domestic hot water in oil boiler

This comes up a lot, and is often the reason for plumbing boilers in series. I fell like I need a better answer than just shrugging my shoulders and talking about how evil series installations are. Thinking out loud:

Given the price of oil and the inefficiencies of using a massive hunk of cast iron as your DHW storage, would a better approach be to kiss the oil DHW goodbye and put in an electric DHW tank with a sidearm and a mixing valve?

Seems like the ideal solution is indirect DHW, but that's an expensive option. Once you've plumbed your boilers in series, it's a lot of work to change down the line. The electric / sidearm approach would make upgrade to indirect much simpler, and my sense is that it would be a more efficient option even in the short run.

Perhaps the biggest issue is the effect of letting the oil boiler go cold. I've heard that some boilers don't like it, but I have no first-hand experience with any problems from doing that.

So, for all the folks out there who actually know what they're talking about, any thoughts or experience with this approach?

I think the easiest solution may be to put a DHW coil in the storage tank if possible. Then pipe the DHW with ball valves so you have a choice how to heat it. That way you could still use the oil coil if you needed to in the summer, but otherwise keep it off and the boiler cold. That way you don't have to go with a series hookup.

I have DHW coils in my Tarm, Oil boiler, and in my storage tank. This gives me a lot of flexibility. I have a series hookup that I am experimenting with, but will probably add the option of also being able to run the system as a primary secondary. All that would take is a couple of well placed tees and ball valves. Some changes to the controls would be necessary as well, but I haven't thought that far ahead. I too worry about letting the oil boiler get too cold. There are a lot of gaskets between those cast iron sections and as it ages the risk of leaks would increase. Just thinking out loud as well here.

I constantly struggle with the series versus parallel installation question- the possible leaks that might develop in letting it go cold is the one concern, and on the other hand the energy losses from heating a big/old unused boiler is a concern the other way. It also seems that in my situation if I went series than I could get away with smaller/cheaper pumps on storage and wood boilers knowing that the main circulator (which is already big) would also be doing a big chunk of the work. In a parallel situation it seems I would need 3 big, more-or-less equally sized circs to do the job.

To my way of thinking though, (and maybe I am missing something), other than the aggravation of hooking it up one way, and then switching, it didn't seem like you would need to do a whole lot of re-piping to switch from one method to the other? I suspect if I thought about it, with a few extra lengths of pipe, and 2 or more strategically placed ball values, you might be able to switch back and forth between series and parallel and just hook it all up at once be able to run it either way - anyone ever try this?

I like DHW preheat coils in storage - I've done it myself. The problem with that is there's a range of tank temps where preheat helps but is not quite enough to deliver satisfactory DHW temps. I run the preheated water into my DHW tank inlet, so that I can add more heat if necessary. I was thinking of the electric tank in the same way.

As usual, I'm looking for the simplest approach that will deliver hands-off operation and satisfactory results.

In cases where an indirect tank is not affordable or where there is a very rare need for supplementary DHW heating, my thought is that an electric tank might be the best way to go. Add a sidearm so that it gets heated whenever the wood boiler is operating, and a preheat coil if you have storage, and life is pretty dang good, I think.

I have a cast iron oil boiler that has operated as a cold-start unit for 20 years with no problems. They make a version with an internal DHW coil, but as far as I can tell that's the only difference. I hesitate to make a blanket pronouncement, but I suspect that many if not all oil boilers can operate as cold-start units without problems.

sawdustburners said:

i try to leave my cast iron boiler cold until i need hot water & then try to extract all the heat from the boiler so to avoid standby losses. no leaks after 4 years.

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I think that a lot may depend on the make, design, age, etc., of the boiler. A friend of mine bought an old house about 10 years ago that had a big cast iron monster boiler, in what looked to be in very good shape. It had a separate propane water heater, but I noticed that the oil boiler was set to stay hot all the time, even when no zones were calling for heat. I pointed out to my friend that he was burning oil all summer to accomplish nothing, and suggested that he shut the power to the burner off during non-heating seasons. several months after he took my advice, his oil boiler went toes up with a big failure in the joints between the iron sections, and the "pros" who came to look at it said that it was a goner- and told him that if he'd left it idling, and it had not experienced the thermal contraction of going cold after being at a constant temp all those years, it'd have lasted many more years.

