Boiler Short Cycling After Thermostat Down For Season - DHW Problem?

merikus

TL;DR: My boiler is short cycling now that it's warm out and the thermostat is set as low as it can go. I think it's an issue of the boiler aquastat competing with the electric DHW heater, but I'm not sure, which is why I'm posting here.

I have a steam system that also provides me with domestic hot water in the winter via a coil in the boiler that is piped directly into an electric hot water tank, pictured here:



As you can see there’s a couple of things of note here. First, there is an aquastat, which is set to 190 F. In addition, I have a circulator pump with a switch that says “summer” and “winter" (currently set to "summer"):



There's a wire that goes from all of these to a box at the top of my electric DHW tank. A few notes about the next picture, I highlighted the wire I am talking about in red. Where the wire goes out of the frame at the bottom of the photo, it terminates at the Oil Burner Controller. Where the wire goes out of frame on the top right of the photo, it terminates at a cutoff switch that cuts off the power to the boiler but not the DHW (the DHW can be cut off using a breaker in my breaker box). The circle is put around the circulator pump switch I pictured more closely above, and the wire goes through that switch to a grey box at the top of my electric DHW tank:



Both the upper and lower element aquastats on my DHW tank are set to 130 F.

So with the scene set, here’s the problem.

It’s finally warmed up here, and I set my thermostat down as low as it could go, as we no longer are using the heat. I read on The Wall that it’s not a good idea to shut down the boiler in the warmer months since it can cause problems, so I have power to both my DHW tank and my boiler.

The problem I’m having is that despite things being set up as I describe, the oil boiler kicks on rather frequently. I would say it kicks on approx. 2-4 times per hour. Each time it kicks on, it can be very variable, ranging from a high of 5 minutes to a low of 1 minute. This doesn’t seem right to me, and I’m at a loss as to why it is calling for the boiler to start boiling given that the DHW tank should be doing the heavy lifting when it comes to the DHW, and the thermostat for the oil boiler is set below 60 (and it is well above 60 here).

To that end, I have the following questions:

1. Does anyone know what’s going on here? Why is the aquastat on the boiler wired into the top of the DHW tank? And what about the summer/winter switch?
2. Should the aquastat on the boiler be set at the temperature it is set at (190 F) or should it be set at the temperature my DHW tank is set at (130 F)?
3. Why would I hear the boiler firing more in the evening rather than during the day (I work from home so I use the DHW throughout the day)?
4. Is it really bad to shut down a boiler during the summer?
5. I noticed that the coil that goes into the boiler for DHW has two cutoff valves (which would isolate the coil) and two drains (which would allow you to drain the coil). Should I use the cutoff valves on either side of where the coil goes into the boiler to stop the flow of water into the coil, and perhaps even drain that coil during the summer? My thinking is even with the circulator pump off, water will still circulate through that coil, and thus my electric DHW tank is still working to keep the water in the oil boiler, used for steam heat, hot. If I did this, what temperature should I set the boiler aquastat at to keep everything OK (if that is even suggested, as per question 4)?
6. During the winter, I had the electricity shut down to my electric DHW tank, thinking that would save me money being that the circulator pump was on. I noticed my electric bill dropped significantly after I did this, but of course I don't know if I'm losing those savings in oil. Am I right in the assessment to shut that off during the winter?
7. Is there anything else I might be missing here that would have the boiler run less during the summer?

Thank you all so much in advance!

Jamie Hall

Since you have an electric water heater... I'd just turn the boiler off. Won't hurt anything. What is likely happening is that the aquastat set at 190 is calling for the boiler to maintain... 190. So, of course, it cycles on and off.

Without tracing out the wiring, I'd not care to say what the summer/winter switch does. Most likely kills power to the circulator in the summer, and lets the aquastat run the circulator in the winter. But that's a guess.

merikus

Thank you so much for the reply, @Jamie Hall, your advice was invaluable when I was working out some issues during the fall when I first bought this house.

My initial impulse was to shut down the boiler in the summer, but I had two reservations, in order of importance:

1) According to this thread and a few others, shutting down your boiler in the summer could cause it to fail earlier. I'm worried about that because it is an older boiler, but a good one. Is that true? If it is--or if it's a risk--are there ways to mitigate that risk while still shutting the system down?

2) Since my DHW is piped into the coil in my boiler, in theory the water should still be moving through the coil throughout the summer, although as a slower rate since the circulator pump is off. Would it make sense to shut down those valves so that doesn't happen anymore? Or does the fact that the water is moving around in there basically address my concerns from above?

