indrect as buffer for non-radiant system with outdoor reset

Rocky

I agree with Joe,
Unless you use a separate circulator for the indirect, you won't get priority. Ie, with a zone valve, the control cannot turn off the boiler pump or else you won't have any flow through the zone valve to your indirect. Therefor, if any other zone is calling, the boiler pump will push through that zone as well. PRiority can only be achieved with a separate circ for the indirect with a tekmar 260. Put in the 260 and I think you will find it will work just fine.
Warm regads from chilly Fairbanks,
Rocky

CC.Rob

indrect as buffer for non-radiant system with outdoor reset

I've been frequenting the Wall for a month or so. You guys have me hooked. Fascinating stuff. Prior to internet forums like this, all of your discussions and the education they provide about something so important to my home (and wallet) but totally outside my field would have been forever hidden, and I would have gone about my merry way with an inefficient heating system that could really be set up better. Well, as they say, knowledge is power. Between this forum and other research, I think I've gotten to the point where I can ask a reasonably intelligent system design question.

Here's the deal: have a 2100sf house heated by a conventional Burnham Series2 boiler (gas). System supplies 3/4" baseboard to two zones (1st and 2nd floors) via zone valves. System also runs a 40gal indirect water heater via a zone valve. One circulator drives the whole system.

Have been thinking about adding an outdoor temp reset with DHW priority (e.g., Tekmar 260) to this system. Previous conversations in another thread suggest some repiping, valving and buffering and/or isolation is appropriate. Recently read the Siegenthaler article in P&M (October 2001 -- linked in another thread) about using an indirect as a buffer tank. Neat idea.

So the question is: with my existing set-up, can I do what Siegenthaler suggests? (And whether the potential cost of doing so is less than ripping everything out and starting over with new boiler, etc.) What I'm thinking is to re-do this system similar to Siegenthaler's design. As I understand it, this would involve: 1) moving the indirect in between the boiler and the heating zone valves; 2) installing anti-scald tempering valve on DHW supply; 3) repiping from indirect to the heating zones using perhaps a 3- or 4-way mixing valve to keep supply temps per the Tekmar's decision on supply temp, and some diversion back to boiler to ensure return temps stay >=140F; and 4) wiring it all together with the Tekmar 260.

Am I even close here? In thinking this through a bit, it's actually no longer clear that if the indirect becomes the buffer tank I need DHW priority as this design just keeps some max temp around 180-200F in the indrect. Would a 40gal indirect be sufficiently sized to work as a buffer tank in this setup?

Obviously I'd want a pro to do this (who I would likely select from this site's Find a Pro service, although the closest couple people are ~30-60 miles away). But I figured a good first step is to pose this question to a knowledgeable group.

Thanks for any insight.

Joe Brix

A regular indirect wont work

Remember the tank is full of the pottable hot water, The heat exchanger has about 4-8 gal or boiler water in the coil. Not much of a buffer for the system. Reverse indirects are like the tankless coil idea. The tank has 20-80 gals of boiler water. A large copper coil sits in the tank with the domestic water running through it to heat up. That mass of boiler water becomes the buffer.

If you don't have a short cycling problem, Why do you need a buffer in the first place?

CC.Rob

OK, understood. Was unclear on regular indirect vs. reverse. What you describe makes perfect sense.

My recollection of an earlier thread was that a buffer might be needed to prevent short-cycling.

But maybe I'm making this more difficult than it needs to be. Would a simpler approach be to give the indirect its own circulator, use the existing circulator (Taco 007) for the heating zones, and have a mixing/bypass valve placed in the appropriate location to ensure proper supply and return temps? From what I understand, the 260 controls all this.

However, I just found something in the tekmar technical essays that make it a bit clearer to me. The literature on the 260 shows a very simple setup like described above. But I think what's probably appropriate is isolation of the boiler loop from the system loop. And it would appear that this can be done by valve or variable speed injection pump. The latter approach above appears to do this.

Sorry for any "stream of consciousness" aspect of this....

Joe Brix

A 260

has a temp sensor to adjust supply temp, doesn't know about return water temp. The outdoor reset would normally be set with a low limit supply of 140 and a max of 180 or so and vary the temp between that based on the outdoor temp. Better to have a seperate circ for the indirect. The 260 will just run the indirect pump when it calls giving priority to the indirect if you want. It then post purges excess boiler heat to the indirect afterwards if the boiler is over 160 dgrees for 4 minutes.

I say get the 260 first and vary the differential to cut down on any short cycling.

CC.Rob

cool. thanks.