Hybrid home heating system, looking for new boiler

Ted Inoue

* sorry, long. lots of information *

Background - I have a ranch style home, ~1800SF/level. The main floor has forced air supplied by a geothermal system. It also has two zones 3/4" fin tubes from the original heating. These are used for backup heat and recovery when quick heat is needed.

The lower level is a single zone of 3/4" fin tube.

Hot water is via electric resistance tank.

Since last december when the geothermal was installed, I've been monitoring all the system usage - baseboard and geothermal run hours. From this and other system parameters measured, I'm computing BTUs. Roughly speaking, daily BTUs = geothermal run * 40k + baseboard hours * 40k. So for example, I might have 8 hours of geothermal runtime and 6 hours of total baseboard runtime = 8*40 + 6*40 = 560kBTUs/day (=23kBTUs/hr) at 35F.

Computed 0F/70F whole house load is ~48k BTU/hr.


The current oil boiler is a 200kBTU New Yorker, boiler room vented to outdoors (i.e. cold). Given it's large size, it runs 5-10 minutes/hr. However, it sucked down about 120 gallons in December for only 164 hours of baseboard runtime. At 40k BTU/hr output for the baseboard, that's 6.5million BTUs of DELIVERED heat. Out of 120 gallons, that's a mere 55k BTU/gallon. Clearly, something is very wrong. (Note that this level of consumption occurs even with regular professional servicing.)

I've been pretty rigorous about my calculations and assumptions and don't see where I could be off by much. If anything, the delivered heat from the baseboard is less than 40k BTU/hr.

I'm now looking to install the following and I want a hydronic control system and boiler that are a best match for my needs.

- Use geothermal for upstairs heating, using baseboard for extra as needed when temperature drops below, 20F or when desired for a fast recovery using a manual override. (I limit geo usage to 12 hours/day because of loop-field capacity limitations.)

- add a dedicated fin-tube zone to an upstairs sunroom. This should have it's own stat (This unfortunately can't be radiant)

- add a dedicated radiant zone for a small mudroom/bathroom that will be added to the house. This needs it's own stat.

- add radiant floor warming to two tiled upstairs bathrooms. This will have to be accomplished via tubes under the bathroom subfloors.

- add a radiant zone downstairs to a bathroom that will be renovated and tiled. This might be wet bed tube installation, though I'm leery of that because it's on slab and adding a couple inches of insulation plus the tile bed and wet bed will raise the floor more than we'd like.

- switch to indirect DHW tank or perhaps just use pre-heat from boiler and keep an electric tank. Not sure which is best solution.

- continue to use fin-tube downstairs for primary heat.

I've been evaluating Energy Kinetics EK-1, Buderus G115, WM Ultra and Biasi boilers. Any unit would be ducted to take combustion air directly so that the boiler room could be sealed up. This will help avoid throwing away standby heat.

. I like the idea of a small, low mass boiler with outdoor reset since load is low. Because of the geothermal system, from the boilers standpoint, the house load is effectivly less than 30k BTU/hr at 0F (not including DHW).

If all the radiant zones are run off the indirect tank, then , I think, the boiler would run very infrequently to maintain temperature in the indirect tank. With a low mass, cold start system, efficiency should be much better than keeping a high mass boiler at a much higher temperature.

I've had four pros in to evaluate the situation and have gotten suggestions across the spectrum. I don't think any have heard of the Biasi. Those that have heard of the Energy Kinetics say that it's unreliable. One says not to bother with the Buderus Logamatic because its so complicated that nobody programs it right anyway.

So at this point, I'm utterly confused. The guys I brought in are highly regarded so I have no reason to doubt their skills. However, I know how tough it is for salespeople to not bash competitors. So I've come to the wall for some more objective analysis of the situation.

Any guidance would be greatly appreciated!

Ron Schroeder

Hi Ted,

I am a big fan of the Buderus but lean toward Tekmar controls over the Logamatic, especially in a hybrid system.

Biasi are also good boilers. I would lean toward the B series because it has lower water content than the SG series.

The Crown Freeport is another good triple pass boiler to look into.

The Energy Kinetics is highly regarded by many and is the lowest mass I think. It is available in both a front access cleaning and a rear access cleaning version. Make sure you get the right one for the space you are installing the heating system. I am not totally fond of their external flat plate heat exchange DHW method since it takes one more circ (a bronze one) than either a normal or a reverse indirect. You don't have to use the flat plate heat excanger and storage tank with the Energy Kinetics, you can use an indirect.

I personally prefer direct vent over power vent but power vent can sidewall vent with longer pipes.

I cold start in the non heating season and have a high differential on the indirect for less frequent running of the burner. Of course, I use a tempering valve on the output of the indirect for safety. During the heating season, I don't cold start since the boiler is in conditioned space.

