I am looking to get input on anyone that has a storage tank for DHW and also one for Radiant floor heat. I currently have a DHW storage tank that is only 50 gallons and works great and then about 4200 sq ft of radiant floor tubing demand. I am going to add a wood boiler to the system eventially since wood is available for free plus my time, and then add a large storage tank but am curious if anyone presently has a radiant system with boiler heating up a large storage tank and then using that water. I am using a condensing / modulation wall hung boiler that does a great job but runs more than I want it to. Any ideas OR better yet actual projects OR before and after examples would be greatly appreciated.

runs more than you want, or cycles more than you want?

with a mod/con, the ideal is 100% on time.

I am down with concept of efficiency and that is what I am after. In all due respect the design and outcome are not always together and therfore the efficiency does not always gel with reality.

Does anyone know of someone / project using Mod Con boiler to get a large storage volume and then use that storage for radiant heat. I have not done it but I feel it is worth exploring. The storage water would also not require full boiler operation just what it takes to keep it up to par

Any comments welcome. I realized going forward with the question would be criticized but I can take it.

This was a recommendation by a Mfg for the cycling issues many face with wall hung boilers that just want to keep adjusting and modulating and kicking on and off. It drives me crazy like I am abusing the equipment and it cannot help with life expectancy. The wood boiler guys have to use this method because they only have a short time to get all the energy they can from the BTU's so I realize it is completely different from an always available BTU source. Can we talk about it?

If the problem is cycling, then the buffer is the answer. Though Tom would say that starts to look an awful lot like a tank water heater, I would say the modulating/condensing combustion efficiency would be much greater with the mod/con.

I don't think you need large storage though. Just enough to run a little longer on the cycles. Seigenthalers' "hydronics design studio" program has a buffer tank simulator so you can play with storage sizes and expected loads.

You can get a demo off of hydronicpros.com I believe.

But just to be clear, the problem isn't that you're running too much, the problem is that you're cycling too much. That's why I asked.

NRT.Rob

Sorry did not follow your original point. It both runs to much and cycles to much but then again I do not want it to run much or cycle much so not much will please me. The cycling is the bigger issue to me. I cannot get in my head how a 40 second cycle does anything to heat water but I am just a dumb white boy trying to do the best I can. Rob I really appreciate your input.

Sorry forgot to ask this: what size tank if a tank is answer for cycling based on a pretty big system - 4200 sq ft. 3000 of which runs always. I would have to dig out the original design paper on BTU's etc.

I have his book but it is way to complicated for my little mind. Someone said use the DHW tank but I keep the 2 water sources apart so not sure what the hell they are talking about. I am just looking for a guestimite. The wood storage tank is in the 200 gallon or more range and efficiency is not a real concern for them. I am think SS 100 gallon and can add but looking for IDEAS. When you get really into the radiant I know 1 size does not fit all so that is why I was looking for an example. I do better with real world examples / pictures realizing they are not the same as mine. Not so good at formulas but I have his hardback $100 plus book, and I think software was in the back. I also attended his advanced controls course way back.

well I was playing with it yesterday, and it depends on the minimum firing rate of the boiler and the size of the load you are servicing.

The higher the load, the smaller the storage.
The lower the firing rate, the smaller the storage.

I was playing with super low load conditions (1000 BTUs/hr) on an Ultra (17k minimum firing) and found that a 30 gallon buffer would give me about 4-5 minute run times, and I think 15-20 minute off times or something like that... reasonable, I think.

The most efficient system runs non-stop. Multiple zones and oversized boilers create short-cycling.

I know you said you don't want to get into the math, but thats where your answer is. 1 btu to heat 1 pound water 1*. 8.33(??) pounds in a gallon (close enough), so at a 20* storage you would get approximately 1,666 btu per 10 gallons. A 5,000 btu/hr zone would be satisfied by 10 gallons for about 3 minutes. 30 gallons would be 9 minutes. This is off time of the boiler. Boiler run time is going to depend on the boiler, and the load currently on it.

If I need some more math done I will let you know. Your input is invaluable. Can you turn that into common speak for me though I did not 100% get it. I am a cookie in left hand milk is right hand kind of guy so go slow and be easy.

Thanks

Jeff, you might want to check into the settings on your controls. It's quite possible that some appropriate changes there could substantially reduce short-cycling.

Ok, different wording...

You have a 10 gallon storage tank (for simple math). You want to store it at 180* (as an example, you choose the temp). Once your tank heats up to 180*, the boiler goes off, and won't kick back on until the tank drops to 160* (20* delta T for this example). So, 10 gallons, stored from 180* to 160* will give you 1,666 btu's. 10 gallons weighs 83.3 pounds, and it would take 83.3 btu's to heat 10 gallons 1*F. So, your delta T is 20* (180-160). 20 x 83.3 = 1,666 btu. 20 gallons would be 3,332 btu.

Gallons X 8.33 (weight of 1 gallon) x DT = btus
10 x 8.33 x 20 = 1,666 btu

OK, I have 2 questions on this now.

First, can typical controls effectively manage a buffer tank for radiant? Or do you need additional controls.

2nd - Is this something that can be easily added later? I am going to have 11 zones, and am trying to size my boiler correctly, but the shoulder seasons are fairly long here, so excessive cycling much of the fall and spring could be an issue if smaller zones are the only ones calling for heat. I would plan on installing my system without a buffer tank, but would be interested in adding one IF it would be beneficial once it is actually running.

Any good rule of thumb for determining how much cycling is too much? (I guess that's three questions )

thanks

This message has been edited. Last edited by: Jeff - NJ,

If this is all low temp, you can either pipe the buffer in-line with the radiant return piping, the boiler return piping, or perhaps use the buffer as a big low-loss header instead of using regular p/s piping. depends a bit on where you are sensing water temperature. No additional controls should be needed, you are simply adding to the mass of the system to even out the firing demands a bit under low demand conditions.

The cycling question has traditionally been a tough one to nail down for sure. I'd be interested in hearing what some of our esteemed guests have to say about that as well, as far as current wisdom goes.

I don't know how much is too much, but there is one thing I know for certain...

As great of a metal as stainless steel is, when its put under the stress of expansion & contraction due to heat, it is going to eventually crack. It will, I promise. The old Hydrotherm HydroPulse boilers had a stainless heat exchanger built like a tank. They crack. Anyone thats been around a while can testify to that. Thats what ultimately kills them.

So, what I'm trying to say is, if you want your boiler to last XX years, the less it cycles the longer it should last.

Thanks for all the input. I follow the formula. Does anyone have a diagram of a system using a buffer tank? that works in a medium size system. Would like to see the diagram of the system for 11 zones Jeff NJ mentioned.