Buffer tanks

[Deleted User]

I recall someone not to long ago mentioning using a indirect tank for a buffer tank. This job I am looking at has a real small heat load like 25,000 one zone yert. There is a munchkin 80,000 installed doing both the domestic hot and radiant. Slab is 100 degrees set by a mix valve and the indirect is set 125 degrees. I really would not like to have to add a buffer tank. I do think there is a way to plumb the radiant return through the indirect to cause a larger load.This would stop the munchkin from turning on and off so much when there is a call for heat. Thanks for any ideas around this, Mike

Mike T., Swampeast MO

Unless the domestic load is quite predictable and nearly constant I don't see how you could do that without using the domestic for space heating as well...

Even if you gave the domestic a BIG differential temperature setting to allow a "dump" for the space load if there is little/no use of the domestic (like at night) it won't be able to "accept" any extra heat unless you also raised supply temperature considerably. Even then you would wind up with "runaway" temperature in the indirect with periods of no domestic load.

I guess you might be able to pipe things such that the indirect COIL itself acts as the "boiler" to the system but I think that would be both impractical AND dangerous.

A "dual coil" indirect???? I believe such actually exists in the solar arena. Check with Dale Pickard at Radiant Engineering.

Mike T., Swampeast MO

If a commercial application

where you have a nearly constant (think close to 24/7/365) domestic load you could probably put the indirect and boiler heat in series in a constantly circulating primary loop with the space heat load in a secondary. Note that ONLY works (prevents the short-cycling you are experiencing) with a constant demand from the indirect.

[Deleted User]

Buffer tank

This is a res system. I have a 502 taco control with no prioity to the domestic load since the system can do both at the same time. I feel the radiant would bring down the domestic and start the domestic pump as well as the radiant keeping a larger load on the boiler. P/S 160 loop temp, mix on the radiant set at 100 for the slab.

Dan_8

Ergomax/Turbomax?

Is it already built or are you still planning?

If you put one of the 'inside-out' water heaters like an Ergomax or a Turbomax in there you'd get DHW and a buffer tank at the same time. Down side is that you wouldn't be able to let the Munchkin run down to really low temps but if you're running your primary loop at 160 anyway maybe that's not an issue.

Here's a link to a Siegenthaler article with a system running this way:

http://www.pmmag.com/pm/cda/articleinformation/features/bnp__features__item/0,2379,78589,00.html

Hope that helps...

Paul Rohrs_2

Buffer Tank Sizing

Precision Radiant Siegenthaler

This is the formula right out of the book. Hope this helps

First formula shower 10 minute cycle time with 30° Delta T.

Second is formula for 15-minute cycle time with 20° Delta T.

Third is formula for 20-minute cycle time with 30° Delta T.

It is easy to plug in minimum loads, cycle times, and Delta T's to see how it affects the tank volume. Residential will be pretty small.


PR

Mike T., Swampeast MO

Why

do you want to keep the primary loop so high in temperature unless ONLY the indirect calls for heat? Perhaps because you're using a FIXED mixing device to mix down for the radiant?

hot_rod

Is your concern

short cycling?

Also the new control would allow you to do the lower radiant temperature and the DHW within the control without a mixing valve. This would simplify things, perhaps.

Your 25K load is probably at design day? So the load will generally be smaller, most of the year?

Unless the heat sourse can modulate way down, to exactly match the load, you will have cycling. How much and how often is acceptable? Tough to answer!

Really, in my opinion, the buffer is the only way to efficiently solve this. Storing BTUs in a well insulated tank, without a flue through the middle, is the best idea.

Heat Transfer Products offers that 20 gallon buffer tank exactly for this purpose. There has been talk of them offering a stand to mount the Munchkin right to that 20 tank.

Buffering a condensing boiler is very simple, as you don't need to be concerned with low return temperature to the boiler. A simple series piping would work.

The back pages of the Munchkin installation manual should have some piping examples.

You COULD run a wide delta T on the buffer, say 100- 140, then use the radiant mix valve off that tank. This would really spread your cycle time.

If the buffer is piped in series that buffer may run up to 180 on a DHW priority call. Still not a problem if you keep the mix valve in for the radiant side.

They also have dual coil indirects in various sizes. For the cost difference, I'm not sure the basic non coil tank wouldn't address your concerns.

hot rod

[Deleted User]

Hot rod

I had built this system knowing the heat load was to small for the step fire of the vision control ( 35,000 btu ) I thought if I did not use the step fire in the vision control it would burn as low as 25,000 btu( this is the old control) but I see I might have made a mistake in my understanding of the 925. This is why I thought I would run the system at 160 and add a mix for the grante slab. 160 is for making domestic. If radiant runs through the tank coil it will only cause a load on the boiler as I could see and never over heat domestic for the return from radiant is never more than 100 degrees. The indirect pump will run sometimes but this would only be to bring the tank back up to 125. Like I had said I was also using a taco 502 without the priority. By the way, yes you are right, the vision control is the bomb, great control stratigy and I have enjoyed using it. Thanks for your help and I have enjoyed your ideas in the past. I have been around here keeping my mouth shut for like 4 yrs now.Thanks , Mike