MASTER VALVE vs. INDIVIDUAL TRV's

OleTwoPiper

In the process of sizing a new steam boiler for a 10-unit, two pipe steam building, so there is sizing flexibility to address the following situation. The building is six stories and has long, 50' horizontal runs between steam main and return. One floor in the middle of the building is significantly hotter than the others (due to sf of Slantfin convectors, insulation, new windows, etc.). The nature of how the individual sets of convectors were piped on the floor, there is a 2" highway in the front and back that feeds four sets of convectors across each 50' span. Millwork covers all the plumbing across the whole span (with access panels and lower/upper grills). The building previously added TRV's to all of its radiators, but due to access issues, this one floor didn't receive any. QUESTION IS: Knowing that zone valves are not recommended at near-boiler piping, in the above case, is there a viable way to valve off the whole floor at the main, so that if the unit is hot (which it normally is), it would get no heat until it needs it, then opens for the steam to come across its 50' run and fill the four convectors when open? (Effectively trying to add a single master Danfoss valve vs. opening the walls in many locations and adding individual TRV's at each radiator)

Jamie Hall

Yes, you can do that. Be sure that whatever power controlled valve you use, however, is full port -- that is, if is in a 2" ID main, the valve must have a full 2" straight through port. Otherwise, your results may disappoint you badly (to put it mildly).

The other thing to consider is that you are changing the load on the boiler by a rather large amount. How is the boiler controlled, and how does its size relate to the existing radiation?

OleTwoPiper

Thanks Jamie.

Total building SF is 1,735 sf (without any pickup factor). The specific unit accounts for approx 216 sf of the total.

We were planning to install a Smith 19HE-6. So would like to confirm if that is the appropriate size, and how the 2" valve cutting overall radiation sf from time to time to +/-1,500 sf would affect the boiler size / brand decision, if at all?

And to answer your question, current control is Tekmar 279. There is a debate whether to keep the Tekmar or consider HeatTimer, despite the cost. Curious about what you and other people believe is the most reliable and accurate control device (when using two indoor sensor front and back zones)?

Jamie Hall

I wonder if the HE-5 (1835 sq. ft) might be a better choice? But you should confirm that with your installer.

Cutting the 216 sq. ft. for this unit from time to time will probably not make that much difference, although the boiler may cycle on pressure. This really isn't that big a deal! Just make sure you keep your pressure low (let it cut out at no more than 1.5 to 1.8 psi, if you are using a Pressuretrol) and it should be OK.

The Tekmar is fine. They're excellent controls.

OleTwoPiper

Is the 1,835 sf before or after the pickup factor?
(ie. our 1,735 x 1.33 = 2,313 sf)
And HE-5 net sf is 1,835 sf and HE-6 net sf is 2,382 sf

The Smith specs say: The net I=B=R Steam Ratings shown are based on a piping and pickup allowance of 1.333

Steamhead

When you order the Smith, get a burner that can fire at a high or low input. This will most likely be a PowerFlame. Control the firing rate with a Vaporstat. That way, the boiler can respond to TRVs closing.

OleTwoPiper

Like that idea a lot @Steamhead. Thanks

Based on the math above, if we assume a 1.5 pickup factor, actual need is for a boiler with Gross Output of 624,000 btu's. With Smith HE-5 at 587,000 and HE-6 at 762,000, we are in between; so which one is optimal? Or should we be looking for a different brand that is closer to the 625k level?

OleTwoPiper

As a follow up to the above, Burnham 5011B has a gross output of 624,000 btu's and the 5012B has 687,000 btu's.

