Buffer tank controls: aquastat with ODR, does it exist?

BBros

To solve a oil-fired boiler cycle length problem (low mass 80kbtu) with my constant circulation system (TRVs only; no indoor sensor or thermostats), I want to add a tank between the system loop and the boiler loop that signals the boiler to fire based on a outdoor reset curve. The boiler already has ODR (hydrostat 3250), but because of permanent boost feature, it doesn't work well without a thermostat signal. I'm running it with a low-limit that works fine for now, but as the weather cools and load increases, I'll have to manually adjust... which is obviously not ideal. I am also trying to avoid the excessive standby losses I'm experiencing right now.

Buffer will be set up four pipe using an electric water heater (I own a used 50gal). Boiler S/R at the element ports, emitter off domestic ports.

What's the best way to control this?

How it's currently piped, mixing is done with a Taco i-series w/odr:

hot_rod

If you want the best drawdown from the tank, consider running the tank to say 180, and pull the loads off the tank with ODR via the i-series valve. The boiler control sensor moves to the tank, if possible, or ad a setpoint control to do that. You might add some boiler protection if you pull the tank down below 130 and the boiler cold starts against that 50 gallons.


What kind of flow rates? 7-8 gpm? A two pipe tank give you some additional advantages, increase the element ports to 1-1/4 or larger with a reducer coupling and short nipple. Some additional reading about 2 vs 4 pipe here. Run some numbers for your actual load, then see what kind of cycling the tank would provide with the formulas.

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf

Add a wrap around the tank to insulate as best as possible.

A ∆T circulator would "wake up" when a TRV calls.

SWEI

You could move the existing ODR sensor to the tank, then run an ODR-controlled mixing valve or mixing block on the system side.

BBros

@hot rod

GPM just under 7.

I'm already running a dP pump (taco 1816), as the dT pump (taco 2218) that I started with went from low to max flow and back somewhat rapidly trying to maintain the delta against the mixing valve. Ended up with extreme flow rates and an unhappy system.

I can't move the boiler control to the tank, as it's tied in with lwco, so that will have to be an additional control.

So seems I'll need to add an aquastat and relay to the mix. Is there an aquastat with built in relay? I've seen circulators on indirect that have built in relays...

I can do boiler protection with the hydrostat (boiler) control, it will stop the primary circulator if temps get too low, though if the tank is maintaining 150 min, theoretically it won't get into trouble. Or I could run the dT pump as the primary, and it too has protection. Might be more efficient than putting a 007 or something there... and I already own it.

hot_rod

Any setpoint control could be the operating switch. I like the electronic versions as they have a lot of adjustability and a nice display. They can switch from 24- 240V, 15A usually.
Honeywell, Johnson Controls, most all brands of controls offer a setpoint controller.

If the tank never drops below 130, no need for any return protection function, sounds like you have low mass emitters.

BBros

I have high mass emitters, original cast iron radiators, so they're sized roughly 2x since the home has been insulated considerably. All run home-run through a manifold. Currently 6, will be 8 when the house is complete. Currently the mixing valve is running about 100F with 20 delta... but it's 40-50F outside. As noted, I anticipate 150F at design 0F.

@hot rod Appreciate the guidance. I've got a reasonable understanding, but much of this is new to me.

I'll look for a setpoint with 24v output then and just tag it into the T-T on the boiler control.

One last question though about the header for the buffer tank. If the boiler loop is mostly in 1.25 iron, should I go up to 1.5 just before the tank and tee off of that? I've got 1" copper running to the mixing valve. I gather the goal is to keep head-loss through the tank at an absolute minimum to prevent the system pump inducing flow through the boiler (as it does now).

billyboy

Tekmar 256, ODR sensor & tank sensor, with its heat request jumpered on. U program the ODR curve & tank delta.
I used this in my Sons new house & it works fine.

hot_rod

BBros said:

I have high mass emitters, original cast iron radiators, so they're sized roughly 2x since the home has been insulated considerably. All run home-run through a manifold. Currently 6, will be 8 when the house is complete. Currently the mixing valve is running about 100F with 20 delta... but it's 40-50F outside. As noted, I anticipate 150F at design 0F.

@hot rod Appreciate the guidance. I've got a reasonable understanding, but much of this is new to me.

I'll look for a setpoint with 24v output then and just tag it into the T-T on the boiler control.

One last question though about the header for the buffer tank. If the boiler loop is mostly in 1.25 iron, should I go up to 1.5 just before the tank and tee off of that? I've got 1" copper running to the mixing valve. I gather the goal is to keep head-loss through the tank at an absolute minimum to prevent the system pump inducing flow through the boiler (as it does now).

Those short connections into a 2 pipe buffer tank become the low loss header, or hydro seperator so to speak, make them as large as possible, keep tees close to the tank.

From Idronics 17