Twin Steam Boilers - 4 Stages of Firing Rates

JStar

So, we just wrapped up another twin boiler setup on a Warren/Webster vapor system. Oil to gas conversion. Since the SMITH 8 is no longer in production, we've been installing a lot of PEERLESS 63 series boilers. The only way we feel good about an atmospheric boiler is by installing a 2-stage gas control valve in them. Robertshaw makes a series 700 two-stage valve that comes in two different sizes up to 720,000 BTUH. Super easy to retrofit and install. The High is 100% and Low is 60%. It's the absolute perfect fit for High/Low on a steam system. As soon as the mains are hot, you can drop out your pick-up factor and run on Low fire until the t-stat is satisfied. On this job, we're controlling 4 separate firing rates. I can maintain 4-8 ounces of pressure with just the Low fire of one boiler.



We installed orifice plates in all of the rads/convectors, as well as upgraded 3 original WW crossover traps. The Air Eliminator check valve is still functional and maintaining a massive vacuum. It takes less than 5 minutes to get 50 feet of main hot.



Each boiler has its own 3" drop header that feeds a 5" system drop header. Dry steam is happy steam!



The picture quality is not great. One day, I'll get some better photos.

JStar

PS

Oh, and over the summer, we'll be installing MarkS's EcoSteam control on this system. I figured...why not add ONE more control to confuse the next guy in 30 years.

heatpro02920

very nice

as always..



I have done a couple dual steam boiler setups, never staged the burners though, just the boilers so 2 stage..



Are them steam pipes getting insulated...

gerry gill

very cool job Joe!

i love multi stage firing. We generally use the maxitrol two stage regulator but i will have to investigate that robertshaw one. Nice looking job you did- as always!

JStar

2-stage GCV

http://s3.supplyhouse.com/manuals/1265283159157/25260_PROD_FILE.pdf



I'm really liking this valve. It has a pilot valve, so it only works with intermittent pilot, but eliminates the need for a separate pilot control. The small size is 300,000 with 3/4" tappings. And, it's a step open valve. Big advantage.

Steamhead

Nice work as always

looking forward to fuel-consumption figures if you can get them. What was in there before, and what condition was it in? 

JStar

Old boiler

The old boiler was a 5 section 78 series Weil McLain oil boiler. It was about 30 years old and leaking from every single section.

nicholas bonham-carter

4 stages

That's a wonderful setup, and I hope you can give us better pictures sometime soon, before the insulation goes on.

How are the 4 stages controlled - pressure, thermostat?

Have you checked the combustion while running on low-fire? That seems to be the main drawback to atmospheric boilers - the apparent inefficiency of variable firing rates. The standby losses can always be controlled by an electrical damper.--NBC

JStar

Staging

Staging is controlled by three Vaporstats and one Pressuretrol. The firing sequence is:



Themostat brings both boilers on at 100% capacity.

Lag boiler triggers Low Fire at 4 ounces.

Lag boiler shuts off at 6 ounces.

Lead boiler triggers Low Fire at 8 ounces.

Lead boiler (never) shuts off at 1.5psi.



I didn't go with a 2-stage thermostat this time because it has been kind of unreliable on another twin system we installed. Although, that system was 1-pipe and this is 2-pipe. 2-pipe seems to respond A LOT better to a thermostat controlling some of the initial staging.



Combustion numbers show that Stack Temps are always above 350F. There is way more O2 in low fire, so we lose combustion efficiency, but our heat-transfer and system efficiency go up by an immeasurable amount.

ChrisJ

Nice!

Another beautiful install by Thatcher Heating and Air Conditioning.



I am surprised you went with atmospheric boilers this time though.

nicholas bonham-carter

2-stage thermostat

I just thought that the 2-stage thermostat could control 2 boilers (without hi-lo firing), using the vaporstat as an upper limit for the lag boiler. With essentially 4 boilers, the 2- stage might not work.

Since you can get to 4 ounces on just one boiler (low fire), would you ever consider an asymmetrical boiler setup, with the lag boiler sized to provide sub-atmospheric pressures, and the lead boiler being maybe twice the capacity? What is the relationship between the heat loss of the building and the EDR of the system. Another words could the 150 degree steam keep up with the heat-loss?--NBC

SWEI

Two-stage retrofits

are something I need to learn more about.  Data sheet above took a little decoding.  2-stage LP starts at 480k, NG at 300k.  Not a lot of info online for these http://www.uni-line.com/modules/catalog/Product.aspx?singlePart=false&prodID=64690



Nice work (as usual.)

JStar

Atmospheric

We haven't settled on the next wet-based gas boiler that we like. Our PR war with several manufacturers is still pending.