Any thoughts on going with either of these Burnham atmospheric designs vs. the Smith HE-6 with the PowerFlame/Carlin burners?

nicholas bonham-carter

You are very fortunate to have a two-pipe system in this building.
You do not want an indoor sensor in an area controlled by TRV's, so leave that area as is, with the sensors there. The TRV's will ramp up in the other areas they control, to keep pace.
Steam Whisperer has written here on the subject of sizing two-pipe systems at very close to the building heat-loss. (Can someone help find this white paper?) This situation may be better served by a twin boiler setup, controlled by vaporstat pressure, and a two-stage thermostat. As the temperature plummets, the thermostat calls for heat on both boilers. As the pressure rises to 6 ounces, the lag boiler would be cut off by the vaporstat, leaving the lead to carry the load. Smaller boilers may be easier to take down to the basement, instead of large chunks of heavy metal sections.
The tekmar controls, like the Heatimer, have a number of variable parameters, whose settings may be difficult for whoever is in charge of maintenance to master, whereas the Honeywell VisionPro could be mounted in the boiler room with the sensor in the cooler part of the non-TRV area. Don't use night time temperature setbacks, but instead select a lower constant temperature, (65 degrees). You can always bump it up a degree or two if people are cold.
Check the main venting/crosssover traps, and improve if possible for a quick steam arrival.--NBC

OleTwoPiper

Like the idea of two boilers in series, and we studied it in the past. Seems like contractors in NYC-area are more comfortable with a single unit install, so we have been sticking with a plan for a Smith boiler (or possibly Burnham as size lines up and simpler atmospheric config). QUESTION: If a modulating burner on the Smith is employed, is there much of a difference with modulating a single boiler vs twin setup?

Regarding your comment about constant lower temp, I think it is a great idea. However, in an apt bldg setting, there are quite a few people that complain if they don't feel the heat on!?@ (vs. complain only if overall temperature isn't correct). Seems to me there are lots of reasons to go with the constant temperature setup (especially less cold start noises!), however, I don't have a solution if the boiler is off for hours on cold days even if the set temp is maintained for unit owner relations...

KC_Jones

You should compare the 1735 number directly to the sqft rating on the boiler. I am with Jaimie it sounds like you are going to oversize this boiler which never turns out well for operations, especially with a 2 pipe where you need to make sure you keep that pressure nice and low (ounces).

OleTwoPiper said:

Like that idea a lot @Steamhead. Thanks

Based on the math above, if we assume a 1.5 pickup factor, actual need is for a boiler with Gross Output of 624,000 btu's. With Smith HE-5 at 587,000 and HE-6 at 762,000, we are in between; so which one is optimal? Or should we be looking for a different brand that is closer to the 625k level?

Why do you want to add so much pickup factor? Again it sound like you are going to oversize.

OleTwoPiper

Just following @Dan Holohan 's formula in Lost Art.
Smith HE19-5 has net output of 1,835 sf and 19-6 has 2,382 sf.
Seems like pickup factor can sway us into the bigger or smaller Smith, or a Burnham for better sizing parity, so not sure what should govern

KC_Jones

The accepted standard is 1.33 and a lot of people are thinking in some situations less is appropriate. Here is a great write up on the topic of sizing.

https://heatinghelp.com/systems-help-center/taking-another-look-at-steam-boiler-sizing-methods/

If you compare your 1735 to the Smith numbers the HE-5 is even a touch bigger than you should need and you are talking about going even larger than that. If you go with a power burner and have a competent tech you should even be able to dial the burner even closer to your load, so possibly down firing the HE-5 slightly to get closer to the 1735 load number.

Jamie Hall

Unless I am hopelessly confused -- which is quite possible -- the net I=B=R rating includes the pickup factor -- which is to say, you try to match that rating to the actual EDR of your system, not the EDR plus a pickup. Therefore, as I look at it, the -5 is even slightly oversize -- and the -6 is way over.

As I understand it, the only time you have to worry about what pickup factor to use is when you are converting your EDR to BTUh, and then comparing that to the BTUh rating of the boiler. If you are going to do that, then you should add a pickup factor to the BTUh you got. If the manufacturer gives you a square foot rating, however, they've already done that for you.

KC_Jones

In short if you go with the -6 unit you will have a calculated pick up factor of 82%.....massively oversized. The -5 will give you a pick up factor of 40%.

nicholas bonham-carter

Can you even downfire 40%?
Maybe one size smaller still would be better. Give the two boiler system another look. Either a 300k and a 200k, or two 250 k boilers. This would give you a little safety net for the future, as they will not both break down in the future!--NBC