Two boilers on one heating system

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Old 03-09-12, 03:30 PM
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Two boilers on one heating system

I have a 25 year old oil-fired Burnham tankless coil boiler with a Honeywell R8182H Aquastat plus two R845A slave relays controlling three circulators on a three-zone radiant heating system.

Iím in the process of installing, in a different location, a Pensotti boiler with an IDWH, a Hydrolevel 3250, and Taco SR504-EXP switching relay to control the same circulators plus the additional one for the IDWH. I have the piping installed so that with a few simple additional pieces I can tie the new boiler into the mains of the radiant heat.

I would like, at least for a while, to be able to use either boiler to run the heating system, and I have installed the necessary check and ball valves to accomplish that. That was the easy part. What Iím now trying to figure out is how to set up the power supply.

I fear that simply connecting the SR504 on the Pensotti to the power supply and feeding an extra set of wires from there into the circulators and from the thermostats into the SR504 and then switching off the power at one boiler before I switch it on at the other would let a lot of smoke out of the relays and who knows what else. So, Iíve come up with a way to isolate the two electrical systems, including the thermostats, with a bunch of DTDP switches. But I wonder if I couldnít accomplish the same thing with blocking diodes. If that were possible, it would considerably reduce the potential for disaster from absentmindedness and the other forms of human error I'm prone to. Unfortunately, I have no idea if that is possible and, if it were, how to go about rigging such a set-up and what bits and pieces to use.

I would be grateful for any comments and/or suggestions you might have regarding all this and, of course, if thereís any other way of skinning this cat.
 
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Old 03-09-12, 05:16 PM
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Boilers for the most part have control systems that are based on 24 VAC.
Diodes are going to change that AC to DC to a large extent.
Some new boilers with computer controls have 5VDC control systems that expect a dry contact closure only.

There are many simple ways to do this, one being a DPDT relay off the SR504, and just power what ever boiler you want to use.

Hope this helps
 
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Old 03-09-12, 05:21 PM
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Hello Vander,
I'd vote for the DPDT switches.

Better might be to wire the t'stats and the pumps to the SR504.
Then from an end switch on the 504, run to just one DPDT switch and then to each of the boilers aquastats.
But i'm not sure i understand the whole piping set-up.


Peter
 

Last edited by NJT; 03-09-12 at 05:59 PM.
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Old 03-09-12, 05:57 PM
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Yeah, you can't use diodes as 'blocking' devices on an AC control circuit.

I can't imagine for what reason you would want to retain the old boiler. Afraid that the Pensotti is gonna crap out ?

I'm with Peter... I would wire the 504 panel up to the t'stats and pumps.
Remove the 845 relays. You don't need them with the 504.

The endswitch wires from the 504 could go through a DPDT switch to fire up whichever boiler you got a hankerin' for on a particular day. Just one simple low voltage switch to wire up...

The endswitch on the 504 would wire to the 'arm' or the 'common' of the DPDT switch, then one side of the switch contacts would go to the T T terminals on the Hydrolevel A'stat, and the other side of the switch contacts would to to the T T terminals on the 8182.

But really... why?
 
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Old 03-10-12, 05:12 AM
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Is this oil fired with a Riello burner?

Is it an "emergency only" type thing? If so, you could use the alarm wire on the Riello burner to trigger a relay that fires the old boiler. I don't think you'd have to change anything else or dig into the SR504.

Just an idea..

Tim
 
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Old 03-10-12, 07:20 AM
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Thanks, Trooper and all, for keeping me out of trouble.

It must have been that Rube Goldberg syndrome (Why keep it simple if complicated is so much more entertaining?) which congenitally afflicts me that kept me from coming up with the approach you recommended. ;-)

Why do I want to hold on to the old boiler? The simple answer would be that I'm a stingy old geezer and just hate to throw away anything that is still working, if not perfectly, at least, pretty well. But, as you are by now aware, simple just doesn't seem to be my schtick. Furthermore, it's going to be a real PITA to get it out of the crawl-space it is tucked away in, and any excuse for leaving it there without having to answer embarrassing question like, "why on earth...?" is eagerly embraced.

The Pensotti became available to me at an irresistibly low price and, since the Burnham is getting on in years and is suffering from a lot of weeping around the tankless coil cover plate; which I may, or may not, be able to take care of; I figured I might as well install it so I can switch over to it without fuss should the Burnham crap out on me.

The Pensotti, however, is only a three section unit - the Burnham is four - and, although I'm quite sure it will be perfectly adequate for the three heating zones that comprise the central first floor of the house, especially since we are just two people most of the winter and also have two wood stoves in the same space, it may fall a little short on the coldest days of winter, or when we have visitors and open up the upstairs and out-lying portions of the place. When that situation arises it may be advantageous to switch to the larger boiler, if it is still available

I also have illusions that some day I'll get around to installing hydronic baseboards in the upstairs rooms and in a wing of the house that consists of a utility and storage area, a workshop, and a tiny apartment, and is now also heated with wood stoves when occupancy or other use so requires.
The Burnham , if still alive then, could be pressed into service for that purpose. If not, and if natural gas comes to this neck of the woods, a gas-fired boiler or water heater might do the job.

