what size boiler woul you recommend?

rich67

I'm converting from oil to natural gas. Having a new boiler installed. Did a heat loss using SlantFin's program & came up with about 40K BTU/hr. It's a 1000 sqft ranch in NJ. Hot water baseboard and an Amtrol indirect 41 gallon with electronic controls.  What  I don't know is how many BTU's the indirect uses.  Does anyone know what the average usage could be. Or maybe what the max  BTU's it's capable of using? I just want to make sure the boiler will be big enough to satisfy the demand.

HomeOwner1

Have you considered a combi?

Sounds like Triangle Tube combi units could potentially be an option for you. Less expensive, less equipment to install. An option that may work for you.



Went through the same thing recently on a much bigger house.



We have a bigger different brand combi, but are pleased with the unit.



You will save a ton converting. 50% just in fuel costs these days plus whatever efficiency gain you get from the new unit.

CMadatMe

The Question Is In Your

Court. It's not about what the indirect can do, it's about what is your hot water demand?



gpm = btu/hr / (Temp Rise x 500) Temp Rise is the temp difference between the incoming cold water and the temp set point in the tank.



40,000/ (120-50 x 500)

40,000/ 35,000 = 1.14 gpm x 60 Minutes = 68.4 Gallons Per Hour Continuous Rating. Add the storage for 1st hour rating.



Don't increase the boiler size if this doesn't handle your demand, increase storage capacity of the indirect.



If you only have 1 conventional shower with no high demands as to gpm then a combi boiler might fit best.

Paul48

Chris

How about using his indirect for storage in conjunction with a combi?

Robert O'Brien

What Chris said

I'd always recommend the right size boiler or as close as you can get to it Find another solution to DHW demand,if in fact it even is a problem. Bear in mind the 40 gallon gas water heater that is pretty much standard in the vast majority of American homes has an input of 40K.Since you already have the indirect,I would be inclined to leave it. I assume it has a mixing valve,if so you can operate at a higher temp which will have the effect of increasing DHW substantially,if that still doesn't do the trick,an aquabooster can be installed to operate in conjunction with indirect. Another option is a tankless water heater or solar with the gas boiler providing backup with a dual coil tanks ( Lots of tax credits!)

Bottom line,they are two separate decisions and oversizing boiler is never the right choice.

Paul48

I Understand

I wasn't suggesting over-sizing the boiler. Merely increasing the capacity of somewhat wimpy output from a combi, using what he has already.

HomeOwner1

Not all combis are wimpy these days!

Triangle Tube has some pretty effective combi boilers for small homes.



That challenger could probably safely handle two people taking showers on the coldest day of the year with unlimited hot water. Plus it is very affordable. The Prestige is roughly twice the price but appears to be much better.



If you need bigger, you can go up to 240kbtu with almost 6gpm at 77 degree delta t on the coldest day of the year with other brand combis. That is roughly 3 to 4 people taking showers at the same time in a house these days.



Why complicate things and make them more expensive? This could potentially knock off a couple k off his parts list and free up a boat load of space.



We went down this same path of thinking when we chose our unit and ultimately this option made sense for our circumstances.



It works just as well as our old cast iron boiler in our home, which was over-sized previously, but for a lot less in operating costs.

Paul48

Combi-Tradeoff

The combi's you're describing have the DHW outputs they do because of high BTU output. You won't find one, sized for a 40000 btu heat loss with those kind of DHW outputs.

CMadatMe

Wasn't Making It Complicated

Was trying to answer the posters question. You should

know better then anyone that you need to know

your actual DHW demand before anything.

Paul48

Nope!

I re-read my posts, and they weren't written in some obscure Lithuanian dialect. I was just trying to clarify possible options for the OP.

CMadatMe

Paul

My last response was not directed toward you. Sorry for any confusion. If he had an indirect he could use it as a storage tank. Just wouldn't be using the coil if I was dumping a combi into it. I'd pipe it so if the indirect ever failed turn off a couple of valves and he would still have hot water.

rich67

Thanks for all the advise!

To keep costs down, I think I'm just going with a standard cast iron boiler. I had one estimate for a Peerless MI3 and another for a Williamson GWA 175. There's a big difference between the two as far as BTU's are concerned. I'm wondering if the MI3 would be big enough to support the heat load and indirect .  But then I suppose if it was wired up with priority zoning there wouldn't be much of a problem.

CMadatMe

I'd Look At Doing

A Crown AWR. Will be price competative but that iron can handle 110 degree return water temps so you could add outdoor reset without having to provide boiler protection as you would with the others. They would be limited to 140 degree return water temps meaning 160 supply water temps. The AWR gives you better system flexibility in my opinion.

