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Normally the tank takes the place of your closely spaced tees. The system pumps in and out of the tank as does the boiler. I have seen these piped and wired many different ways all of which were proper in their application.
I have also seen applications where it was not installed properly. Most common mistake is an aquastat on the tank to maintain tank temp but affects the ODR temp and maintains temp year around.
TK,
I was not aware that standard buffer tanks need/use an aquastat. I thought that buffer tanks are piped using the four ports and the cavity served as a storage facility. I'm guessing the boiler circ pulls from the tank to maintain flow for the boiler while the zone circs pull from the tank to satisfy the zone stats. I was thinking the aquastat was used when and if the tank functions as an indirect water heater. Am I way off base here ? Thanks.
Depends on the tanks and application as I stated earlier. I personally would not use an aquastat. Check out boiler buddy. They have a pretty good website for buffer tanks.
I assume this would mean only turning down the fan speed on the heating side and not the DHW side? How would I calculate what to set the new fan speed max to be?Originally Posted by tk03
Also if the rate of modulation is shipped at 3. How do I know if I should lower it? And should I lower it to 2 or 1?
When in doubt, go lower (for highest efficiency and fewest burn cycles), especially when you think/know it's oversized for the application. Start at 1 and only bump it up if it doesn't keep up.
David: There are buffer tanks that operate as "reverse indirects", with aquastat control, but that's not what we're after here. (ErgoMax, TurboMax, Everhot EA all make reverse indirects). Any insulated tank plumbed in series would do.
BoilerBuddy, or ErgoMax buffer tanks are essentially high-mass hydraulic seperators- useful in some system configurations but an expensive way to go for 1-2 zone simple residential systems. In a small primary/secondary system installing an (unpowered) electric tank hot-water heater anywhere in series with the boiler loop an appropriate (and far cheaper) way to add the necessary mass to the system in such a way that it participates in every burn. The electric tank adds only a very modest amount of head to the boiler loop, and won't affect pump sizing.
Normally I use the higher number for modulation. The faster setting is going to react to turn down faster. The fan speed is as Dana stated start lower and work in the direction you need to.
Remind me what the heat loss and boiler size you have. I can give you a good starting point.
Dana,
Does the electric tank have large enough tappings for the system piping? Aren't they normally 3/4"?
Dana,
I would be interested in looking at a piping diagram for the electric water tank. If a buffer tank sits across the loops, (2) supply outputs and (2) return inputs, how does a standard hot water tank work ? I'm thinking the electric hot water tank has (3) available ports (supply, output and drain). I have watched the "burn cycle" a few times on the boiler at different OATs. It looks like the boiler fires up on average 5-7 minutes. I am thinking that a 10 minute burn would be better, any comments ?
It does appear that the boiler is oversized but I think the min mod output is even with a one zone call. At a quick look, I have about 300 feet of 3/4" copper. At 2 gallons per 100' that suggest I have 6 gallons in the loop and less than one gallon in the boiler. From what I can gather, copper baseboard (existing) is about as low mass as you can get. I'm thinking the hot water slugging back does not help me out very much.
TK,
I plotted my way thur a heat loss calc (3 hours for somebody that does not know what they are doing) and came up w/ 70,000 loss. The boiler that was installed is the Alpine 150. From what I read, it has a min mod ouput of 28-30 K with a max of 120 K. I have two loops split evenly.
David1: The boiler only has one loop- putting the electric tank in series with either the output or return side (between the zone manifold & boiler) would allow the mass to participate in every burn, independently of how the rest of the system is configured. A short section of 3/4" plumbing on the boiler loop isn't going to present enough head to ANY pump to cause issue with operation of the boiler at the flows you'll be looking at. The 1" taps are only necessary to be able to run it on longer runs at full-fire without running out of margin on the safe delta-T. (see discussion below) The tank is a lot fatter than 3/4"- so you're really only looking at the length of the tank's dip-tube.
300' of fin-tube is capable of delivering ~75KBTU/hr with 120F average water temps, so with your zones are split evenly 150' would still be above min-mod, you should be able to run ~125 or 130F output water without inducing short-cycles, but you may need to reduce the flow (tweaked with a ball valve) to get it to actually RUN at mininum moduation. As long as the return water is ~120F or below you'll be in the 90%+ range. If it doesn't allready have a ball valve in series with each zone, adding ball valves to be able to back off the flow and tweaking each zone to run the boiler at min-mod when it's the only zone calling for heat is much easier & cheaper than adding a buffer tank on the boiler loop- I'd go there first. With that much fin tube in you should be able to achieve continuous burns whenever there's a call for heat (from either or both zones) and stay in condensing mode the entire time.