Reasonable? Probably not, but there you have it.

Thanks again!
 
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Old 03-10-12, 07:40 AM
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Wow, Tim, that's ingenious! But since I'm thinking of more than just emergency situations, I'll stick with Trooper, et al,'s recommendation.

Is this oil fired with a Riello burner?

Yes it is.
 
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Old 03-10-12, 08:22 AM
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Good enough reasons for me... since I'm somewhat in the same boat regarding hanging onto stuff that still has some life in it. Me frugal? naaahhhh, just CHEAP!

When all is said and done, stop back and let us know how it worked out for ya...
 
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Old 03-10-12, 11:19 AM
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The weeping coil sounds like it could be a problem. If it is leaking water from the boiler side of things, that means fresh makeup water is continuously being introduced. That equals constant addition of oxygen to the system. Something to consider. You can always leave the bears in place and have it isolated from the reast of the system. Then if need be, you can easily fire her back up.
 
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Old 03-10-12, 04:11 PM
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The weeping coil sounds like it could be a problem. If it is leaking water from the boiler side of things, that means fresh makeup water is continuously being introduced.
The weeping appears to be at the gasket and pretty much evaporates as long as the boiler is running. When the boiler is shut down it stays sort of wet around the edges of the cover plate, but not enough to require much makeup water. When I drain the boiler in the summer it stays dry even if I run water water through the coil.

I have to open it up to see if there's a remedy, but the bolts are so corroded that it will be a slow and careful process to get them out without twisting them off and. Even so I'm likely to get myself into trouble. That's why I want to have the new boiler with the indirect up and running before I try to fix the old one.
 
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Old 03-12-12, 11:35 AM
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Trooper, one more question. I had planned to wire the Pensotti the same way I have the Burnham: circuit breaker on house main panel -> manual switch near boiler -> Firomatic thermal switch -> separate conduits to Hydrostat and SR504.

I assume that with the setup you and Peter recommended (a DPDT switch after the 504 to allow connecting either to T T in the Hydrostat on the Pensotti or T T in the Aquastat on the Burnham) I should wire in a separate power supply to the 504, ahead of the manual switch to the Pensotti which, I assume, should be off prior to activating the Burnham, therefore, killing the 504. Iím also assuming that I should put a DPDT switch ahead of the 120v AC input to the 504 so Iíd be able to select power either from the regular supply via the manual switch and the Firomatic thermal switch or the separate power supply without causing back-feed of power into the Hydrostat.

In other words, Iím not sure if diverting the T T connection from the 504 to the Aquastat on the Burnham is sufficient to shut down everything on the Pensotti, making it unnecessary to manually switch off its power supply before switching it on to the Burnham.

I hope you can follow all this Ďcause Iíd sure appeciate your response before I go ahead with the wiring.
 
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Old 03-12-12, 04:07 PM
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I think I understand what you are saying...

There really should be ONE 'MASTER' Emergency switch that cuts off EVERYTHING. This should be located in a location that is easily accessible in the event you need to cut power fast for some reason. This switch should be RATED FOR 20A operation! Don't be tempted to use a 99 cent HD light switch for this purpose!

It's not real clear to me whether you were planning on running two separate circuits out of your breaker panel, one to each boiler, and I don't think that's necessary or desirable.

I THINK (NOT a professional electrician, mind you!) that it would be OK to run a single 20 AMP circuit (12 ga wire) and split that into two runs, one to each boiler, each with it's own switch. Please verify this is OK by codes before you do.

Where the problem would lie in using TWO separate breakers in the panel would be that it would be possible to get 240 VAC between the two hots of each circuit. Wiring stuff with two circuits can be vewy twicky, ya gotta be careful which circuit you are on.

With only one circuit feeding both, there's little fear of that.

As for the endswitch ... in the 504, that is a 'DRY CONTACT'. There is NO VOLTAGE coming out of the 504 on that set of terminals. It is only a contact closure which when applied to the T T terminals would energize the relay in the aquastat.

The T T terminals on either of the aquastat are 24VAC low voltage. The DPDT switch between them would be sufficient isolation there. No worries about 'backfeeding' anything. (as long as the switch is wired properly.)

I believe that simply flipping that single switch would be all you would need to do. NOT sending a contact closure to the T T on either aquastat would be enough to prevent it from firing on a heat call.

Your old boiler has a thankless coil, and as long as there is AC power to the aquastat, that will fire the boiler when needed to maintain temp. When not using that boiler, you WILL need to turn that power off if you don't want to keep it warm 24/7.
 
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Old 03-12-12, 04:10 PM
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What are your plans for OIL supply to each of the boilers? You probably need check valves on those as well.
 