Thanks again!

Jamie Hall

On the summer shutdown... I can see the argument for keeping a boiler warm, sort of. There are actually two arguments, one of which didn't appear in that thread. The one which didn't appear regards potential corrosion, and is based on keeping the oxygen level in the water very low. There is some validity to it -- but if no fresh water is added to the boiler in the summer (which it wouldn't be, in steam, unless there is already a leak below the water line or in a wet return) that shouldn't be a problem. Any oxygen which was in the water left the last time it was run up to steam, and there will be almost no oxygen in the overlying gas (it would be mistake to call it air at that point!) either. The other argument was mentioned: that the boiler will cool (true) and potentially joints will loosen. This, however, has nothing to do with time -- if they are going to loosen, they will the first time the temperature drops. If the boiler was made up properly in the first place, that shouldn't be a problem -- both the forces holding it together and the changing stress from heating and cooling are well below the elastic limit of the steel and iron.

So I can't see it as a problem.

On the coil. Do I understand that your hot water system is piped so it always goes through the coil first, and then to the hot water tank? Or is it that there is a hot water circuit from the boiler to the tank (and a heat exchanger in the tank) and then back to the coil? In either case, I think I'd just let it be. If the coil isn't being heated (see above!) there's very little chance of scale formation in it, even with hard water.

Ctoilman

Ok, I think I see the installers strategy of a steam boiler/electric hot water tank combination.  First off, the boiler aquastat setting of 190 is ridicously too high.  That alone will make the boiler cycle a lot (wasteful and shortens equipment life).  So I'd turn that temp down to 150, maybe even 140.  Second, under the cover of that boiler aquastat should be an adjustable temp span differential.  To reduce cycling even further, set the white dial to at least 15 degrees, maybe more.  If the control doesn't have an adjustable differential, then this option is not available to you unless the aquastat is changed out to the usual Honeywell L6006.  Moving on, the firing rate as listed on the handwritten nozzle size and pump pressure is A LOT of btu's going into that boiler....like 250,000 btu.  My god, your home like 6000 sq ft?  That much fire is going to contribute to short cycling bigtime when there's no heat load (summer).  I'd suggest, IF applicable, less firing rate.  Now whether it's less money to use oil or electric for heating domestic hot water depends on the fuel costs.  Electric rates where I am, Connecticut, is 20 cents per kwhr when all the fees are added in ($233 for 1119 kwhr)...it's BRUTAL!!  Oil is wayyyy less money at $2.35/gallon.  And lastly, whew!, the aquastat on the electric tank controls the circulator, something has to tell the pump to run when getting heat from the steam boiler.  The summer/winter switch is only needed should the boiler be turned off in the summer.  I'd say that's not a good idea either.....save that for another post...LOL!

Jamie Hall

@Ctoilman -- " Electric rates where I am, Connecticut, is 20 cents per kwhr when all the fees are added in ($233 for 1119 kwhr)...it's BRUTAL!! Oil is wayyyy less money at $2.35/gallon."

Which is why, in the structures I care for, the hot water is generated by stand alone tank type oil fired hot water heaters (Bock). An option which seems to be very rarely considered.

We don't have enough money to be able to afford to go with the cheap solutions...

nicholas bonham-carter

I believe the boiler water level should be raised in summer, so as to completely submerge the coil.—NBC

merikus

Thanks again for your insight, @Jamie Hall. It seems that the best bet would be for me to simply turn down that aquastat so I don't have a lot of short cycling during the shoulder season (we're now under a Winter Storm Watch so the lovely weather we've been having is over!), and then shut it down during the summer once we're truly done for the season.

As for your question regarding how the hot water is piped, I am not 100% sure. I can see the water is piped into the hot water tank, and then I have a pipe running from the hot water tank to the boiler and then it completes a circuit (running through the circulator pump) going back to the tank.

Based on looking at the manual, I do not think there is a heat exchanger in the tank, as it is a traditional electric hot water tank. I think the water in the tank is running through the coil in the boiler and then back into the tank.

merikus

@Ctoilman, thank you for your take on this. I agree the aquastat is set insanely high, I'll be adjusting it today. As for the BTU issue, I have a very old, approx. 4000 sq. ft. farmhouse. I actually ran the numbers when I first bought the house, calculating the EDR of my radiators and comparing it to the size of the boiler. The numbers actually worked out for the power of this boiler. So far this heating season we've used 1,477.9 gallons of oil, though I'm not sure how many heating degree days we've had this season, so it's hard to say whether that's good or bad.