John Mills_5

175K?

Holy cow! That's enough for 4 houses of your size!

heatpro02920

Rich

How old is your indirect?

Whats your budget?

Are you thinking high Efficiency or std?



You like a lot of home owners are in the sub 55 group, where under 55K BTUs your choices get slim if you are looking for High efficiency...{but this is changing}...



If it were me I would go with a boiler properly sized for you heat load, and never mind combustion efficiency... I would go Weil McLain CGi3 with a priority control for your indirect water heater, use an ODR control and then when the amtrol quits go with a condensing rinnai tankless water heater... Or if its old, scrap it now and get an ru80i or a 98i depending on your water needs...

rich67

Indirect around 10 years old

I'm just going to replace the boiler with standard style cast iron. Hoping to reuse items like air scoop, zone valves and ball shutoffs if installer sees fit to do.

I don't really have money for this but have the opportunity to get gas to the house so don't want to pass it up. Hopefully will see some savings in fuel usage over oil.

Sounds like from what I've been reading, I can go 80 to 100000 BTU range on boiler size and have plenty of hot water available.



Thanks everyone for your comments and suggestions.

SWEI

Size the boiler to the heating load

DO NOT oversize the boiler.  Really.



If your load is 40k and you want a standard efficiency boiler you might look at one of the Crown/Burnham units Chris suggested, as they will tolerate 110F return temps so you can take advantage of at least a partial outdoor reset.  A 50k boiler will heat water 20-30% faster than a standard tank heater and provide even greater annual fuel savings.



Burnham offers a 50k version of the Series 2 that should be a nearly perfect fit.

heatpro02920

Rich do yourself a BIG favor

And do not over size that cast iron boiler. You can make a cast iron 86% boiler very efficient and save yourself a lot of fuel.... But try to ...



Size as close to your heat loss as possible {I would rather be 2K BTUS under than 2K over}.

Use delta T circulators.

Use an ODR control

Priority DHW

Programmable T-stats

Direct vent {if not possible, at least fresh air intake}

electronic ignition {kind of a moot point now a days}



You do as many of them things as possible you will be fine, the thermal shock and condensation swei is talking about is a good point to, Buderus boilers are good with low return temps too, but I use the Weil Mclains piped primary secondary and never have any problems, I find with normal baseboard 140 supply temps are about as low as you are going to go, unless the house has a lot of extra board. {don't get me wrong, you can go as low as matching the desired indoor temp, but......} For what you are looking to do, I think any boiler will be OK, just get it as close to 40K as you can, thats going to save you over time, and save you on the initial install as well...

rich67

So I don't really need extra BTU for the DHW

I am reading the post from John in Buffalo, he is asking the same question as me basically.

When I look at spec sheets, I want to use the net output rating of the boiler, correct?

RobG

Picture this

A 40,000 BTU boiler in combination with your Amtrol tank (properly installed) will give you more hot water than a conventional 50 gallon gas water heater. A 50 gallon conventional water heater will typically supply enough hot water for a 2 1/2 bath single family home.

Just putting it in lay-mans terms. Don't worry! size the boiler to the heat load of the home, you'll do just fine. Just pick the right installer.

Rob

SWEI

output rating

Use the DOE or gross output, not the I=B=R or AHRI number.

rich67

Ah-ha! I think I get it!

Thanks everyone for all your help. This is a great forum to get good advice.

JStar

DHW

Sounds like a job for the Triangle Tube tank-in-tank. Those babies have outrageous ratings at lower BTUH's.

Rich_49

Too much thinkin goin on .

   Use the boiler that is large enough for the Space heating load . 40 K is fine .   Increase the storage temp in the indirect and employ a mixing valve , in this way you can conceivably have the output of a 50 gallon high recovery .  See this PDF for explanation   https://taco.box.com/s/xxknuh1nzgik688gqjof .  Don't drive yourself crazy . And guys when will we quit trying to reinvent the wheel to solve a simple problem , ?

Rich_49

Too much thinkin goin on .

   Use the boiler that is large enough for the Space heating load . 40 K is fine .   Increase the storage temp in the indirect and employ a mixing valve , in this way you can conceivably have the output of a 50 gallon high recovery .  See this PDF for explanation   https://taco.box.com/s/xxknuh1nzgik688gqjof .  Don't drive yourself crazy . And guys when will we quit trying to reinvent the wheel to solve a simple problem , ?