JStar

Staging

The 2-stage t-stat can control two boilers, but it will only stage them based on indoor temperature. If we added the 2-stage t-stat functionality into our system, we might see the Lead boiler running alone for 90% of the season. Sometimes, the indoor temperature can fall behind for a long enough time to notice and become uncomfortable. With both boilers firing on every call for heat, the piping heats up very quickly and the Lag boiler only runs for 5-10% of every cycle. We've had better performance without the 2-stage t-stat so far. Although, it seems to depend entirely on the individual system. Who knows, I might integrate into this system at some point to see how it works.



Haven't done a full heat loss yet, but plan to for the EcoSteam control installation. The house holds its temperature VERY well, and the last few weeks of mild weather has seen no lack of indoor comfort.

ttekushan_3

Boiler size and Effective EDR

By installing the orifice plates, the effective EDR drops. Are the boilers sized to the total radiator EDR? Or did the orifice plates allow you to size closer to the heat loss of the structure? It may be irrelevant due to the firing rates available.



I've come to like orifices such that the effective EDR is on the slim side for the supposed heat loss, but boilers sized and modulated (or staged) over that amount so that the pick up factor is quite large but drops out quickly. In practice, this gives great distribution and very low fuel consumption per square foot per degree day. It appears that this is what you have done.



In other words, you've just done the ideal steam setup for comfort and efficiency.



I'd be wary of reducing the cold start available pick up factor in warmer weather via a multistage thermostat. The hot water mindset of half the boilers running being directly equivalent to half the heat is problematic with steam if distribution suffers. I've compared and it seems more effective to just let the whole shebang fire up and then drop out quickly, cold start pick up factor being the most demanding of all.

Unless you could alter the orifices based on outdoor temperature. :-)



I'm working up a design to allow one boiler of two to fire on a large system, but the wrinkle is that we'll be controlling the amount of vacuum applied to the system based on outdoor temp, once the new vacuum pumps arrive from Oak Services.



BTW, I wouldn't worry too much about the combustion numbers on low fire. Experience has shown that, for some reason or another, you should see excellent fuel usage that belies the combustion numbers. Ideally, fuel and air should be tightly controlled but low fire on atmospherics works better than it should.



Outstanding work.

JStar

EDR

The boilers are matched to the orifice plate sizes, but the orifice plates are not sized to the home's heat loss. That is something that I've yet to expriment with. My instinct tells me that this house, that has had no insulation or air sealing upgrades, needs the radiators at the size that they are. Eventually, I will be conducting a heat loss and that will give me a chance to compare the numbers.



I agree that the Low Fire ratings don't bother me. The combustion efficiency is lower, but we're only using something like 20% of the total BTUH's for the majority of a run cycle. That has to equate to reduced fuel consumption. With staged controls, even if fuel use isn't reduced, comfort is always increased.



Interesting idea with the modulating vacuum. Are you controlling the actual pump speeds or run times? Are you essentially modulating the time that it takes steam to reach the radiators?

ttekushan_3

steam size.

If there's been no tightening of the building then keeping the current EDR in play is a good choice. They way you've designed it, there's a lot of flexibility and resilience there. Getting control of air infiltration and introduction of attic insulation, storm windows or anything else that affects heat loss can be compensated for by simply changing an orifice. And it can be done on a room by room basis as changes to heat loss are made. A good situation.



Regarding the vacuum, I'm cheating. What I failed to reveal is that once upon a time this building had a Warren Webster Moderator system. The 8" Moderator modulation valve is still there and works perfectly, although I have to replace the diaphragm stem seal. The original operator is gone, but I'm going restore it with a modutrol motor with a linkage. I want a constant differential between supply and return but want it to slide up or down with respect to atmospheric pressure.



Still looking at control options. I've done my own relay-logic set ups before, but the world of vacuum modulating controls is radically different now. I'm afraid a simple solution no longer exists. Cost is a factor since the heating fuel consumption on the place is very low as it is, it makes it harder to justify a large expenditure for diminishing returns (no pun intended)

nicholas bonham-carter

Oriffice control

Wouldn't it be easier to control the steam flow by inlet valve adjustment, and then remove the valve handle. Or is that not exact enough?

Changing the orifices to readjust could be a bit of a chore, in the future.

This thread is one of the most interesting in quite a while, as it deals with multi-boiler installations, along with vacuum systems.

The vacuum pump could be wired into the control circuit, so that by the time the boiler is firing, there is already a strong enough vacuum to speed the steam. In the shoulder season where you may want low temperature steam, the control could maintain the vacuum in an inverse of the outside temperature:

Cool weather-deep vacuum,

Cold weather-less intense vacuum.--NBC