On the value front, I'm currently paying $2.10 (locked in for the season) for oil and about .18 per KWH. It sounds like I might be better off, from a financial standpoint, of setting my aquastat to 140 and then running my heat all summer off the oil boiler. Do you have any idea if there's a tool online somewhere where I could figure out what the tipping point is between using oil vs. electric for heating, based on what I'm paying for each?

Finally, you say "The summer/winter switch is only needed should the boiler be turned off in the summer. I'd say that's not a good idea either.....save that for another post...LOL!" Why do you think it's bad to turn it off during the summer--putting aside the financial question we've talked about above?

Thanks!

merikus

@Ctoilman, well, I answered one of my questions on my own. I found this tool online: https://www.efficiencymaine.com/at-home/water-heating-cost-comparison/

It's a little hard to make the calculation for me, because I can't figure out how the water gets hot--meaning, whether the water in the coil running through the oil boiler is piped directly into the DHW tank (what I think is going on) or whether it is an indirect coil in the tank. I have no evidence I have an indirect coil in the tank based on the manual left for me by the previous owner, and based on the fact that I was told that the electric DHW heater was added a few years ago as an afterthought when oil prices were very high (my understanding is that before that all hot water was just done through the oil boiler).

Anyway. Given I am currently paying .18 per KWH, this tool tells me that I would pay about $594 a year for hot water using electric. If the efficiency of my oil system is similar to an oil boiler with indirect tank, then my annual cost would be $185; if it's similar to an oil boiler w/tankless coil, it would be an annual cost of $268. Holding the cost of electricity at .18/KWH, then the cost of oil would need to get to $6.76 for the indirect tank efficiency, or $4.66 at the tankless coil efficiency.

Given that, does it make sense at all to use my electric DHW heater? I'm starting to think it makes more sense for me to set the aquastat to 140 and shut down the electric DHW heater until the economics of electric vs. oil are different.

merikus

@nicholas bonham-carter, that's a really interesting point. Why is that? I assume I can just look at the sight glass at the back of the boiler and compare the water level to the level of the coil?

bburd

@merikus An aquastat setting of 140° would work fine to preheat the water before it enters the electric tank. That is how most such systems are set up.

However, if you use the tankless coil as the only heat source you may need to set the aquastat higher, depending on the incoming cold water temperature. The aquastat has a differential, and most will only turn on when the tankless coil temperature is 10° below the setting. It takes a long time to heat water to 125° using a 130° heat source.

Ctoilman

merikus said:

@Ctoilman, well, I answered one of my questions on my own. I found this tool online: https://www.efficiencymaine.com/at-home/water-heating-cost-comparison/ It's a little hard to make the calculation for me, because I can't figure out how the water gets hot--meaning, whether the water in the coil running through the oil boiler is piped directly into the DHW tank (what I think is going on) or whether it is an indirect coil in the tank. I have no evidence I have an indirect coil in the tank based on the manual left for me by the previous owner, and based on the fact that I was told that the electric DHW heater was added a few years ago as an afterthought when oil prices were very high (my understanding is that before that all hot water was just done through the oil boiler). Anyway. Given I am currently paying .18 per KWH, this tool tells me that I would pay about $594 a year for hot water using electric. If the efficiency of my oil system is similar to an oil boiler with indirect tank, then my annual cost would be $185; if it's similar to an oil boiler w/tankless coil, it would be an annual cost of $268. Holding the cost of electricity at .18/KWH, then the cost of oil would need to get to $6.76 for the indirect tank efficiency, or $4.66 at the tankless coil efficiency. Given that, does it make sense at all to use my electric DHW heater? I'm starting to think it makes more sense for me to set the aquastat to 140 and shut down the electric DHW heater until the economics of electric vs. oil are different.

Before I installed a dedicated indirect tank I had the setup as you merikus....what you have is an aquabank setup and they work fine.  There is no coil in the electric tank, just pure circulation throughout the tank and into the boiler tankless coil.  In effect, it ends up being a tankless coil setup WITH a tank.  My circulator was actually activated by the electric tank thermostat so no electric elements were used at all.  It worked great until the electric tank got to be 16 yrs old and they typically don't last too much longer than that.  Heating water with electric is the most expensive way to do so.....seems like you have high electric rates which makes using oil instead a no-brainer.  If it were me, I'd turn off the 220 volt electricity breaker to the tank and heat the tank with the oil boiler.  Set the boiler aquastat about 150 and see how everything works out.  See if the there's an adjustable differential on the boiler aquastat as I mentioned too.