who knows if they were right. my uneducated guess is that modern and semi modern boilers may have been built around the assumption that they may spend time cold, but the much older ones may not have been built with that in mind

The arguments I hear against series piping is, for me anyway, specious at best. For the most part, we're not talking about the boilers-of-old, with huge water contents & surface areas, but more likely very small, by comparison, size/water content (most are 3-6 gallons). The benefits of keeping the boiler hot are well worth whatever stand-by & chimney draft losses you'll have (an air inlet shutter can be added to most oil burners, to prevent heat loss up the chimney during standby). I see boilers leaking from every imaginable source , when they go cold. esp. @ tankless coil plates.
If we can't sell an indirect to a customer, we can give them one on the cheap by incorporating an inexpensive glass-lined elec. water heater w/ the boiler coil. using the lower t-stat on the tank to power a bronze circ pump. The boiler coil acts as your heat exchanger & you have 40 or more gal. of HW on standby. Works pretty good, as long as the coil isn't scaled up too badly, but, then again, they usually complain about running out of hw when the coils plug up.

Are talking about keeping the oil burner hot all summer long too? I could see where perhaps during the heating system the losses may not be huge, but to keep an oil burner up to temp all summer long to get DHW (especially if its oil), does seem like expensive insurance. When we are talking about letting an oil boiler go cold, are we really concerned about extended periods of time, or is even just the summer too long time to leave it cold?

MrEd said:

Are talking about keeping the oil burner hot all summer long too? I could see where perhaps during the heating system the losses may not be huge, but to keep an oil burner up to temp all summer long to get DHW (especially if its oil), does seem like expensive insurance. When we are talking about letting an oil boiler go cold, are we really concerned about extended periods of time, or is even just the summer too long time to leave it cold?

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No, I was talkng about the heating season (series vs. parallel connection of wood to fossil)
Yes, leaving the boiler idling all summer is wasteful. I installed solar at my house to eliminat that, for the most part. Only had to kick it on a few times during extended cloudy weather & right back off after showering, etc.
I have a 4 yr old Buderus oil & found a couple of valves leaking when it went cold. I've seen lots of older boilers leaking @ t-coil plates & between sections when shut off, for even a day or two.

We are going to run this winter with DHW in the Oil boiler and the wood boiler is in series with the oil boiler with its task to keeo the oil boiler hot at all times and keep its burner from kicking on. Next summer will install an indirect DHW heater and hopfeull a storage tank with the idea of being able to run the boilers in parallel and not keep the oil boiler hot. If we can fire the wood boiler every other day or so just for DHW with the small storage ill have that will be great. If that works then i will look into pulling the oil boiler out and go to a cold start on demand propane heater.

mpilihp said:

We are going to run this winter with DHW in the Oil boiler and the wood boiler is in series with the oil boiler with its task to keeo the oil boiler hot at all times and keep its burner from kicking on. Next summer will install an indirect DHW heater and hopfeull a storage tank with the idea of being able to run the boilers in parallel and not keep the oil boiler hot. If we can fire the wood boiler every other day or so just for DHW with the small storage ill have that will be great. If that works then i will look into pulling the oil boiler out and go to a cold start on demand propane heater.

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I'll predict that running your wood boiler in the summer for DHW will get old pretty quickly.

I'm retiring my oil boiler and going with a propane tankless for DHW and space heating backup.

Given the rough parity in $/BTU between electric and oil, I'm wondering if an electric is less expensive. I'd expect standby loss to be less than for an oil boiler, even a small one with a flue damper.

In our case, the oil boiler doesn't run at all(!) between October and May unless we go away on winter vacation, and only runs a couple of hours (in 12 minute bursts) during the summer for DHW backup when there's not enough solar.

I put a 42 gallon galvanized steel tank inside my 1000 gallon storage tank that my DHW goes through from the well. I also put a tempering valve on the DHW so I don't scold anyone. At the moment my tank is 172 degrees ( I fired last night) so that 42 gallon tank is also 172 degrees near the top. Works great.

My oil boiler has been cold start for 9 years now, no leaks.

I’ll predict that running your wood boiler in the summer for DHW will get old pretty quickly.

