dual fuel source

albo

I have a coal boiler with 5 automag zone valves and 5 thermostats. I have a gas boiler for backup which uses the same zone valves but has a separate thermostat controlling it's own circulator pump. Presently to use the gas boiler I switch off the coal boiler which opens the zone valves and switch on the gas boiler. So when using the gas boiler I have no control of the zone valves. When the gas thermo calls for heat, gas circulator pump turns on and pumps to all 5 zones.

I would like to be able to control the gas boiler independently, either by using the existing thermostats or installing 5 new separate thermostats. I am able to run new wire for the additional thermostats.
I can direct the heated water through either boiler or both boilers by closing or opening various valves.

Any advice would be appreciated.
albo

Jamie Hall

Seems to me that from the control standpoint this shouldn't be that hard. The thermostats control the zone valves. Their end switches would signal -- probably through a couple of relays -- both the coal boiler and the gas boiler that heat is wanted. In turn, their aquastats would determine whether to fire the boiler or not, in conjunction with your manual switch to one or the other. You might also want to incorporate in their a relay which would switch the gas boiler off if the coal boiler is providing enough, and hot enough, water. Where things get interesting is in the piping -- and I would very strongly suggest a variation on primary/secondary -- which connects both boilers to the manifold serving the zone valves.

albo

Thank you. There is only one aquastat. It is on the coal boiler. The gas boiler only gets used when I am away from house for long periods of time and unable to fire the coal boiler.

Jamie Hall

No aquastat on the gas boiler? Say again, please? I hope that there is, at the very least, a high temperature limit shutoff?

albo

there is a 30 lb pressure relief valve

Jamie Hall

A 30 pound pressure relief valve is not a substitute for a temperature and pressure relief valve, which is required for a hot water boiler.

If it just pressure relief, or if there is no high limit shut off for the boiler, you have an extremely unsafe situation and, at least around here or when I used to inspect, that boiler should be shut off and left off until the situation is remedied.

SuperTech

> @albo said:
> there is a 30 lb pressure relief valve


Thats a safety device installed in case the aquastat fails it prevents explosion. It's not meant to be used as a primary temperature and pressure control! You are operating the equipment in a very unsafe manner. Where are you located? I hope you are not putting other people in danger with this setup. Sounds like you need to scale back on the DIY stuff.

ratio

Sounds like you need a pro to go over everything & verify all the proper safety equipment is installed & operating properly. They should be able to set you up to change over fuels, with as much automation as you want.

I would not expect the system to operate differently depending on fuel source—you shouldn't be able to tell which fuel source is operating without checking.

Solid_Fuel_Man

Post some pictures of what you currently have please.

You may just be killing power to all the controls when in gas mode. Automags are usually normally open, power close.

albo

this system has been in operation for about 30 years. It was installed by a certified master plumber. Perhaps I am not explaining things properly. I simply want to be able to use my gas boiler more efficiently when I am on vacation, by not running the coal boiler circulator and pumping hot water through the coal boiler to zones that are not calling for heat. I thank all of you for your concern in this matter.

albo

I know automags stay open when there is no power, that is what I am trying to prevent. I want the gas thermostat to open and close the zones. If necessary I would install 5 new thermostats to control the five zones. As for power, when I shut the coal boiler down the circulating pump on the coal boiler is inactive but I believe the aquastat is still functional

albo

albo

zone valves above coal boiler

albo

gas boiler relay

albo

gas switch on left, coal switch in middle, aquastat and draft control on far right?

Jamie Hall

Your descriptions are a little unclear, however.

1. Does your gas boiler have a temperature and pressure relief valve, properly plumbed to it, certified for the boiler input?
2. Does your gas boiler have a high limit aquastat on it which will shut all power off to the boiler if the high limit is exceeded?

If the answer to either of those questions is no, I honestly don't care if the thing has been running that way for 30 years. It's in violation of several different codes and is an immediate hazard to life. It must be red tagged and the gas turned off until the conditions above are met. No ifs, ands, or buts.

After we get that straightened out, we can talk about modifying the control system so it and the coal boiler play nice together and the zone valves work as you would like. Not until.

EBEBRATT-Ed

GG I had that same CRANE boiler in my house. I think the control on the side of the Crane is an aquastat.

albo

The gas boiler does have a temperature and pressure relief valve. the coal boiler also has a temperature and pressure relief valve. The aquastat on the coal boiler dumps to all zones at a temperature that can be manually set. I keep it at 162. So if the water is circulating through both boilers, wouldn't that cause the aquastat to dump to all five zones that open when the aquastat engages no matter which boiler was firing and heating the water? If you have an aquastat that shuts off all power at a set temperature, wouldn't that cause the circulator pump to not distribute the overheated water through the 5 zones that open when the temperature reaches 162? I am not trying to pick a fight here. I am asking a question.

