buffer tank ideas

hot_rod

the boiler modulate to the load demand, based on the delta t it senses? So if a small load draws from the buffer the boiler should cover it on low modulation? I think.

I've used ErgoMax tanks, with extra nipples welded in for that buffer application.

hr

scott markle_2

seeking buffer ideas (scott m)

I'm looking at designing a system for a very small load, probably around 18k.

Since most of the time this will be below the min. modulation of the Solo 60 (that I am planing to use) I have concerns about chronic short cycle.

I'm planing to use a reverse indirect as buffer and HX, the ground floor radiant loop that was laid with non-barrier pex (not by me).
If the budget permits the radiant will incorporate a motorized mix valve with ODR and indoor feedback (tekmar 360). The upstairs will have 3 panel rads with TRV's and and differential bypass valve. The radiators will draw directly from the tank and the boiler will feed directly to the tank. The non-barrier radiant will pull from the coil, with lots of rohmar in solution.

The tank it self will have a setpiont control with a wide differential probably ending up at around 100-140 on/off. I feel that this 40 deg. rise will provide a decent condensing runtime regardless of what the system is actually pulling.

Here is the issue if I set my boiler target at 140 , what will happen when a call for heat is generated by the 100 deg tank? My concern is that the boiler will modulate up to high fire to satisfy this demand, when a more efficient mode of operation would be for it to fire only as high as the highest anticipated system load of 18k.

Can anybody think of a work around on this? The vitodens has a potentiometer on the control board that will limit the firing rate on a call for heat. I spoke with TT about this and they said they had no such option. Any MCBA experts that might have an insight on this?

scott markle_2

I figure that 35 gallons with a 40 degree spread is 11,690 btus.

At around a 23k this would give a 30 minute burn in the condensing range. Just the kind of steady state operation I'd like to see. 24 on cycles a day.

I'm concerned that the boiler sees the mass of water as a large demand and ramps up to high fire. However even at 60k it's going to take around 10 min to make it to 140 from 100. and thats with no load from the system.

Mark Custis

Scott

If you control the flow the boiler will not see how much is in the tank.

scott markle_2

I considered this, but I'm concerned about boiler flow rates I'd be down around 2 gpm not sure the boiler would like this.

Mark Custis

I was thinking

pipe size, not pump curve.

Uni R_2

Fan Speed

The MCBA has parameters for maximum fan speed for both CH and DHW that can be changed through the installer parameters. In this case you would probably only need a maximum of maybe only 2100 rpm for CH. Then the boiler won't fire too high regardless.

In addition, lower maximums keep the sound level down and prolong the cycles and condensing. For 110s, a nice maximum fan speed for DHW is 3800. About 75 MBH and the boiler is much quieter than at 5300 and the flue temp stays much lower.

I don't know why T-T doesn't suggest these options to help people save fuel and less stress and duty cycles on the equipment. They warn users that these settings should not be changed from factory defaults in such a way as to protect themselves from anyone who might mess it up, but it really doesn't help these boilers run at their best. Floyd was the first one to get into the MCBA, but a capped fan speed is actually what you need. However, the manufacturer doesn't condone you doing this, even though it would seem to be best for a really low CH load house.

The parameters are different on the Solo 60 and aren't the same 42 parameters most other MCBA equipped boilers have.

As far as it short cycling, my 110 will only do that if I allow it only 1 80' ½" XPA loop. With a second open it won't... so it'll short cycle if doing a mild circuit air purge if the system has to be refilled.

The heat exchanger is the same for both models and is 2½" gallons. If the minimum modulation is 20 MBH net that's only 16* per minute heat rise, and would probably go 2 or 3 minutes. Surely any zone will have a bit of volume. If the zone doesn't then it really should be run off the buffer. On a proper zone or a few zones, the MCBA will run non-stop for a full 10 minutes and then the controller will vary the off cycle time to try and match the load and the manually set ODR curve. The Ultras tend to run a combined 10 minute fire to fire cycle but the Prestige fires for a full 10 minutes and then coasts. Kind of like pulse and glide for car drivers that try for really high mileage.

A lower budget way to do the low temp might be an iSeries 3way mixing valve from Taco drawing directly off the buffer. The zone itself would have its own ODR curve. If you want 2 temps, get another i Series 3way valve and pump. I think the 3 way valve and ODR are under $300. That and a pump.

On my Prestige I added a monoflo bypass on the return so part of the return is like a 3/4" monoflo branch. The partial flow allows the buffer tank to mix down the return temps for a longer portion of the burn cycle and store more BTUs between cycles. Any buffer tank directly on the return loses all of its coolest water right off the bat when the boiler pumps starts, the monoflow flow is quite low and it helped widen the delta T on my monoflo fintube system. The tank can be easily isolated if needed and it was the former electric tank so the price was right and piping it like this means the bottom 3/4" drain and the former 3/4" hot work perfect for the flow (no need to tap into the 1" element holes). The buffer wasn't needed but I had pre-piped it to be accommodated once the indirect was fully functional and crossed over (my heating guys weren't plumbing guys). Anyway, buffers should be on more systems, they do help.

scott markle_2

Thanks uni,

I was not aware of the i series valves. Thats a pretty unique offering. I actually like the simplicity of the reset ratio dial, you could even have a capable client tweak this them selves (constant circ.) Do you think the motor is robust and the valve well made? The price is so good it almost makes me nervous about the quality.

Any guess what that parameter number might be? I'll try and get this from TT but for what ever reason they don't seem to want to give this information out. Perhaps Its important for the burner or HX to see some high fire operation? If so my DHW demand should take care of this

My proposal is a bit different from the way a buffer would generally be used. Let the buffer tank run the boiler via a set point control with a wide differential (that is within the condensing range.) And mix down and use trvs to control output to the system.

I Also considered using a heat manager with this set-point control. The heat manager would see the rate of decline of supply from buffer and allow the tank to drop below is low setting when it indicated a very low system load, not so sure the programing of a heat manager would work in this application but the concept makes sense.

As building envelopes improve and people start to question large houses I thinK this issue of designing intelligently for low loads is going to come up more. The logic that say "it's going to be so cheap to heat this little place why bother with high efficiency" will change. Unfortunately todays best residential combustion technology is not really being designed for these types of structures.

UniR_4

Scan the parameters

The maximum fan speed on a 110 is 5300. The MCBA splits the limit for max ch fs, min ch fsm dhw fan sp, ignition fan speed. When scrolling through the parameters, there will be two times you will have a number like 53 appear immediately followed by 00 (these are the other 2 digits of the fan speed 45 then 50 would be 4550 rpm). Anyway, if you did drop one of those maxes, you just have to HOLD the "PLUS" button and then press and hold the MENU button until it brings the boiler up to a 10 minute duration high fire. You can see from the fan speed if the CH speed changed. Otherwise, twist the indirect's heat up and then watch for the fan speed on the heat call.

You can use whatever size pump you need. Hopefully very low wattage.

Yes, you buffer is the warehouse that takes the heat orders. This should be standard on anything oil using lower temp emitters.