I am thinking you are right. I purchased a 50 gallon electric hot water tank off of craigslist last week to hook and use temporarily while I dismantle my boiler piping and the plan was to sell it once I was done. Even with a family of 6, the 50 gallon is providing with more and hotter hot water than we have ever had in this house and I really love not not listening to the oil burner kicking in all day long and all night long to stay warm.

So now I am thinking I will keep my electric tank, and take the summer off from wood burning and wood handling. Perhaps if electric spikes up in the years ahead I will re-think and re-plumb, but for $200-$300 (my estimate) per year in extra electric, it may be worth it.

MrEd said:

I’ll predict that running your wood boiler in the summer for DHW will get old pretty quickly.

I am thinking you are right. I purchased a 50 gallon electric hot water tank off of craigslist last week to hook and use temporarily while I dismantle my boiler piping and the plan was to sell it once I was done. Even with a family of 6, the 50 gallon is providing with more and hotter hot water than we have ever had in this house and I really love not not listening to the oil burner kicking in all day long and all night long to stay warm.

So now I am thinking I will keep my electric tank, and take the summer off from wood burning and wood handling. Perhaps if electric spikes up in the years ahead I will re-think and re-plumb, but for $200-$300 (my estimate) per year in extra electric, it may be worth it.

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Maybe some day you could add solar to cut back on electricity usage.....

Here's a system that I'm doing with a friend. His homemade panels circulate potable hot water. The circulator turns on whenever the panel top exceeds 160 degrees. In his case, Hot Water Tank 2 is indirect heated by a propane boiler, but electric would work too. We had to remove built-in chack valves that were in Tank 1 in order to allow reverse flow during charging from the panels.

Your probably right that it will get old but I can make time to cut and burn wood, I cant make money (any more legally) so burn wood is what we will try. Im looking into indirect hot water tanks that have two coils, one to be fed by Storage tank and a second fed from solar (in the future) Ol based products will only get more expensive and as people get off of oil/propane, other alternatives such as electricity will increase in cost as well. We dont even use an electric dryer, cloths get hung on the line and in the winter we dry them in the basement beside the woodstove (havent figured out how this will work with switching over to a wood boiler)

Im young enough now to cut haul split and burn wood, ill save $$ now so when im old and feeble I can pay to heat with oil and electricty.

~ Phil

Take a look at my site in my signature. I have a storage tank hooked up in series, but it is unpressurized. Simply remove the plate hx from the picture and pipe directly in and out of the top and bottom of the tank and it should give you something to start with. The trick is keeping it stratified. That is why there are two pumps pointing in opposite directions going into and coming out of the tank. Could also be accomplished with separate charging and drawing lines (ie 4 connections to tank instead of 2). Draw something up and post it in a separate thread. I am sure you will get some constructive feedback, though some here are not fans of the less efficient series hookup.

I think one form of boiler failure comes from condensation that develops inside the boiler on cool-down. There are super efficient boilers called condensing boilers that are designed to handle the moisture that can rot other boilers out (the combination of water and exhaust chemicals is pretty corrosive). From what I understand, most boilers are designed to be kept warm to prevent condensation and rot.

That said, I do shut my boiler down when we leave for more than a couple days and its been going strong for 6 years. I bet I just jinxed myself!

I've read this thread before and remember that it made me lean away from letting my oil boiler go cold. Now that I've considered the drawbacks of a series hook-up more closely, I'm leaning the other way. Now, I've just read this thread again and I wonder if the three day power outage we had with the recent ice storm is proof. The boiler went below 70*, no leaks. Or is this meaningless?

Rick

I believe what is bad for oil boilers is a cycling of wide temperature changes from hot to cold(off) and back over and over. I dont think turning the boiler off and leaving it off for a month or more and then turing back on will become problematic. We turned ours off in October, totally off and dont exptect to turn it back on until some time later this month when it gets too cold for use to keep up with the wood boiler alone as we are gone 9-10 hrs a day. House cools off quite a bit during then.

I bought a used Boilermate 40 gal unit for $200 and that now serves our DHW needs, I see them for sale often in Craigslist. My system is plumbed for series but while the Oil boiler is off I shut the valve going to the oil boiler and this forces the wood boiler to circulate through the boilermate whenever its running and none of the zones are calling for heat. Im working on a relay control panel that will automate the turning on/off of the oil boiler when the wood boiler looses heat and the house calls for heat with a dual temp range thermostats as NoFo did (he has some great ideas)

~ Phil