Automatic staging of two boilers with different fuel types

GroundUp

I'm toying with the idea of adding an electric boiler to a property I own, to take advantage of $.03/kwh off-peak power they're offering. The old WM HE-5 that's in there now is on borrowed time plus is oversized anyway so I was looking at replacing it with a CGI-4 or a GV90+4 and redoing the piping, zone valves, controls, etc at the same time but then the off-peak rate came into play and that's about 1/3 the price of burning propane at current prices. So in theory it'd save me $1000ish per year in fuel, which would rapidly pay for the electric boiler and leave the HE-5 as backup for on-peak hours. I've got no concerns about making that work and am familiar with the metering devices used by the power company, BUT I've only got 100A service so I'd be limited to a 20kw boiler which is going to fall short on design days and would need the LP to step in for the heavy lifting on those days. Is there a way to set up an outdoor sensor so that maybe when it falls below -20*F that the LP would still come on while the electric stays running all-out during off-peak hours? Say the load is 85k on a design day and the electric is giving me 68k, I'd just need that little nudge to finish the job but otherwise the LP boiler would stay off (aside from on-peak hours). I'm obviously not a controls guru so maybe there is an obvious solution and Tekmar probably makes it, but I don't want to break the bank either if avoidable. Perhaps outdoor sensor through a simple NO relay to interrupt the TT signal to the LP boiler in parallel with the dual fuel controller/metering device?

STEVEusaPA

Couldn't you just do it with a 2H thermostat.

Jamie Hall

KISS. There are lots of ways you can do this, and they range from a couple of simple devices on up to oh my gosh that looks like it should have been made by Apple. What I would do -- and I'm a VERY simple minded chap -- is run the two boilers -- with suitable check valves and so on -- in parallel so far as the piping went. Then I'd control them with two thermostats, with the LP boiler's thermostat set a few degrees cooler than the electric. Then I'd put a simple time switch with a relay in the circuit from the electric boiler's thermostat, so that it was closed (thus enabling the electric boiler) at those times of day when the juice is ridiculously cheap and open -- cutting the electric boiler out -- when it wasn't (wish I had $0.03 electricity -- mind is 10 times as much no matter what time of day it is -- and is going up in January).

GroundUp

STEVEusaPA said:

Couldn't you just do it with a 2H thermostat.

Sorry, I failed to mention that this is 6 zones in an apartment building. So no.

EdTheHeaterMan

I am wondering if a storage tank is in order. like solar systems, when the heat is cheap, get as much as you can into a storage tank. Never let it cycle off during the low cost time. When the rate goes back up then use the stored heat from the tank until it is depleted, then turn on the gas (if needed)

You may be able to store 250° of heat in that tank at 20 PSIG.
Now we are getting away from KISS

GroundUp

Jamie Hall said:

KISS. There are lots of ways you can do this, and they range from a couple of simple devices on up to oh my gosh that looks like it should have been made by Apple. What I would do -- and I'm a VERY simple minded chap -- is run the two boilers -- with suitable check valves and so on -- in parallel so far as the piping went. Then I'd control them with two thermostats, with the LP boiler's thermostat set a few degrees cooler than the electric. Then I'd put a simple time switch with a relay in the circuit from the electric boiler's thermostat, so that it was closed (thus enabling the electric boiler) at those times of day when the juice is ridiculously cheap and open -- cutting the electric boiler out -- when it wasn't (wish I had $0.03 electricity -- mind is 10 times as much no matter what time of day it is -- and is going up in January).

I'm all about simple, but I failed to mention that this is an apartment building with 6 zones so the twin thermostat idea or even a 2 stage thermostat will not work here unless I'm missing something. I will not be touching the thermostats in each unit to double them or replace them, we need just basic 2 wire heat-only stats because, well, renters. The timer thing seems reasonable, except under design conditions I'd need them both to run simultaneously. Also, they would need to be in series with the electric first due to thermal mass/buffer for the small zones. Pricing remains 3 cents 24/7 EXCEPT when the grid is overloaded and they need to cut someone off. They can turn it off for up to 8 hours per day, but a maximum of 400 hours per year. The standard dual fuel rate is $.056 and that is up to 4 hours per day, 200 hours per year.

GroundUp

EdTheHeaterMan said:

I am wondering if a storage tank is in order. like solar systems, when the heat is cheap, get as much as you can into a storage tank. Never let it cycle off during the low cost time. When the rate goes back up then use the stored heat from the tank until it is depleted, then turn on the gas (if needed)

You may be able to store 250° of heat in that tank at 20 PSIG.
Now we are getting away from KISS

That's not how this power company operates. Thermal storage is a completely different animal and rate, plus I don't have the physical space for thermal storage anyway. The only thermal storage I will have here is the CI sections in the gas boiler.

ratio

Do you have any kind of outdoor reset, or is the boiler just one steady discharge temp? I'd pipe them in series, propane into the electric, put both the propane and electric temp sensors on the outlet of the electric, set the electric a few degrees higher than the propane, & use a timer to enable the electric during off-peak rates. The electric will run during off-peak times, and when it can't keep up—or we're on peak—the propane will automagically take over. You might need to play with the setpoint separation to keep the propane off while the electric is cycling.

