Another boiler sizing question.

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DrPcFix

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I just refinanced my home and have a few dollars left over which I plan on spending on updating the boiler and DHW.

Home is 5,000 sqft with spray foam insulation installed in 2008 (2x6 walls), including rafters.

3000 feet are Warmboard radiant with remainder baseboard and cast iron radiators. I installed Tekmar controls to run everything and have 14 heat zones (9 radiant, 1 baseboard, 1 cast iron (2 bedrooms), basement cast iron radiators (don't use really) and 2 coils in AC units which never get used (put in just in case the radiant was not enough to maintain temps -- but it does -- worked real well until I got the radiant hooked up!

According to last winter's billing, I used 552 Therms over 61 days (Jan-Feb) with a total of 1704 HDD.

Current boiler is a 200K monster cast iron (hot water) WM which I know is grossly oversized.

Design temp for my area is 15 degrees.

Based on the above, If I'm understanding what I've read, I calculate a need for only 54K BTU boiler on the replacement which really scares me as you all know what will happen if I put one in and the house is ever "cold".

I'm also interested in replacing the hot water heater (gas fired) 75K unit as we frequently run out of hot water when everyone is trying to shower in a short period.

So I'm thinking of the following:

Burnham ESC4 for heat and a tankless heater for DHW
In addition, due to having some very small radiant zones (2K BTU), I'm thinking of also adding in a buffer tank of 40 -50 gallons.

Alternatively, I could upsize to a larger ESC5 or ESC6 and a dual coil tank for both buffering as well as DHW?

I'm posting here because none of the local guys will do anything other than eyeball the house and offer to put in another 200K unit.
 

Jadnashua

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IMHO, for most of the northern half of the USA, a tankless system for DHW is either a mistake, or an expensive proposition. If you have a cold-fire capable boiler, an indirect WH is usually a much better idea.

Unless you're in a situation where you are using lots of hot water all throughout the day, you do not need to upsize the boiler to reheat that water tank as you make the IWH a priority zone.

Dana's better at running the numbers, but 54K sounds about reasonable to me for a well-insulated house.
 

WaldenTonic

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Don't want to steal the thread but for the sake of not starting another duplicate post...

I am thinking to do the same thing just in a new construction.

I have 1000 sqft of inslab radiant in a 2000 sqft home. All the btu calculations i have been able to make have come out around 44k BTU for the heating load for the house. no engineer here. am planning to balance loads with flow controlling valves but thats aside the point

I am interested to use boiler in primary secondary configuration with a second primary loop with priority for a IDWH which is currently 30 gal in the design.

I have narrowed it down to a 56k Weil mcclain to run everything and i cant see any reason why this is a problem. I am thinking to go directly to the floor with the boiler water through a mixing valve. Eventually i would like to use an indirect tank as a buffer tank for the floor mostly so i can use solar panels also to heat water with when possible.

To anyone with experience:
Does this seem like a good size on the boiler? considering a well insulated home.
should i upsize my btu load based on the addition of the water heater?

Thanks for any additional information in this thread!
 

Jadnashua

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Unless you're running a spa where you need copious amounts of hot water all through the day, it is rarely necessary to upsize the boiler to heat your indirect.

Is the boiler you're thinking about able to modulate its output? You'll rarely need the max, and things are more efficient running constantly at just the right output verses cycling on/off.
 

WaldenTonic

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Thanks for the reassuring reply.

Yea im looking at, what seems like, one of WM's more sophisticated condensing boilers - http://www.weil-mclain.com/products/wm97-ct-wall-mount-gas-boiler

My understanding agrees with the benefits of no short cycling hence i narrowed it own to that one(only slightly oversized). also it can modulate and control 3 pumps and heating curves independently.

i decided to go with a 30 gal IDWH instead of the 11 gal add on unit they are trying to pair with this boiler for that same reason as well. To me it seems like this thing would heat 11 gal in a heart beat, but maybe those WM guys know something i do not.

I have no experience with boilers; but i think i have a fair handle on this project. i just want to make sure my boiler is sufficient which it seems as though it is.

A few more questions if anyone wants to jump in:
From what i can tell using a buffer tank for radiant is inefficient unless you are taking advantage of multiple heat sources, Is this true? Im having problems reconciling the fact that people say to not have cold water return to the boiler but is that really avoidable in the first several minutes of boiler firing when cold plumbing is being filled with hot? i suppose a mixing valve can mitigate this but if the run is long or the plumbing to the IDWH is significant then cold water is bound to hit that heat exchanger at some poinbt after fire. i guess im just looking for some reassurance here

Might be hard to determine from the information i have provided but... Would anyone see it as a possibility to add a radiator or two to this system in the future? or would this system be tapped on the floor?

From what you say jadnashua i might have some slack in those heat loads and the boiler will be able to do more than i think. But considering boiler modulation and variable speed pumps that are basically designed to under throttle the system. Im curious how it will actually look when installed i.e. will the boiler basically cycle all day if well matched with floor? or is it significantly less than that.

Thanks for any responses!
 

Jadnashua

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Under ideal conditions, the boiler would run constantly, just meeting the needs of the structure. But, that is pie in the sky wishful thinking. If it did occur, you'd have the most comfortable situation. By using an outdoor reset and an adjustable, modulating boiler, up to the point that it can throttle itself down, that can happen. Most days, though, will not require it to run constantly since it won't be able to modulate down far enough. IOW, nearly all of the time, it will have excess capacity to heat the indirect. From my experience, I've never had the house cool off while reheating the indirect (it's generally installed as a priority zone...it gets all of the heat until it is up to temp.

What people don't often consider is that if say you have 55K btu, and for maybe an hour or two, you really need 60K btu...the house doesn't immediately become cold...it does (hopefully!) have insulation! The temp will drop slowly, and then, it will recover. Since that usually occurs at night, most people will never notice since things tend to warm up after the sun comes up. And, that's only on the very coldest day, that may not happen for years.

Excess capacity can speed up the recovery from a setback, but you pay for it. I solved that issue on mine with a WiFi connected thermostat...I can monitor and adjust the setting. I use this when I'm away from the house for vacation. FWIW, radiant tends to work better if you don't use a setback.
 

Dana

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The 552 therms over 1704 HDD/65F is 32,394 BTU-in per HDD, or (/24)= 1350 BTU/degree-hour. In an 80% efficiency boiler that works out to (x0.8=) 1080 BTU/degree hour. The 65F presumptive balance point and 15F design temp adds up to 50F heating degrees, for an implied load of 50F x 1080BTU/degree-hour = 54,000 BTU/hr.

With a cast-iron boiler using ASHRAE's recommended 1.4x muliplier you'd be looking at a boiler with about 75,600 BTU/hr of output. The DOE output of the ESC4 is 78,000 BTU/hr, and would be the LARGEST of that series to consider installing.

The fact that it's broken up into 14 micro- zones, and has at least a couple of low-temp radiant zones means you will be FAR better off with 60-80K modulating condensing boiler than with a 75K-out cast iron beastie, and it can even be cheaper up front than an ESC4. Fire tube mod-cons such as the Triangle Tube Prestige Solo 60 or HTP UFT-080W would be good candidates. The Solo-60 costs about the same as an ESC4, the UFT-080W is cheaper than an ESC4. Mod cons can use plastic venting, which is pretty cheap. Venting a mid-efficiency direct vented ESC4 requires stainless steel in most installations, which is a real cost-adder.

For the napkin math on sizing a boiler for the radiation in multi-zoned systems (and why an ESC4 would be a bad idea here), see this.
 
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