Combustion efficiency in condensing boilers is all about the return water temp- if you can get it to run 100-110F return-water into the boiler and 120-130F out you'll be in the mid-90s. I'm not sure what the max delta between output and return the Alpine is speced for, but 20-30F deltas are safe for any boiler. At 2gpm a 30F delta between output & return is ~30KBTU/hr, so that's a decent way to guage when you're balanced at or near mid-mod you are when tweaking flows. Even if you have to bump up the output up 140F out with 110F return for consistent heat in every room that's still fully condensing, but 140F out with 120F water on the return would be a few percent lower efficiency.
While 6 minute burns aren't exacting terrible toll on efficiency on a low-mass boiler, reducing the number of burn cycles by a factor of 2 or 3 saves considerable wear & tear, and you can probably get there without adding mass with 150' of fin-tube per zone.
Most heat loss calcs tend to run 25-35%on the high-side of reality- you may actually be closer to 50K. If you have a year's worth of fuel use to work against weather data you can get closer to the true design-condition heat load, but that won't much change how you set up the system. Average water temps still need to be in the 120F range for consistency with fin-tube, and that's more than enough to meet design-day conditions at 120F AWT.
Fin Tube Length
Dana,
I may have mis spoke .......
the 300' is the total run of copper on both zones. The emitters for each zone, (zone 1 = 60', zone 2 = 65') total 125 '. Is that what you understand ?
Ah- I mis-read that to mean 300' of baseboard.
To get ~28-30K out of it in 60' your talking ~450-500BTU/foot, which would usually require average water temps ~160F. The same 60' of baseboard should still be able to deliver ~10-12K at 120F water temps though, and if your heat load calc is correct that'll be close to the heat load for that zone roughly half the time, so you should still get a significant seasonal condensing benefit.
The cycling solution is to add thermal mass (buffer tank), then tweak the outdoor reset curves to where it's putting out ~150-160F water @ 0F outside. Then bump the curve temps down a bit if it's satisfies the thermostat in less than 30 minutes when it's 15-20F out. The burner shouldn't be cycling at all during thermostat calls at 25F or below if your heat loss calc & curve is correct. It might at warmer temps when the whole house load is below the min-modulated fire, but with even 10-15gallons of additional water to work with the burns should exceed 10minutes even when it's 50F out (very light loads.)
Adding thermal mass
Dana,
When the P/S piping was installed we took into consideration adding an indirect heater in the future. The take-offs are there w/ ball valves ( I pulled the handles off). If I am adding 15 gallon electric tank (unpowered), should it be installed just after the two return legs from the zones and before the return leg from the future indirect ? I'm thinking I do not want the indirect plowing through the (buffer tank). If I understand this correctly, only zone 1 and/or zone 2 will "use" the added thermal mass. When the indirect is added and the Priority is wired, the zones will shut down on a call for DHW. The indirect will fire up and bypass the buffer tank. Do I have it right ?
If so, does it matter how the tank is piped, inlet vs outlet ? I do not want to install it backwards. My guess is that it does not, just asking? I'm thinking the drain remains and the pressure relief is installed. Thanks again for your input !
Your fan speed for the Alpine 150 with a 70k heat loss should be turned down from 5500 to a starting point of 2800 rpm's and fine tune down from there.
TK,
The fan speed was reset to 2800 about two weeks ago. I have seen a marked improvement on the cycling. I have not adjusted the mod rate yet, still thinking about that. The min fan speed is set to 1300 (factory), when the gas valve modulates down it drops to 46% (min mod). I was wondering what I cabn do to increase the burn time to 10+ minutes. It makes sense that a 15 gallon tank adds the mass the mod con is missing. Reading back on my past posting, it probably does not matter if the indirect uses the added mass. Just thinking about the piping considerations. As long as the 15 gallon tank is installed on the return manifold and before the "closely spaced tee", I should be fine ? Right ?
I would drop the fan speed another 500 rpm's and see what that does for you.
Let's review again what the ODR info is including the minimum water temperature.
Last edited by tk03; 03-10-2011 at 08:32 PM.
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