Oversized Boiler Options

deyrup

I know this has been asked before, but I can't find it with the search function. I calculated the EDR of my radiators; 135 EDR known and I am guessing another 15 EDR for the radiator in the wall which I can't get access to, so 150 EDR. If I do the math that means 150 x 240 = 36,000 BTU/hr, the boiler is heating the first floor (1200 sq ft) of a 2 family house. Let's round that up to 40,000 BTU/hr. Looking at the boiler in the basement it is rated up to 105k BTU. So my boiler is about 2.5x times to large for the space. What are my options for the boiler?

Jamie Hall

Much easier to look at the steam square feet rating on the nameplate than to do fuzzy math... the nameplate says your boiler is rated for 271 square feet of steam. So... your boiler is almost twice as big as it needs to be.

The best solution -- and it has been discussed a number of times in different ways, with slightly different approaches -- to this is to hook up a timer relay which, when triggered, will not allow the boiler to refire for a set time (you'll have to fiddle with the time to make sure it can keep the place warm, but that's minor). The trigger could be reaching the pressure limit on the pressuretrol or a sensor for steam reaching the last radiator or... lots of options.

ethicalpaul

You can make things easier on yourself by ignoring BTU and sticking with EDR vs "net sq feet of steam". Is that what the 271 is on the rating plate? I can't read it.

Setting that aside, you have a very small radiation load for a steam boiler. The smallest boiler is about 75K BTU input for about 200 sq ft of steam (EDR)

Is your boiler currently cycling on pressure? What is the timing of the cycles? (running time to first cut-out, running time to later cut-outs, number of cycles per call-for-heat)

Here are some ideas:
1. make SURE your main venting is operational. For such a small system, a single MoM or Gorton #1 should be fine (it actually depends on the size/length of the main, but it should be fine).
2. Don't run a thermostat setback. Keep it at whatever setting. The cycling will be exacerbated by a temperature recovery. You'll waste fuel trying to recover with an oversized boiler.
3. Find a good tech with a combustion analyzer who knows how to use it to try to reduce the manifold pressure maybe to about 3" wc instead of 3.5, and see if there's a burner that can be plugged while keeping good combustion.
4. Implement a time-delay relay with a low-pressure switch such that if the pressure is reached (say 5" of water), then shut off the boiler for some minutes to let the radiators give up their heat to the house without burning gas uselessly.

deyrup

Is the idea that there is enough heat in the system even though the thermostat isn't satisfied, so you can shut the system off before it thinks it can?

ethicalpaul

I would word it like this: The idea is that the radiators are all full of steam so it's a waste of fuel to just try to push more steam into them. If they are nice cast iron radiators, they will have a lot of heat energy in them by the time they fill up with steam, and that heat continues to be released into the living space for many minutes after the firing ends.

motoguy128

If your system has had radiators removed and still has a descent length header, then the pickup will proportionally be much larger than a larger system. So the same 1.33 I don’t think will be accurate. It will be more like 1.5 or higher if the main is uninsulated. I’d still want some fairly fast vents on the main. It’s still going to be 60’ of 2” pipe or more.

That being said, you are still big.

I agree with the others, using a timing relay to manage duty cycle should work well.

Jamie Hall

deyrup said:

Is the idea that there is enough heat in the system even though the thermostat isn't satisfied, so you can shut the system off before it thinks it can?

On a properly sized system, that is exactly what the anticipator in the much maligned, old fashioned, not all singing and dancing, mercury T87 does. And, assuming someone took the time to set the anticipator properly... it works spectacularly well.

Some of the new digital thermostats also do a so-so job at it, but since most of them use a time-based, not heat loss based, system, they don't work quite as well.

acwagner

I like the timer relay. BUT, you need a balanced system before you do that, and generally address other issues before you add the timer.

Are there specific issues you want to address? Is this a one pipe or two pipe system?

deyrup

We insulated the pipes and replaced the main vent with a gorton #2 this year and it is a single pipe system. There are no issues with venting or water hammer as far as I can tell, the systems runs with 82% combustion efficiency and all the rooms have balanced heat. Except for one room that is a bit drafty. I am trying to optimize the system for efficiency and comfort. Here is what the gas usage looks like https://snapshot.raintank.io/dashboard/snapshot/s24F5FJKSPGQvWTypaid9aSMt4W7oHfl?viewPanel=6&orgId=2 The resolution is 2 minutes and is coming from the gas meter. It seems like the system is always using gas. I would have expected it to turn for a bit and then turn off for a bit but that doesn't seem to be happening.

There is no stove on the gas meter, but there is a 40k dhw as well that uses gas, which I am not able to pull out of the gas usage graph, but as far as I understand the boiler gas usage is negligible.

Jamie Hall

Nice and even. It's the overall slope of the graph to which you want to pay attention.

It's probably obvious, but -- if your system is running as well as you say, there probably isn't much you can do to improve things on the heating side. It takes a certain amount of gas to provide the BTUs you need -- and that is governed entirely by the weather outside and how well insulated and tight your structure is.

acwagner

@deyrup I'd expect your graph to be much more stepped. It looks you're getting around 4-5 cycles per hour.

What was the boiler doing during the time period on the graph--was it cycling on pressure?

What kind of thermostat do you have? Most have a way to adjust the cycle time, either through setting for "steam" or adjusting the temperature range that it triggers itself on/off. My instinct is the thermostat is set in a forced air setting, which has a tighter temperature response range. Which would make the boiler run more frequently but for shorter duration for each fire.

Which isn't necessarily bad. Some people purposely run their systems that way. But those are well tuned systems. Most recommend 1-2 cycles per hour for steam. Whether that is more energy efficient is a matter of debate.

deyrup

I have a honeywell thermostat, specifically this one. https://www.mclendons.com/2742405/product/honeywell-rth6350d1000e1 from reading the manual it doesn't look like it has an option to change to have longer cool down periods. I can't really tell what is going on with the boiler pressure wise because my pressure gauge is broken; it reads around ten psi all the time. I have the pressuretrol differential set at .5, 1psi.

I would expect there not to be constant gas usage as well and I expected the gas usage to be more stepped. Does keeping the pilot light on somehow use a fair amount of gas, or could it be the domestic hot water heater?

Jamie Hall

Check you manual on the thermostat. I think that under item 200 on the system setup you should select option 3 and under item 205 option 12.

deyrup

Interesting! I did not know about the secret advanced mode on the thermostat. It did assume I was using forced air.

acwagner

Let it run for a few days and see what difference changing the thermostat setting makes.

You'll probably start cycling on pressure. It's worth getting a 3psi or 5psi pressure gauge to add to your boiler so you can see what pressure it's running at and to make sure the pressuretrol is functioning.