oil boiler? When the pellets run out, I assume. How does the oil boiler know when to come on, and what's with the auxiliary boiler, whatever that is?
If you're out of pellets, and you're away from home, what's so bad about running the oil boiler to heat up the buffer tank; it isn't 1000 gallons, and maybe it'll prevent short cycling.
Just thinking off the top of my thick head.

 

[ewdudley]

When considering a separate small buffer, need to keep in mind that arriving at the right solution is a case-by-case decision process. If it works out well to connect the existing boiler to the new main wood boiler storage as in Bob's drawing at the top of the thread then so much the better. All the bases are covered and the design is better off without added complications, although one enhancement might be to insert a DHW coil into the top of storage with a port added for that purpose.

But in other situations like mine where the existing boiler is tucked away in a basement fifty feet or more from the new wood boiler a nexus tank configured as a reverse indirect DHW tank with an integrated "tankless coil" could work out well for connecting the two systems together.

The European style tanks like those from Fröling are very very nice but seem to cost more than they should. I found that building my own with all the ports custom located for my installation was a lot easier than I thought it would be. Got a 30 gallon propane tank off CL for $30, a couple 'tankless coil' necks with flanges for $60 each, plus all the forged steel (not cast iron!) half-couplings for ports and sensor wells. A good DHW coil is going to set you back at least a couple hundred dollars depending on your luck on ebay.

I contoured the half-couplings a little with an angle grinder, laid out the ports, and then cut the holes with rotary saw bits (Lenox is a sure bet). Tacked everything into position and took the assembly to a welding shop and asked them when I could pick it and they said just sit tight and we'll have it ready in a few minutes, $25.

So here's the result, which I've posted a couple times before, in a 'Danger Will Robinson' pose:



Here's a schematic as configured in the system: (Note how 'spent' return water from the high temperature and DHW loads is pulled from the bottom of the nexus tank to feed the in-floor PEX zones. This 'you-saw-it-here-first' innovation helps a lot in maximizing system deltaT, which maximizes the heat capacity of storage.)



If the DHW coil is not needed I think a smaller 10 gallon tank would do the trick as a simple hydraulic separator for connecting the existing and future loads, the existing boiler, and new wood boiler system together.

 

[velvetfoot]

The European style tanks like those from Fröling are very very nice but seem to cost more than they should.

All that stuff is too expensive.

In my little personal case, I got up today about 6:30 and took a shower, both zones kicked on at 7:00 from overnight setback, and the pellet boiler is running and the top of the tank is 140 and down 1/3 is 134. The low efficiency baseboards are warming the place up, but they could be faster. I have the tank set at 175 (down 1/3) with a 25 differential, so the sensor at 1/3 down from the tank gets down to 150.

I guess my point in a pellet system, the buffer tank can give an instant blast of heat, but unless the tank is pretty hot, the zones could still be a lttle slow in warming up when the pellet boiler is starting up, although setbacks with hydronic heat aren't as instantaneous as with hot air.