High efficiency boiler for inefficient house

nystateofmind

Contractor suggested that a high efficiency system may not be a good match for our house (4900 sq ft, old and drafty) and recommended a System 2000 instead. Said this was because the house is a "high mass" system but the boiler would be "low mass". I'm not an engineer so I can't connect the dots. Does this make sense? Wouldn't a high efficiency system always be more efficient than a standard efficiency in the same setting?

Thanks,
FB

EBEBRATT-Ed

In general high efficiency systems are not "high efficiency" all the time. During cold weather there efficiency is similar to a standard system. People see "95%" but you only get that in a system running with a water temperature low enough to produce condensation.

High efficiency modulating boilers have other advantages and can condense (run high efficiency) during the shoulder seasons spring and fall. They also modulate the boilers input to match the load. This reduces cycling which helps efficiency and reduces standby losses.

High efficiency is a more expensive system to install (rebates may be available).

So in short everything needs to be analyzed , the system, the boiler, how long you will live in the home etc

A good installation with a properly sized boiler has more to do with efficiency and a good contractor to go with it is most important

Steamhead

Also, if the house is old and drafty, IMHO you're better off tightening it up first. Then, you can buy a smaller boiler when you replace it.

What boiler do you have now?

nystateofmind

It's a 76 series Weil McLain, which I think is a commercial boiler. A different proposal we received was for two Weil McLain WGO-6 boilers. Is that common to run two boilers to handle the load, or is that just asking for problems?

John Mills_5

What is your radiation?

nystateofmind

Cast iron radiators, about 20 of them.

Jamie Hall

I presume hot water? You don't say...

KC_Jones

What is the building heat loss, need that to properly size the boilers.

I will say 2 WGO-6 is probably double what you need at least. That would be about 86 BTU per sq ft and that is ridiculous. Well unless you plan on heating with all the windows and doors open.

That being said if you do go with 2 boilers it gives you more flexibility with capacity. Heat loss is for the coldest day so the rest of the time you would be oversized. With 2 cast iron boilers you could run at 50% on milder days and increase efficiency of the system. That is as long as they don't massively oversize like they are planning.

bob eck

Have the heating contractor do a heat loss to see what size boiler is needed.
Then do a heat loss as if you have new windows and doors plus wall and attic insulation.
See how much of a BTU difference that would be.
Putting money in new windows, doors and insulation will make a big difference.
Are you staying with oil or going to natural gas.
If staying with oil how oil is your oil tank? If that needs to be changed now or in the near future that will be an extra expense.
If going to natural gas there are some gas utility companies and some states offering rebates for high efficiency boilers.
A high efficiency gas boiler loves systems with large cast iron radiators because of the water volume and most homes with cast iron radiators they are way oversized for the rooms they are in.
Older systems worked with higher water temps of 160 to 180 degrees to heat the home. With a condensing gas boiler with outdoor reset you can use lower water temps to heat your home.
If staying with oil take a look at using the Trio oil boiler from F W Webb and you can also do outdoor reset on this boiler and run lower water temps to heat your house.
With 20 cast iron radiators on one zone take a look at going with two or three heating zones that way if there are rooms you are not always using you can keep them a little cooler saving some money.
An oil boiler with an indirect water heater will save you money vs a oil boiler with a domestic coil in it.
When replacing your boiler there are many options. I would work with a professional heating contractor that can go over several options so you can make a good choice.
A good oil boiler cast iron like the Trio oil boiler or even the system 2000 should last you 20 to 30 years with proper maintenance.
Condensing high efficiency gas boilers their heat exchanger warranties are from 10 to 15 years. These boilers if sized right and installed right and if the home owner has annual maintenance on the condensing boiler they should last 15 to 25 years. The parts warranty on most condensing boilers is 5 years and as these parts fail out of warranty they can be expensive.
There is a lot to consider when replacing your boiler. Remember there is a cost to put a new boiler in but what is your cost to operate that boiler vs another boiler for the next 20 to 30 years.
A lot of home owners go with the lower upfront cost of a boiler installation but in the long run they are paying more because their heating system is not as efficient as it could be. Most times spending a little more money up front will save big dollars over the years.

nystateofmind

It's hot water. Regarding heat loss calculation, here's what I was just told by one contractor, let me know if this holds water (haha): the reason you use manual J is to determine heat loss so you can size your radiators and boiler. Since radiators already exist in our house, they can count up the radiators (accounting for size, type, etc.) and use that to back into a boiler size. They could do a true manual J but if that result says we need smaller radiators and we don't want to change them out (we don't) they have to heat the radiators that are there. Does that sound right?

We definitely plan to convert to gas and I'm leaning toward HE (any comments on the Weil McLean Evergreen?) so the main question is how to size it optimally. I will say that the former owners were burning about 3500 gallons of oil per year as a reference point.

Leon82

That sizing method about adding the radiators output at 180 degrees it going to oversized almost every time. you may only need half that on design day.

With the manual j you will find out what temp water you will need on design day and can work back to find needed btu

You don't change radiators, you can lower your water temp.

Steamhead

nystateofmind said:

It's hot water. Regarding heat loss calculation, here's what I was just told by one contractor, let me know if this holds water (haha): the reason you use manual J is to determine heat loss so you can size your radiators and boiler. Since radiators already exist in our house, they can count up the radiators (accounting for size, type, etc.) and use that to back into a boiler size. They could do a true manual J but if that result says we need smaller radiators and we don't want to change them out (we don't) they have to heat the radiators that are there. Does that sound right?

