Condensing boiler with micro-zones

[BillTheEngineer]

I am looking for some input on using a condensing boiler when I am pretty sure I have zones that likely have lower heat output than the min fire rate of most if not all condensing boilers.

I trying to plan ahead so I don't need to scramble if and when my boiler and or water heater die.

First off I know I need to done measuring of my existing fin and tube baseboards to see how much capacity there is on each zone. And do a fuel use/heat load analysis.

But, I would suspect most of my zones are on the mirco side. It a 2300 sq-ft house (high ranch) with 3 zones (zip code 11788).

What I am thinking of doing is use a condensing boiler to heat an indirect how water heater. Add use the stored heat in the indirect hot water heater for space heating.

Questions I have:

1. Would the heat exchange within the indirect tank be sufficient to supply heat to warm the home? I know I would need to know my homes heat load, but is there a way to extrapolate what the heat output could be if the water in the tank were kept at 125°F - 130°F? (I chose this temp so the boiler would stay in the condensing range)
2. Each thermostat would control a circulator for each zone. The only trigger for boiler to come on would be from the indirect. Any thing I am missing that might cause an issue?
3. I prefer to set back my thermostats at night when sleeping. With learning thermostats (I think mine are already) is it likely that it may take a long time to warm up the house in the morning with 130°F water going though the fin and tube base boards? I am guessing there is sufficient heat output for most times but not sure how it will be for the extremely cold times. Is there somewhat simple way to adjust the indirect temp to adjust for the heat load?
4. Plumbing the boiler and zones so that water flows correctly I think can be done with use of check valves. I am thinking the flow resistance of the boiler will be higher than the indirect. And the circulator on the boiler to indirect will prevent/limit any flow though it when a zone is calling for heat and the boiler is off. In the case where a zone or zones are calling for heat and the boiler is also calling I don't think would be an issue (the boiler should modulate to supply ~130°F water, should not make difference if it goes to the indirect or zone).
5. Is there something basic or not basic that I am missing?

 

[Dana]

The flow resistance of fire-tube boilers is quite low compared to water tube boilers, and can often be pumped direct (no hydraulic separator.) You may or may not need a hydaulic separator, but with a 10:1 turn down boiler you can use a fairly modestly sized buffering hydraulic separator such as a Boiler Buddy as the buffering thermal mass, which is pretty easy to design with.

Are you sure you have enough baseboard to heat the place with 130F entering water temp at your 99% outside design temperature? The heat load of a 2300' raised ranch on Long Island is likely to run in the 25-30,000 BTU/hr range, lower if it's tighter and better insulated than average. It would take on the order of 125-150' of typical baseboard to emit that much heat at an entering water temp (EWT) of 130F. Run your fuel use load numbers and measure up the baseboards to get a handle on the storage temperature it would take on design-day.

You may be better off using a "smart" ECM drive pump operated in constant-pressure mode w/zone valves for the radiation, and a separate pump for the primary loop. A Grundfos Alpha1 can often be had for under $150 these days, and zone valves + Alpha1 are probably cheaper than three pumps. If you go with multiple pumps, circulation pumps come with an integral check valve option that can be used or discarded when installing the pump. At the flows you'll be using for micro-zones there is no need for a more rugged separate check valve.

If using an indirect as the buffering thermal mass, use a "reverse indirect" such as an ErgoMax or TurboMax as the central buffer. In this configuration the boiler's only "zone" is the indirect. The tank is full of heating system water, the potable water is in the internal heat exchanger. When combining the heat & hot water into one thermal buffering device you can't give priority to just the domestic hot water. At condensing temperatures for storage you may have to oversize the indirect or boiler to get reasonable tub-filling capacity out of a reverse-indirect even without zones calling for heat. I have a 48 gallon Ergomax set to 130F, which is just enough to fill a standard tub as long as the largest zone isn't calling for heat. But for showering it'll deliver a 105F shower all day and night with all zones calling for heat, with the performance boost of a 4" x 48" drainwater heat recovery heat exchanger.

If domestic hot water performance going to be an issue, use a simple buffer tank for the heating system water (an unpowered electric water heater can be a pretty cheap buffer), or massive hydraulic separator, and run a standard indirect operated as the priority zone for the domestic hot water.

