Hydronic loop off of steam boiler or dedicated hydronic boiler for addition?

OilfieldHippie

I recently purchased a ~2000 sqft 2.5 story brick and masonry house that was built in 1922. The house has a single pipe steam system (interestingly, it was converted from 2-pipe steam at some point in the past) that looks to be done correctly. The boiler is a Weil McLain EG-75 (300,000 BTU input, 240,000 BTU output, 750sqft steam) which was installed in 2016 and is in good shape. There are 13 radiators that I estimate to be 625sqft EDR. There are also 2 in-wall bathroom radiators that I have no idea how to measure and 2 radiators that were removed during poorly conceived renovations of the kitchen by the prior owners.

This system seems to be way bigger than needed for the heat loss of the house, which I understand is common. My heat loss calculation is ~112k BTU on a design day. The boiler does seem to be sized somewhat close to the radiation taking into account the removed radiators. I have not spent a winter in the house but the system seems quiet and even so far.

We are planning an addition to the house of ~800 sqft. I'd really like to have radiant floor heat in the new kitchen, which got me thinking about how best to heat the rest of the addition. Hydronic heat seems to be the best option because of the ability to zone and do an ODR. I'm estimating less than 50k BTU of potential hydronic load for the addition and adding baseboard in the basement while I'm at it, so that is well within the 60k nameplate pickup on this giant boiler.

I understand that we can add this size of hot water loop to the boiler, but should we? Is a dedicated hydronic boiler a better choice? Better in terms of economics but also long-term reliability and comfort. If we do go with a loop off the steam boiler, what should we specify to make it work as well as possible for as long as possible?

I'll have a pro do a real heat loss and sizing before making any decisions, but I want to understand what is realistic while we are in the design phase.

Thanks, y'all.

ScottSecor

I just checked and your EG75 is rated at 733 EDR. While this is more than you came up with, it is not grossly oversized. As a matter of fact, with two in wall radiators, the boiler may be the correct size.

If the budget allows I would suggest getting a small condensing boiler for the new addition. Might consider adding an indirect tank for dhw. How are you making dhw now?

OilfieldHippie

ScottSecor said:

How are you making dhw now?

We have a separate gas hot water heater.

ScottSecor

Just a guess here, gas fired water heater efficiency about 65 percent. Standard tank is forty or fifty gallon in my area for a house your size. If you choose to heat the new addition with a condensing boiler, rated at 95 percent, it might make sense to also make dhw with this same new boiler and ditch the standard tank setup.

Three are some potential issues when taking low temperature radiant off the EG steam boiler. Some of them are:

1. Ideally, you would have a tankless coil for just the radiant, most EG boilers do not have tankless coil openings these days.
2. Assuming there is no tankless coil, you would have to pump hot boiler water to a mixing valve and perhaps a heat exchanger to provide a stead low temperature for the new radiant. Unfortunately, boiler water is typically rusty (at least near the bottom) and can wreak havoc on pumps, small diameter tubing and mixing valves.
3. You might use the new addition much more than the rest of the house. You may find that you end up using the radiant zone all of the time and the steam some of the time. Why run the "big" steam boiler when you can run the "small" radiant boiler instead?

offdutytech

I have had a few customers in similar situations where it was an addition or a kitchen remod where steam radiators weren't wanted in the kitchen by the customer. 
Could you run a loop off the boiler for some form of hot water? Sure, but it's not ideal with infloor radiant heat and not my first choice. We have either done a small electric boiler for the radiant heat or installed a mod con wall mount. With 800 sqft a mod con would be overkill unless you also used it for an indirect dhw tank. I would investigate the electric boiler option for a small area like that. 

EBEBRATT-Ed

@OilfieldHippie

I agree with @ScottSecor

While you can certainly make a hot water loop off the steam boiler it takes a lot of parts and pieces to get it right.

expansion tank
heat exchanger
backflow preventer PRV
controls etc
2 circulators (1 bronze construction) and a lot of valve s and fittings.

If you can afford it and since you have gas available, I think a separate boiler is the best way to go. If the steam boiler fails, you will have a place to stay warm and the option to heat domestic hot water with the new boiler is a good idea.

Mad Dog_2

Can't add much more than The Sharp Gents above said.  One of my Life Guides in the Trades, Jimmy The Gent Burke (Semi-retired in Fla), always had great ways to get very close results on a shoe-string budget.  Being the oldest of 15 (yes! 15) and coming up through the oil companies, he was a master of using the bare minimum in materials, valves, electric to get something to work. 

We did many, many, hot water and radiant zones off the bottom of a steam boiler with no HX, expansion tank, et cetera.  The people got to enjoy radiant floors on a budget.  But as Scott and the guys said, after 7 or 8 years, the circs go, the mixing valve clogs, the loop loses its prime. It can be done, but if you have a few extra bucks now, a small, dedicated boiler or water heater is better for all the above reasons.  Mad Dog 🐕 

BobC

My steam boiler is 10 years old this month and from the start I've used Steam Master to keep the boiler water PH high and deter corrosion. That water is clean, very little rust in it so I would think all the valves are pretty happy and if therer were a pump in there it to would be pretty happy.

If you decide to use the steam boiler for a HW loop you might want to consider using something like Steam master to keep the boiler water clean, that would keep everything in that loop nice and clean and should extend the life of those items.

Bob

OilfieldHippie

Thanks for the advice here. We are pretty happy with the steam heat, and I get a kick out of being one of the last houses in the area that hasn't been "upgraded" to forced air. We plan on this being our last house and are looking long-term when we do this addition.

I'll look into the electric option, @offdutytech.

offdutytech said:

With 800 sqft a mod con would be overkill unless you also used it for an indirect dhw tank.

Just so I'm clear on this point, the mod/con would be "overkill" as in $/BTU? Meaning it would be more expensive than necessary for this load but still function in a reliable way?

EBEBRATT-Ed said:

@OilfieldHippie
If the steam boiler fails, you will have a place to stay warm and the option to heat domestic hot water with the new boiler is a good idea.

This is a point I hadn't thought about, @EBEBRATT-Ed. Having a warm space in the event of either boiler having an issue is very appealing.

Mad Dog_2

Like Ed said, the redundancy of having a seperate heat source is a Godsend when one goes down.  A totally seperate heat source (small boiler) is perfect for a den or extension.  With the great turndown ratios, of today's Wall Hung boilers & combis, its harder to oversized but if the load is That small, a Hydronic zone off the steam boiler condensate will make the most sense.  Mad Dog 🐕 

ScottSecor

If you decide on hydronic boiler you might want to consider heating some other areas of the house or even the garage? You obviously know the benefits of steam heat, but if you're thinking of replacing the steam boiler someday, you could use a smaller model.

offdutytech

@OilfieldHippie with a modcon you can absolutely do a radiant floor loop with it, but is it cost effective for your situation. With old 800 sqft it might be nice, but will it fit the reno budget? If the thought is to have a backup heat source or have the means to add more zones in the future like having it do DHW, then yes it's a good value. Each situation is different. I've quoted similar setups for customers and it's gone both ways. 
1. No that's too expensive for the budget for whatever reason.
2. We don't care on the price, that's what we want and like the diversity of it. 
At any rate get a load calc done on the space, get quotes so that you have all the info on hand.