Jamie Hall

Ah. Well, no. My idea wouldn't work. However, all is not lost. If you can find a thermostat which has a low enough range, however, there is no good reason why you couldn't mount that outdoors and, using a relay to invert its action (instead of on when it's cold, off) use that to control the LP unit in series with the rest of that unit's normal thermostat control. Since the electric rate isn't a time thing, there's no use for a times.

EdTheHeaterMan

OK then your design is simple. all the controls need to be in the boiler room. Are you currently using outdoor reset of any kind? Simple outdoor reset controls are common in commercial boiler rooms.


Set up the piping Parallel as mentioned previously. Set a control to measure the outside temperature on the north side of the building. Make sure it is not near any exhaust vents or other devices that will alter the sensor's measurement of the outdoor temperature. That control will make on temperature fall. Set it at the temperature that will bring on the gas boiler when it is below the temperature of the electric boiler capability.


You will also need a relay with normally closed contacts. That will be powered by the electricity that is going to be turned off when the demand event happens. When the electricity to the electric boiler is turned on, the relay contacts will open leaving the gas boiler inoperable until the outside temperature is below the set point. If the electric to the boiler is off, the relay contacts will close to allow the gas boiler to operate until the electric demand event is over.

Matt_67

Could you just use a Johnson controls a450 control or similar to make tt on the lp boiler when the OA temp falls below your setpoint? Then let the Lp boiler cycle on its aquastat.  I’m assuming there aren’t end switches on the zone valves.

EdTheHeaterMan

https://www.supplyhouse.com/White-Rodgers-230-22-Line-Voltage-SPST-Open-On-Rise-Temperature-Control-20-120-Degrees-F-Range-14414000-p

https://www.supplyhouse.com/Rheem-42-25104-08-Relay-SPDT-230VAC-coil

Your electric boiler should be equipped with a 24 VAC thermostat control system.

hot_rod

Can the electric boiler run without a circ running? If not pipe it to a 6 or larger gallon buffer tank. An electric water heater with two a couple 10kw elements would be ideal. I know you can get 6kw elements to fit. 10kw may be too long.

Boilers piped parallel, P1 on electric boiler buffer, P2 on LP boiler.
∆T controller, Sensor T1 on top of the buffer, sensor T2 on return piping from system.

If sensor T1 is 3° warmer than sensor 2, P1 is activated.
If temperature differential drops to 1° LP boiler and pump activate, P1 is dropped off.

If sensor T1 gains in temperature the electric contributes again. A 6, or larger, gallon buffer would solve short cycling also.

Basically the ∆T control determines when the electric is no longer keeping up. If or when it can cover the load by itself again, which sensor T1 determines, it comes back on LP drops off. So it may still pulse in some energy even after the LP kicks on. The larger the buffer, the less on/off cycles.

I think I sent you a ∆T controller years ago to experiment with on wood systems?

If the electric only runs at time of day rated, a basic digital timer with batter backup controls that.

GroundUp

ratio said:

Do you have any kind of outdoor reset, or is the boiler just one steady discharge temp? I'd pipe them in series, propane into the electric, put both the propane and electric temp sensors on the outlet of the electric, set the electric a few degrees higher than the propane, & use a timer to enable the electric during off-peak rates. The electric will run during off-peak times, and when it can't keep up—or we're on peak—the propane will automagically take over. You might need to play with the setpoint separation to keep the propane off while the electric is cycling.

The electric unit will have ODR, but the LP will have nothing including temp sensor. They would be in series, but electric before CI as I need the thermal mass to prevent short cycling. It's not a constant circ system so that whole "timing the peak" thing will not work for this application. There needs to be an external control with no guesswork or reliance on the other unit.

GroundUp

EdTheHeaterMan said:

OK then your design is simple. all the controls need to be in the boiler room. Are you currently using outdoor reset of any kind? Simple outdoor reset controls are common in commercial boiler rooms.


Set up the piping Parallel as mentioned previously. Set a control to measure the outside temperature on the north side of the building. Make sure it is not near any exhaust vents or other devices that will alter the sensor's measurement of the outdoor temperature. That control will make on temperature fall. Set it at the temperature that will bring on the gas boiler when it is below the temperature of the electric boiler capability.