It might be simpler & cheaper still to run the whole thing with a modulating condensing water heater such as the HTP Phoenix series (even the "Light Duty" version has a 76,000 BTU/hr burner- pick the tank size for the biggest tub you need to fill) and an external heat exchanger:

 

[Sponsor]

 

[NY_Rob]

Hi Bill....

First off... are you self installing or is a HVAC company doing the install?
I live about 10 mi east of you and had a heck of a time trying to find a contractor in our area willing to to replace my old cast iron boiler with a modern mod-con. And when I found one- they wanted to oversize it to the point where all efficiency would be lost. If you're going with a contractor- they generally don't take advise from the homeowner (even if you show them the math!).

Second... if you can keep it simple, you'll be much better off.
Is there any way you can combine some of the micro zones so they meet the minimum requirement of say 8K BTU's with 110F water? Use 160BTU/ft of fin tube element for your calculation.
There's generally no savings micro zoning four upstairs bedrooms for example when they can all be on one decent size zone that will let the boiler operate at low condensing temps.

Third... your radiators were sized to heat your home using 180F water, 170F AWT (Average Water Temp) radiates 510 BTU's per ft of heating element. A 2,300 sq/ft home could have 46,00K BTU's heatloss- that would need 90' of fintube at 170F to make up for lost BTU's at 15F outdoors. If you run 120F water (110F AWT) through that same 90' of fintube you now get 9'900 BTU's output. Your house will be very cold inside. You will need to add more radiation (where you can), replace standard fintube with low temp fintube and or run the mod-con at higher temps where it will not be condensing any longer and will be at or below 90% efficiency. Hopefully you'll be over-radiated like most homes and the numbers will be workable.

So, to get a better picture of what you actually need.. as Dana mentioned "Run your fuel use load numbers and measure up the baseboards to get a handle on the storage temperature it would take on design-day."


Regarding night setbacks.... I use a mild 2deg setback in our upstairs (four bedrooms + one full bath) and it takes about 2hrs to recover. I have the t-stat turn up the heat from 67F to 69F at 4:30an so the upstairs is up to temp at 6:30ish when we get up.
I don't use any setback on the downstairs zone at all- it's 68-69F all the time.
You're heating with 120F water vs. 180F water... it takes a long time to bring up the heat if you use setback.

 

[BillTheEngineer]

Dana & Rob, thanks for all you input and recommendation.

I can do the install myself, as long as I plan it for the non-heating season. I know that anything I do is going to take longer than plan for and going to take a few more trips for all the parts.

I know I need to do some measuring of my baseboards and determine how much heat they can deliver.

How efficient are condensing boilers when operated in a non-condensing range? Are the same/better/worse than a non-condensing boiler?

If I find/figure out that ~160°F (just a guess) water is needed to heat the home for the better part of the heating season a condensing boiler does not make sense correct?

I have done a fuel use heat load in the past and I think I was in the 40k btu range (i'll have to look for it and redo it for one of the colder winter periods). I have 135k btu (input) boiler that has 2 stages, I think output is 110k and 55k btu. Currently it's only the lower 55k btu, it would kettle on warm startups so I pulled the wire off so the second stage doesn't fire. Even with the 55k btu output it keeps the house warm. I do have to watch on the arctic blasts to not turn the thermostats back to far or at all otherwise it does take some time to get the house back up to temperature. I am pretty sure it's baseboard limited not boiler output limited.

 

[Dana]

In the non-condensing range it'll be about 85-86% efficiency at the maximum temperature output of a mod-con, about 87-88% efficiency when the entering water temp at the boiler is just above the condensing range. Which is to say they will be at least as efficient in terms of raw combustion efficiency as a cast-iron boiler, but probably more efficient as-operated due to the higher duty-cycle achieved from modulation, with less abandoned heat lost to standby between burns.

Even if the boiler needs 160F water at the 99% outside design temperature it's unlikely to need more than ~130F at the average winter temperature, which if properly dialed-in would be in the condensing zone. If that is your situation it's definitely worth getting a condensing boiler and setting it up under outdoor reset control rather than a fixed-temperature output.

If you are using gas to heat the domestic hot water and a fuel use calc showed 40K during a fall or spring time period it's likely that a winter period fuel use based load calculation will come in under 40K, since the size of the domestic hot water error will be smaller. A 40K load @ +15F for a 2300' house on L.I. would be on the high side, indicating perhaps a fair amount of low-hanging fruit remaining on the air-sealing and insulation front.