Expansion tank for hot water loop off steam boiler?

jsavage

I am adding a hot water loop to my steam boiler for an indirect water heater with future capacity for additional hydronic zones. When run off a hot water boiler this would require an expansion tank, but does it if it’s a steam boiler and therefore open to the atmosphere?

EBEBRATT-Ed

If your using a heat exchanger between the steam boiler and the new hot water loop then ....YES you need an expansion tank.

If their is no heat exchanger then you do not need an expansion tank.

keep in mind the indirect water heater and any other hot water load can not exceed the steam boilers pick up factor

jsavage

@EBEBRATT-Ed Yes, I was planning on running it without a heat exchanger. If I add zones that could benefit from a heat exchanger later (ie > 30’ up), I’ll add one in along with an expansion tank.

Just to check my math, household Manual J and connected radiation are both around 200k. My boiler input is 385k BTU at 79.5% efficiency. So my output BTU should be 306k BTU, which would mean that the pickup factor is 106k BTU. Correct?

Thank you!

Jamie Hall

Partially correct on the pickup factor -- steam boilers are sized only with reference to the connected radiation -- heat load (Manual J) of the structure has nothing to do with it. Since you say your connected radiation is around 200K, I presume you measured it at an EDR of around 800 square feet? Then check the nameplate on the boiler for it's EDR rating and compare them. You can safely convert the difference into BTU/hr and compare that to your water heater or other loads. You can add to that 1/3 of the boiler's EDR rating, converted to BTU/hr, and get away with it -- if your steam pipes are insulated.

jsavage

Ok I’ll look for the EDR rating and compare it to my radiators and see where I’m at. Thanks!

EBEBRATT-Ed

@jsavage
What @Jamie Hall said is correct. You can use any "leftover btus" (difference between the boiler rating in sq ft edr versus the edr of the installed radiation +the pick up factor for any hot water load.

I would guess you boiler is good for about 1000 EDR or square feet

mikeg2015

Why is it so critical to live in the pickup factor? Why does if matter if for 5-10 minutes the boiler stops steaming or is only partially steaming? Could just set up domestic priority like a hot water boiler if that was such a big deal.

I ask this because the only way to get most indirect tanks below 80-120k BTU input is to either 1) reduce boiler loop flow rate or bypass more flow and end up with return water temp <120F or 2) use 2 pumps, which is ridiculous.

Plus on a vapor system at least, I contend you don’t need pickup anyway and can even undersized a little and use TRV’s. Coldest radiators will create a partial vacuum and self balance. It becomes more of a “pull” system rather than a “push” system.

Getting ready to add an indirect on mine this Fall and will probably use a DPDT relay to open the heating call and use only the aquastat heating call circuit. Then I can minimally bypass as well since it won’t be steaming while heating the indirect.

Unless there’s another reason? I will note that you will probably be limited to maybe 50% output on hot water since your using a supply and return location thats not optimal for hot water heating and the boiler isn’t filled with water.... although the only thing not wet is mainly the steam drum area. Steam boilers in hot water use only gain 1-2% efficiency.

Jamie Hall

It only become really critical, @mikeg2015 , in the sense that it will affect occupant comfort, at or near design conditions for the heating system -- which, granted, don't occur very often. Under those if the net BTUhr output of the boiler is less than the heating load of the structure the structure is going to cool off. No way around that. Similarly, it will affect how long it takes for the building to recover from a setback.

Now... if you are above design conditions, or the building is over radiated (a lot are) then it's less of a problem. There have been a lot of discussions over the years about controlling the firing duty cycle of steam boilers and how various strategies affect the overall system fuel use -- with no really satisfactory solid answers to the problem. Thinking about it, though, I think that I would probably arrange the control strategy -- assuming I wanted to get fancy -- so that I had essentially a two stage arrangement. If it had only a heating call from the hot water system, I'd use an aquastat to maintain the proper boiler temperature for that If the steam system were calling, however, I'd have two vapourstats for control. One, set lower, would be make on rise and would be in series with the hot water section thermostat. When the pressure rose enough to allow it to make, and the hot water thermostat were calling, it would run the hot water circulator. The second vapourstat (break on rise) -- set at whatever working pressure your system uses -- would shut off the burner, but not the circulator for the hot water.

Got to admit that I've never been really convinced about the radiators pulling a partial vacuum, though the will self balance to a certain extent. Why, Because if the trap is open (or in an orifice system, no trap at all) the radiator is exposed to the dry return, which (unless you have vacuum main vents) will be at atmospheric pressure, and thus so will the radiators. Just a thought.

mikeg2015

Show me a larger home with steam that isn’t over radiated anyway. Most were designed for coal so intentionally over radiated since the fire dies down. On top of that many older homes are high mass.

If a 250k boiler even runs a 50% duty cycle it costs roughly $25-30/day in natural gas to operate.

That all being said, I could see issues on a smaller boiler and smaller system like a row home or townhouse because it would be more dynamic due to the larger surface area to volume ratio.

jsavage

So just to follow up on this, here was the math from the boiler.

It says:
385k BTU input
288k BTU “for steam”
900 square ft EDR

Relating BTUs and EDR, 288k btu / 240 btu per sq for EDR = 1,200 sq ft EDR. 1,200/1.333 = 900 EDR. So pickup factor is 300 EDR or 72k BTU (as expected).

My house has around 200k BTU of radiation (or 830 sq ft EDR). So the boiler’s more or less properly sized and I have 88k BTU of extra capacity with the unused capacity and the pickup factor.

In weighing my options, I was looking at doing an indirect tank or a gas tankless HWH. I ultimately made the decision to go with the gas tankless as a result of looking at this math (thank you all for your help!). My reasoning was that the gas tankless I was looking at fires up to 199k BTU, is 95% efficient, and can modulate down to 15k BTU. The boiler has a lot of water in the system and takes maybe 20 mins to get up to temperature. It’s efficiency is 79.5%. Although I like the idea of benefiting from the boiler having already heated the water, if this becomes important, I can always buy a heat exchanger to heat the water before the inlet to the tankless. The gas tankless works out to be about 50% of the cost of the indirect fired HWH. Both access to gas and venting are very simple given the location.

... and finally, while I’d like to think that all of that logic was what led to my decision, my several-hour-long failed effort to remove the hot water plugs in the boiler probably had something to do with it as well!

Thanks Again!