Opinions on Reverse Indirect Water Heating

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  #1  
Old 11-11-20, 08:45 AM
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Opinions on Reverse Indirect Water Heating

My boiler is oversized and typically short cycles, burning just 3 to maybe 5 minutes at a time. This isnt all that surprising given I have 126k BTU boiler for an 1800 sqft house with 4 relatively small load heating zones. All zones calling = good long burn. Three or less zones calling = short cycling.

Short of redesigning the entire system, my research has found the idea of adding in a holding tank to increase the thermal mass of the boiler and prevent short cycling. However, my boiler also has a tankless coil for DHW. This has led me to the thought of doing a "reverse indirect water heater", like the Turbomax, which will act as a "large tankless coil" for DHW and a buffer tank to increase the boilers thermal mass. This way, I can go cold start, have a large coil that should be ample for the two residents at home, and buffer then boiler so it doesn't short cycle.

What are peoples take (or alternative recommendations) for this scenario and on this set up?

edit: I should add, the space around boiler is limited (two tank set up might be tough). Also, the system has glycol in in it due to long runs through unconditioned crawl space.

Thanks.
 
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Old 11-12-20, 07:06 AM
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I think that the tankful of water acting as a thermal mass will cause erratic domestic hot water termperature and will sometimes not accomplish solving the boiler short cycling.

The boiler cycle is lengthened by "sinking" the heat of the FHW in the tankful of water. Before this is accomplished the tankful of water will be supplying cold water to showers which is obviously not satisfactory.

Alternatively the tankful of water needs to be maintained at shower hot temperature. Then it won't absorb much more heat from the boiler trying to run a longer cycle and the cycle will stop short.
 
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Old 11-12-20, 08:30 AM
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Thanks, Allan. I am not sure I described everything correctly. Based on all the reading I have done, the reverse indirect seems like exactly what I need (but I could be wrong lol). I have another thread going (here) that kind of describes my issues.

Page 33 here shows and describes much better than me what I am thinking of.
https://www.caleffi.com/sites/defaul...nics_17_na.pdf (see figure 7-2)

Thanks again
 
  #4  
Old 11-13-20, 07:30 AM
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Is your boiler oil fired? If so, consider down firing the burner, you should be able to find firing rates that are specific to your unit. I had 110K BTU boiler in 1900 SQ ft home DWH coil too, same issue, short cycling. As per the operating manual I was able to down fire it from 1.1GPH to .75 GPH. I also added an Intellicon HW+ to it. HUGE savings on fuel and nice long burn times. Hot water supply was not compromised either. If possible, insulate the long runs and dump the Glycol, it's not an efficient heat transfer fluid.
 
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Old 11-13-20, 11:25 AM
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Thanks, Tom.

The previous owner did a gas conversion on the boiler back in 2018. She probably blamed oil for her high heating bills when in fact, it was boiler short cycling and her technician setting the boiler to maintain 200F 24/7 because he did know how to properly wire a new zone he installed back to the boiler and to tell it how to fire.

Since I have a tankless coil, I need to maintain boiler temp at 165F in the winter to get sufficient hot water. And, on our coldest days, 185F is sufficient to maintain the house comfortably. That small operating range results in short burns and wipes out any benefits from outdoor resets. I had looked at the Intellicon HW+ last winter but the thing was impossible to find. I ended up installing a Hydrostat 3250 plus; partly for its features and partly because the Honeywell stat I had on there was wildly inaccurate. The downside with the 3250 is the low limit differential is fixed at 10. This means short burns when maintaining boiler temp. The pre-purge feature is useful. I thought the economy feature was a good idea on it but it actually seems worse because it "intelligently" adjusts the high limit based on heat call frequency. This often results in, if anything, lower high limit temps which means more short cycling. Yes, I am not burning as much gas since high temp limit is lower, however, because my boiler is oversized, this just results in even shorter cycles.

Tom, were the longer burns due to the intellicon or the decreased firing? Since adjusting the burn isn't possible on my gas conversion, I would have to all come from the intellicon, which I am not sure what features of it could do so. I am not sure I see a controller approach to fix the thermodynamic issue that my system presents.

Regarding the glycol, insulating will be a lot of work. I assume I would have to use a fiberglass insulation and that would add up quickly. Based on the horrible set up of my system (all because the basement flooded once in 100 years so the previous owner relocated the boiler to the farthest part of the house), I would probably need a couple hundred feet of insulation; so I am not sure that's worth it economically speaking.

All that said, that's what lead me to the reverse indirect set up. It will allow me to go cold start and ad an ODR, if will decrease the short cycling and will give me steadier dhw since it will be a much much larger coil.

 
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Old 11-14-20, 06:51 AM
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One advanced feature of a reverse indirect system is a delayed cold start. The boiler can start after, as opposed to at, the same time as the thermostat call for heat. The system saps the heat from the thermal mass until the latter gets down to a minimum threshold temperature and then the boiler starts. If the thermostat call is satisfied before the threshold then the boiler does not come on at all for that cycle.

Now if the thermal mass is the domestic hot water tank then the warmth of your shower may be compromised occasionally.
 
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Old 11-14-20, 08:40 AM
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Keeping boiler temperature at 165 F to compensate for insufficient tankless coil DHW is expensive solution.

In winter to increase DHW capacity from tankless coil I use small Taco 007 circulator activated by temperature control aquastat to feed 50 gallon water heater. In summer, with boiler off, direct fire water heater.

During heating season Tekmar 256 out-door-reset aquastat keeps boiler minimum at 135F with hot water tank aquastat controlling Taco is set for 115F.

System has provided adequate DHW for years. Just replaced 18 year old, oil fired water heater with $500 gas fired one.

Valves allow either boiler or water heater to be used separately for DHW. Worked great when replacing heater.
 
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Old 11-16-20, 07:42 AM
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The longer burn times were obtained first by down firing the unit, when I put the Intellicon on, the burn times increased even further and that's when I saw significant fuel savings. As far as insulation goes, just go to your local big box store and get the black foam pipe insulating sleeves. They're cheap and do the job quite well, I've put them on most of my hot water pipes as well as most of the supply lines from my boiler including a run up in my unheated attic.
 
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Old 11-17-20, 06:43 AM
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Thanks, Tom. I never would have thought it before experiencing it but it wasn't a good thing that the old homeowner convertered the boiler to gas. Well atleast its easier to say when oil is so cheap.

Either way, I just noticed something. Something pretty obvious and dumb. The last owner's pro (or the owner?) elected to put gylco antifreeze l in the whole system. Several plumbing runs for heat go through unconditioned basement/crawl space and atop slab under the kitchen hardwood floor. I assumed this is why we are using the antifreeze mix. However, I just thought about it and adjacent to all of these runs are the hot and cold DHW feeds. The DHW lines ARE NOT insulated. They have also never frozen. In my mind, this negates the need for antifreeze in the hydronics, right?
 
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Old 11-19-20, 07:25 AM
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I think your on track with the removal of the antifreeze, if those domestic lines aren't freezing then you could pretty much conclude that your heating lines aren't going to freeze either. For piece of mind, put that foam pipe wrap on as much as you possibly can, it will also eliminate heat loss from those runs through unconditioned space.
 
 

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