Theoretically, I lean toward a reverse indirect (inside out indirect) or a buffer tank if the loads are substantially smaller than the boiler's capacity. You could take all of the loads from the reverse indirect or buffer, using it as a hydrolic separator. You might also be able to use the de-superheater from the geothermal to the indirect or buffer tank. The reverse indirect is advantagious over a seperate buffer tank, especially in the non heating seasons since you can better use the de-superheater.

PJO_5

How about...

something like a Burnham LEDV (direct vent) with an Ergomax or Triangle Tube "tank in tank" for the DHW? This would give you the buffer tank you will obviously need with those small zones. The Ergo would have at least 26 gallons of boiler water, and the TT (also sold as Weil-McClain Ultra/Gold) would have about 6-12 gallons depending on size.

Install primary/secondary with the Ergo/TT as the "heart" of the system in the primary loop, and the LE as a secondary loop (along with all the zones of heat). The temp switch on the Ergo could control the circ for the LE if you want to have control on the boiler return, but then you would have it one the "end" of the primary loop and it would hold less BTU's. This piping arrangement would prevent the boiler from seeing any flow until the larger volume of water needs it.

There was a "Signature System" that Siggy wrote about a few years ago, and I re-did my own system similar to it two years ago. Can't find the thread though :-(

I would keep the electric HWH as a pre-heater of sorts for the Ergo/TT, and it would be a domestic back-up in a pinch if you forgot to check the tank level. You could put it on the "far" side of the indirect for extra tankage in case you have a monster tub or one of those new-fangled 20 gpm showers, but whatever you do don't pinch the piping down anywhere. Ask me how I know! :-)

I believe you can get a 0.60 gph nozzle, and at 87% AFUE (for what it's worth) that's not bad. The Ergo will still do you justice with the lowered boiler - especially with the water going up 10 or 15 degrees through the (existing electric) preheater.

I am not a contractor by the way...

Hope this helps. Take care, PJO

Ted Inoue

reverse indirect?

Hi Ron,
Thanks for your thoughts. I'm not familiar with a "reverse indirect". Are you talking about something like the Ergomax suggested by the other poster? Or is that really just a buffer tank?

By the way, would there be any advantage to using the Ergomax with one system or another?

With the Ergomax, would one then use an outdoor reset controller to modulate the storage tank temperature of the Ergo to the temperature needed by the fin-tubes and then use other methods to modulate the temperature needed for each radiant zone?

Sorry for the basic questions. I'm always learning.

Ted Inoue

Buffer tank with small boiler

PJO,
thanks too for the thoughts.

No room in my boiler room for boiler, indirect/buffer and my electric tank. I'm comfortable relying on the setup for everything.

I've got another question for you guys. Boiler sizes come in fairly large quanta. For example, the Burnham mentioned comes in 64MBH and 102MBH models. 102 is too much, 64 should be ok but could be on the edge if my calcs are off or the instantaneous load is higher, i.e. several zones running simulaneously.

The Energy Kinetics EK-1 looks like it can run 83 to 121MBTU gross, I'm assuming with different nozzles. I've heard that other boilers don't run optimally significantly out of their design point, so the EK seems pretty versatile in that respect.

A buffer tank would seem to solve issues of instantaneous demand load higher than boiler capacity. A super low mass boiler also seems to mitigate some of the issues of boiler oversizing. It runs, heats the water in the buffer tank and stops. Minimal energy lost since there is (almost) no standby energy in the boiler, just the buffer tank.

Have I got this all right?

Ron Schroeder

Hi Ted,

Yes, the Ergomax is one example of a "reverse indirect". Sometimes they are called "inside out indirects".

By having the boiler water in the tank and the DHW in the coil, they can perform just like a buffer tank.

So far, I have seen no disadvantage of using a "reverse indirect" like the Ergomax compared to using a conventional indirect other than possably cost and the need for a larger expansion tank. It seams that the performance is equal to or better than a conventional indirect even if not being used as a buffer.

The reset control would regulate the "reverse indirect" or the buffer tank to the needs of the highest temperature emitter.

Ron Schroeder

Hi Ted,

With a properly designed system, you should be able to eliminate the electric tank. Depending on the fittings on the indirect/buffer, you might even be able to add an electric heating element to the indirect/buffer as a backup but that would be a little overkill.

I haven't seen any boiler that can't be underfired a little and many can be under fired a lot. If setup and tuned properly, every boiler that I have underfired had at least a small improvement in efficiency.

A buffer tank of some kind and proper controls can greatly reduce the loss of efficiency that most systems see durring the majority of the heating season when the load is lighter.

The higher the ratio of thermal mass in the buffer/indirect to the thermal mass of the boiler, the closer you can get to steady state efficiency even at light loads so a low mass(low water content)boiler would have an edge since it wouldn't need as large of a buffer.

I have seen fuel reduction of up to 50% by adding a buffer.

By the way, I am currently doing some studies on using a conventional indirect as a buffer with some unique piping and controls. It's not as effective as a "reverse indirect" for thermal storage but I am seeing savings with it.