You will also need a relay with normally closed contacts. That will be powered by the electricity that is going to be turned off when the demand event happens. When the electricity to the electric boiler is turned on, the relay contacts will open leaving the gas boiler inoperable until the outside temperature is below the set point. If the electric to the boiler is off, the relay contacts will close to allow the gas boiler to operate until the electric demand event is over.

No ODR, the HE-5 is a 35 year old CI boiler. This is not really commercial, it's just a 2500 sq ft building with 6 small apartments. What sort of temp control are you recommending? I can't really pipe it parallel because I need the thermal mass of the CI boiler to prevent short cycling of the electric, but that shouldn't change the control method. I have a very comfortable grasp on the off-peak control and typically utilize the circulator output through a RIB relay to interrupt the TT for the gas boiler, but adding the outdoor temp back into that mix has me befuddled again.

GroundUp

Matt_67 said:

Could you just use a Johnson controls a450 control or similar to make tt on the lp boiler when the OA temp falls below your setpoint? Then let the Lp boiler cycle on its aquastat.  I’m assuming there aren’t end switches on the zone valves.

Perhaps? I'm not familiar with that controller but I will do some research. Does it have an OA sensor? The ZV do have end switches and the new ones will as well.

GroundUp

hot_rod said:

Can the electric boiler run without a circ running? If not pipe it to a 6 or larger gallon buffer tank. An electric water heater with two a couple 10kw elements would be ideal. I know you can get 6kw elements to fit. 10kw may be too long.

Boilers piped parallel, P1 on electric boiler buffer, P2 on LP boiler.
∆T controller, Sensor T1 on top of the buffer, sensor T2 on return piping from system.

If sensor T1 is 3° warmer than sensor 2, P1 is activated.
If temperature differential drops to 1° LP boiler and pump activate, P1 is dropped off.

If sensor T1 gains in temperature the electric contributes again. A 6, or larger, gallon buffer would solve short cycling also.

Basically the ∆T control determines when the electric is no longer keeping up. If or when it can cover the load by itself again, which sensor T1 determines, it comes back on LP drops off. So it may still pulse in some energy even after the LP kicks on. The larger the buffer, the less on/off cycles.

I think I sent you a ∆T controller years ago to experiment with on wood systems?

If the electric only runs at time of day rated, a basic digital timer with batter backup controls that.

I'm going to need an hour or 20 to digest what you've said here, but thank you. I do have one of those controllers from you, yes, but it's being used on my shop system for BTU metering. I can't grasp what you're getting at with the buffer, but I definitely do not have space for it. This is all packed into a tiny closet with barely enough space for what's already in there, much less another boiler and a tank. Maybe I put the cart before the horse here and need to take a step back

hot_rod

Does  the electric boiler modulate? With 6 zones it will be tougher to keep it from short cycling. Thats why I tried to sneak in some buffer to the system

Do you want the electric boiler to run whenever it is able or only on off peak times?

GroundUp

hot_rod said:

Does  the electric boiler modulate? With 6 zones it will be tougher to keep it from short cycling. Thats why I tried to sneak in some buffer to the system

Do you want the electric boiler to run whenever it is able or only on off peak times?

I haven't narrowed it down to a specific model yet, but it will modulate to a certain extent. 2 of the zones are only 8ft of BB so there will need to be some buffer regardless, but I was hoping to use the mass of the CI boiler for that purpose since it's already there. I'd prefer the electric to be able to run whenever there is power, given the cost of operation in comparison. So during a design condition, ideally the electric boiler would be running wide open and the LP just kicking on for supplement if/when necessary.

hot_rod

Im not sure piped in series that you get much buffering. Warm up will be delayed by the mass if the CI, but once all the calls go away flow stops and the warm mass is stagnant 

If you could pull from the warm cast without the electric boiler involved, then you have the thermal storage to buffer

Either way the control logic still works. The delta t control is not looking at a specific temperature, just a difference 

so the system starts and SWT is 80 degrees supply is 65 or whatever

if the control sees the return keeps dropping it knows the boiler is not able to meet the load, so it fires the lp boiler. Both can run until the heat call goes away or the return rises 4 degrees and the lp shuts down

Maybe the electric can now keep up, maybe not. If it can’t the control will see system return temperature drop again and fire the lp
It doesn’t matter if the system is running 160 supply at this point, return at 145, the control just watches the difference to switch on.

think of how this control runs a solar thermal. When the collector is hotter than the tank by some temperature, say 10 degrees, the pump turns on
When the tank exceeds the collector by some degrees, say 8 degrees it turns the pump 

in your case the control just toggles the back up boiler on and  off

 

hot_rod

The 261 tekmar would work also. It may have more features than you need? ODR, rotation, exercise, etc.