Last edited by NJT; 03-12-12 at 04:31 PM.
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Old 03-12-12, 06:50 PM
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Thank you, Trooper. I think I understand what you are saying, but I may need a bit more time to digest all of it, and I may come back to test your patience some more. Meanwhile:

There really should be ONE 'MASTER' Emergency switch that cuts off EVERYTHING. This should be located in a location that is easily accessible in the event you need to cut power fast for some reason. This switch should be RATED FOR 20A operation! Don't be tempted to use a 99 cent HD light switch for this purpose!
Actually, there is and it is located just inside the door to what we, not without irony, refer to as a "basement." I didn't want to clutter up my already confusing question with mention of more bits of hardware.

It's not real clear to me whether you were planning on running two separate circuits out of your breaker panel, one to each boiler, and I don't think that's necessary or desirable.
No, that's not the case - one circuit to a junction box where it splits: one run to the old boiler, one to the switch upstairs, and where another will be added to go to the new boiler.

The switch I have near the old boiler and plan to have near the new one are mainly there to be able to switch off power to the boiler when I need to do minor work on it or its appendages without having to climb steps to shut off the power. Major and/or prolonged work involves flipping the honcho switch upstairs.

Your old boiler has a thankless coil, and as long as there is AC power to the aquastat, that will fire the boiler when needed to maintain temp. When not using that boiler, you WILL need to turn that power off if you don't want to keep it warm 24/7.
That is understood.

The information you have provided will save me putting in the extra wiring, at least two junction boxes, and one DPDT switch. Not to mention several sleepless hours around midnight tracing imaginary electrical circuits on the inside of my eyelids. ;-)

Thanks again!
 
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Old 03-12-12, 07:05 PM
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Thanks for the heads-up regarding the fuel line, Trooper.

I'm not that far along yet, but I'll probably run a separate line from the tank with its own valve so I can open and close either one or the other as required. The switch-over from one boiler to the other won't be a frequent occurrence, if it will ever actually happen. It all depends on what condition I find the old boiler to be in, but I want to have all the hook-ups in place in case it turns out to be worth the effort.
 
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Old 03-13-12, 04:05 PM
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Good... you'll run the 504 and the pumps direct off the 'honcho' switch, then split the power through individual switches for each boiler... sounds like a plan.

hours around midnight tracing imaginary electrical circuits on the inside of my eyelids. ;-)
Perhaps we should start a 'club' ? or at least have a 'syndrome' named for the condition!

Remember to use only FLARE FITTINGS on fuel oil lines. NO compression fittings allowed!
 
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Old 03-14-12, 06:30 AM
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Remember to use only FLARE FITTINGS on fuel oil lines. NO compression fittings allowed
!

I've got that covered. I've got a handy-dandy, British flaring tool that makes perfect double flares every time in one simple operation. Professional Brake Tubing Flaring Tool - Brake Flaring Tool - Brake Tube Flaring
 
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Old 03-14-12, 04:30 PM
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Generally we use single flare on fuel lines, but I can't think of a reason not to use double flare connections... never thought about it much though. Usually the double flare is applied to steel tubing (as in brake lines) for use in high pressure situations. I'm not sure if the soft copper tubing will actually 'take' the double flare, or if it will crumple instead.

Just looked at the website... says it works on 'soft metal' so that's OK, and does say that it will do a single flare, so I guess if ya have trouble with the double on the copper, just do a single and be done.

You got a classic that goes with that tool?
 
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Old 03-15-12, 08:19 AM
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I'm not sure if the soft copper tubing will actually 'take' the double flare, or if it will crumple instead.
It works. I've done it.

You got a classic that goes with that tool?
A 1962 Volvo PV544 and a 1971 Volvo 142E. Replaced the brake lines on both with copper-nickel last winter.

I suspect we may be getting off topic, but then, you're the moderator.
 
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Old 03-15-12, 03:29 PM
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Yeah, a little off topic... I was just curious... we won't go much further than to say that I sure wish I had me a 544 ... always loved them. I know it would be sacrilege, but I've always thought under that hood would be a great place for a Ford 289... A Duetta would be even cooler!

Ok, that's enough of that! Keep us posted on the progress of the install.
 
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Old 03-16-12, 05:51 AM
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What I have done a half dozen times where the owner wanted to keep a fairly new oil boiler and installed a new gas boiler, so they can run either one. Run the Zone relay end switch to both boilers and a two position center off switch. Whichever boiler is powered is the only one that runs. No need to do any changes with the zone panel other than wire to the second boiler. One position runs boiler one and the other position runs boiler two.
Using a two position switch will keep both boilers running at the same time.
 
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Old 03-16-12, 07:08 AM
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Run the Zone relay end switch to both boilers and a two position center off switch. Whichever boiler is powered is the only one that runs.
Thanks. That confirms what NJ Trooper recommended, and that's the way I'm going. I wired the DPDT switch in yesterday and am about to wrap up the rest of the wiring, except for the final connections. Those will have to wait till the end of the heating season and shut-down of the old boiler. Here in Maine, that is usually mid-May, but it feels like it might come a month earlier this year. However, we've been fooled before.
 
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