Can you run a CI oil boiler <130 as long as you ensure hotter temperature afterward ?

Dave Carpentier

I have a Ranco tstat with it's probe strapped (and insulated) to the outlet pipe down inside the cabinet as close as possible to the exchanger. The Ranco controls the TT on the boiler so that my boiler operates 130-155 and the aquastat is basically a high limit set to 160. My slab tstats switch a relay that operate primary and secondary circs.
There are occasions where the Ranco will cycle the boiler even when there are no heat calls to keep it "always hot".
Could I run this at 120-150 (I think this Ranco is max 30deg diff) ?
There is never a time when the boiler doesnt hit the high temp, but the run time to get there would vary based on no-load or high-load.
My current no or light load run time is 5 mins.. pretty short.

Ironman

What brand and model boiler?

hot_rod

You are running some slabs off this boiler? How are you mixing that temperature?

Dave Carpentier

Its an Olsen boiler, circa 2000.

3way mixer is a Honeywell/Sparco. It seems to hold 110f.

My primary/secondary interface is a bit magical.
Picture a standard "closely spaced tees" setup - but separate the tees by about 18", and put the primary circ inside that tee space. The boiler's RWT is always pretty close to SWT, and heat gets distributed without issues to the slab. It was a strange mistake by my pipefitter buddy, but it seems to work fine.
So.. if I did a "cool" start each cycle, I think the RWT would track the SWT fairly close.

Adding: its a 115k (out) with a max load of 70k.

hot_rod

All thermostatic mixing valves need a hotter temperature on the supply H port then the mixed requirement.
Most brands and models show 25- 27 degree.

in other words, to accurately mix at 120 you need 145- 150 supply to the hot port. One HW model shows 180 is required on the hot side to mix to 120? So it depends a bit on the exact model you have.

So running the boiler at 120- 150 would present mix valve problems.

As for low temperature operation of a non condensing boiler, the key is that the boiler return rises above dew point before it shuts down. If not you may have condensation in the boiler and vent piping. This moisture combined with flue gas by products leads to low ph conditions which will eventually destroy the various metals, like the thin aluminum liner in b-vent flue pipe.

EBEBRATT-Ed

Rule of thumb on a cold start the return should not be below 130 for more than 20 min or so

hot_rod

This is a good read on boiler protection. Notice the corroded vent ell after only 1 year in service.

Dave Carpentier

Thanks guys, this is some great info.
Once we get into some actual cold temps where I have a decent heat load going on, I might fiddle and monitor.
I think I should try to get into that 1-wire monitoring stuff so that I could track my temps better.

mattmia2

why not put a thermostatic bypass between the tees to keep the return temp to the boiler up?

Dave Carpentier

The primary loop return into the boiler is always hot to touch.
The secondary loop where it joins to the primary is always cool.
I think I just got lucky with pump sizing.

mattmia2

Do you have the supply and return tees in the right order? The return should be upstream of the supply.

fentonc

@Dave Carpentier - I used 6 of these for my 'Boilertron' to monitor the supply and return on all 3 zones for my boiler. Very easy to wire up and get working with an arduino or similar. I put some thermal grease on the probe where it contacts the copper pipe, attached it to the pipe with aluminum tape and then zip-tied foam insulation around it. It's been running for a year straight without issues.

hot_rod

In my minds eye, a circ between the closely spaced tees seem like the boiler circ and that circ are in series? Essentially doubling the head, maybe resulting in low delta whenever the system runs.

I don’t see it providing hydraulic separation as the closely spaced tees allow two direction flows, depending on boiler vs distribution flow rates. The pump between the tees would prevent that.

Hydraulically speaking it must be working, thermo dynamically possibly not ideal.

Dave Carpentier

Here's the basic diagram..
The Alpha and the "P-1" come on together via end switches.
It never really occurred to me until just now that the air handler return has no boiler protection.
The AH typically only comes on once it gets quite cold, and long before then the slab zones are running (and thusly the P-1 is circulating anyhow).
Still, since the AH could run on its own now and then, maybe thats a good reason not to turn the Ranco down any further.

hot_rod

Primary secondary piping, to me, would be closely spaced tees no more than 4 pipe diameters apart, somewhere in the piping. I’m not feeling the P/S love in this drawing🥰?

You have an assembly of parallel loops, as I see it. I’m not sure the purpose of P1, a boiler bypass to accomplish what?

An excellent read here:

Dave Carpentier

Oh for sure I recognize the closely spaced tee thing now. 20+ years ago I didnt know any better so I didnt catch his mistake, and when we fired it up everything worked. Kinda fortunate that it doesnt seem to deadhead the secondary flow even when there's minimal flow (my looong garage loop). The "primary" loop stays hot, the mixer mixes quite well and the house heats.
Maybe we invented something.. lol

A, B and C always seem the same cool to hand-touch. D is always hot.

EdTheHeaterMan

I agree with @hot_rod about the need for P1. I wonder if P1 were to fail would you ever know it. It is one of those things that seems like a good idea when you installed it, but you may come to find it is not really necessary some day.

Dave Carpentier

I should clarify that there are only 3 circs in total.. the Alpha, P1, and the one inside the AH.

I think if P1 was removed (but the pipe was there), the house wouldnt heat or be poor heat as the secondary flow would go up the P1 pipe instead of thru the boiler.
If the P1 pipe was removed, the house would have normal heat, but the boiler would be receiving full return (cool) temps (and start condensing).

EdTheHeaterMan

So P1 is really not there?

hot_rod

I don’t think p1 or the pipe needs to be there. There is no primary loop in this piping. The radiant and or the ah would pull from the boiler

What is the boiler, again?

Dave Carpentier

The P1 is a Grundfos UPS 15-42 on low speed.
P1 was originally a Taco single speed. During some slab zone calls, pipe C would warm up a little, signifying a bit of backflow. The 15-42 on low doesnt do that.
Boiler is an Olsen OBC3