Jamie Hall

Actually the circulator to the zones and the boiler can be on a separate circuit, and that isn't a problem. The idea for the high limit aquastat is to kill power and gas to the boiler if the normal control aquastat -- the one set to 162 -- fails closed. They do. (The same thing is -- or should be done -- on steam boilers: there should be a manually resetting high pressure pressuretrol which kills power to the burner).

Your initial comment on the pressure relief valve was that it was set for 30 psi -- which is OK, provided it also has a temperature relief function which will open if the temperature gets too high (typically around 210). Perhaps I misinterpreted your comment.

The coal boiler should have a fail closed draught control for the temperature over limit (that is, the intake draught damper should close if the limit trips or the power fails), and its own T&P valve.

The idea with the T&P valve, rather than just pressure, is again safety -- the last thing you want is for the boiler water temperature to go well over boiling and then get a pressure release (even from the valve, never mind a leak), which will flash a good bit of the boiler water into steam -- which can get really exciting really fast.

Ideally there would also be low water cutoffs for the burner or the draught, as the case may be -- but for reasons which I do not understand they are not always required.

And you are quite correct in your thinking -- should one of the high limits trip, it would be good to open all the zone valves and keep the circulator running to help things cool down.

Now having said all that... it would not be difficult to set your system up for essentially automatic operation. The thermostats should control the zone valves. They, in turn, would close two relays through their end switches. One relay would be for the gas boiler and its primary circulator, the other for the coal boiler and its circulator. But not directly -- the one for the gas boiler would go through both an aquastat on the coal boiler output, and would only allow the gas boiler to run if the coal boiler output was too cold, as well as though the gas boiler's own aquastat and high limit cutout. This essentially sets your coal boiler up as the primary heat, but automatically brings on the gas boiler if the coal boiler isn't doing its thing.

Solid_Fuel_Man

Im really not following your description of what happens when you run the gas boiler.

Basically you turn off one switch for the coal and turn on the switch for the gas? If that's the case, then all your valves default to open and the gas just runs uncontrolled off its aquastat.

For clarification, each boiler has it's own aquastat, gray box on it. The silver box on the beam by switched is a switching relay most likely used to control a circulator.

You need to keep the zone valves powered and operated by the thermostats. Each boiler will have it's own circulator and a call for heat from the valves needs to be transferred to the boiler which is operating. This can be done with a double pole bobble throw relay. I like to keep things simple with a 3 way switch to select the fuel source which also would operate the relay and transfer the "TT" the respective boiler. Gas boiler being the normally open contacts as it's the backup source. That way you dont have a relay energized all the time the coal (primary) is selected.

albo

Yes, turn off one switch for coal and turn on the other switch for gas. As you stated that causes all zones to open. That is the situation I want to correct. Yes, each boiler has it's own aquastat. Silver box on the beam controls coal draft and circulator pump.

How do I install the double pole bobble throw relay and 3 way switch?
Thank you

Jamie Hall

Easiest way to do it is to take the power for the thermostats and the zone valves off before the switch for the coal burner, rather than after it. I have only a rather hazy idea of your wiring diagram, but I suspect if you go through it you'll find that the power for the thermostats and the zone valves is connected in after the switch for the coal burner. It should be connected in before that switch.

You don't really need either a double pole double (not bobble! Dang autocorrect!) throw relay nor a three way switch. In fact, you might not even want them, should you ever for some reason want to have both the coal burner and the gas boiler running at the same time. But if you did, the sequence would be power from the circuit breaker to a junction box. One line from there to the transformer for the thermostats and zone valves. Another line from there to the double pole double throw switch. A line from one side of that switch to the gas boiler, and one from the other side to the coal boiler. The end switches on the zone valves would be connected in parallel from one side of the transformer of the zone valves and thermostats, then to the coil of the double pole relay, then back to the other side of the transformer. One set of contacts in the double pole relay would be connected through the safeties to the gas boiler; the other set through the safeties to the coal boiler.

albo

Thank you. Remember, the gas boiler currently is controlled from a sixth thermostat. That stat wire runs directly to the gas boiler.

Jamie Hall

If you want the gas boiler controlled from the thermostats controlling the zones, wire it as I suggested. If you want the gas boiler to run from the sixth thermostat with all the zones open, leave it as it is. I would not set it up for the gas boiler to run from the sixth thermostat and the other thermostats to open the zones; sooner or later the gas boiler is going to turn on with no zones open -- or someone is going to ask for heat from the zone thermostats an be dismayed when nothing happens.

albo

Jamie, power comes into junction box, the power goes to the the switch that shuts off the gas boiler and the switch that controls the coal boiler. Coal switch controls coal aquastat via a 3 wire cable and and another 2 wire cable feeds the relay for coal circulator. That relay also controls the draft for coal.

The aquastat is 3 wire. Black wire feeds power to the Automag 40VA MB transformer which controls the 5 automag zone valves.

Where would the end switch be on the Automag 40VA MB? The 40VA MB has a R, W, B terminal blocks.