That guy obviously doesn't know how easy it is to do a heat-loss calc on a computer. All you need is the right program.

bob eck

With a heat loss you might see that your radiators are way over sized. That way even on design day you might only need 150 to 160 degree water temp going out to the radiators.
The evergreen is a good condensing gas boiler. I like condensing gas boilers with fire tube SS heat exchangers.
You might want to look at installing two smaller condensing gas boilers that way most of the heating season you could run on only one boiler.
How are you going to get your domestic hot water?
Take a look at some of Lochinvar’s condensing gas boilers. The Noble is a combi boiler that will give heat and domestic hot water. Also their Knight boilers are very good you need to use an indirect water heater with Knight boilers. One model Knight boiler is wall hung and one is a floor standing model. All three have an SS heat exchanger.
Condensing gas boilers are good but they need to be piped in the correct way plus vented the correct way. Water quality is also critical on condensing gas boilers have the contractor install a dirt magnet filter. You can buy a high efficiency condensing gas boiler and if not installed right the boiler can have a short life. A heating contractor or plumber can also install a regular gas or oil boiler not according to manufactures instructions and that also can shorten the life of a boiler.

nicholas bonham-carter

Download the SlantFin Ap, and do the calculations yourself.
The program will even suggest the model of boiler.—NBC

EBEBRATT-Ed

If you had a steam system sizing by the radiators would be correct

SINCE you have hot water you size the boilers by the heat loss of the home.

There is a lot of good advise from the posters above. Use it, have a proper heat loss done (you can download the Slant Fin App) and do it yourself and consider insulation and window replacement.

This is a long term investment. You want to make the right choices

mikeg2015

Sometimes a more accurate heat loss method is taking heating degree days and working your gas bill backwards. The math is a little tricky, but it's very good.

Or when it's 35F outside, time how long your boiler runs per hour. Double that, and figure out what size you would need for the boiler to run continuously. It's a lot smaller than you'd expect.

You also probably only need 160-170F water temps, with 145F return temp at design temp (coldest weather). One olution then, is to is to install a mod-con for about 1/2 the capacity, and a cast iron boiler for the remaining 1/2. If you have a primary/secondary piping, then you can pipe the boilers each as their own primary, then have them in series on the secondary loop. The mod con takes all the load above about 35-40F, below that outdoor temp, the cast iron boiler comes on periodically to raise the water temp. You might be able to experiment with flow rates and get the delta T above 20F, closer to 25F. Then you can be party condensing at 135-140F most of the time weather, maybe 88-89% efficient. The Mod con would raise the water 15F, the cast iron boiler the next 15F (as measured on the secondary) and it would be at maybe 86%.

3rd, the mod con would be a "swing" boiler. You could set it to control to secondary loop return temp and swing up and down as the cast iron boiler cycles on an off.

The trick is getting the outdoor reset sorted out.

mikeg2015

I'd use the modcon for the indirect water heater. Cheaper to operate in the summer for domestic water.

KC_Jones

It doesn't hold water he's being lazy and sloppy. Imagine what else he might cut corners on doing the actual work.

What @Steamhead said is the truth with the proper program it isn't that hard. Even the free apps would get you pretty close. A lot closer than what those contractors are proposing.

SeymourCates

@nystateofmind

We can do quite a bit with your previous value of 3500 gallons/year of oil.

Permit me to make a few assumptions:

1) You live in NY and we use LGA as the point of reference for degree days. For 2016, LGA had 4200 degree days with a base temperature of 65F (the point at which no additional supplemental heat is required).

2) You use 200 gallons of hot water per day. Requires 38M BTU annually.

3) The current system efficiency of the oil boiler is 65%.


So, the usage of 3500 gallons is 483M BTU which equates to 314M BTU net. Subtract the 38M BTU for hot water and the house requires 276M BTU for heat annually. 100K degree hours equates to 2760 BTU/degree hour.

On the design day (assumed to be 6F), you require 162,840 BTU to heat the building.

If you utilize a new modulating and condensing boiler, the system efficiency is going to be at least 85%.

Therefore, the boiler requirement is not more than 191,600.

Personally, I'd look into a pair of UFT-100's which will operate seamlessly together in a cascade arrangement. No need to run the second boiler for the majority of the heating season:

http://www.htproducts.com/UFT-Boiler.html

These boilers will work perfectly in the high mass system that you have and they are very cost effective.

Also, your "contractors" are basically incompetent.

nystateofmind

Thanks so much for all the great input. Seymour your example was very instructive and helped me understand the basic math involved which feels very empowering!

I got the slantfin app and will tackle that tomorrow.

Is the efficiency of a two boiler system really that much better than a larger single boiler to offset the greater costs, both up front install and ongoing service considering twice as many things to fail?

We do plan on doing an indirect water heater.

Thanks very much

bob eck

When you have two smaller boilers vs one larger boiler and let’s say it 0 degrees outside and your one boiler system that boiler goes down guess what you are out of heat and hot water. With a two boiler system you can at least run the boiler that is still working your house might only heat up to 55 or 60 degrees but you still have heat and hot water until your heating professional can catch up with all of his / her service calls and fix you other boiler. That my friend is PRICELESS

njtommy

Yes two boilers are more efficient, will you see a pay back for running two boilers for utilities? Probably not, but it’s more effective and efficient to run two boilers at lower firing rates then 1 boiler at higher firing rates. Like Bob said a back up is very nice thing to have.

DZoro

Perfect application for a Lochinvar Noble 199, covers your domestic and heating, great stainless fire tube system. 20,000 to 199,000 btu range

GW

I stopped being amazed years ago regarding how many heating people don’t understand the European heating strategy. If it’s a high mass system all the more reason to use a high-efficiency boiler. The hard part is finding a contractor that can properly set up the control system to operate at Constant circulation.I promise you will get a dumb lok from nine out of 10 heating contractors.

mikeg2015

Those HTP UFT's have excellent specifications. Domestic Indirect integrates seamlessly I tgink with a tank sensor. They can cascade and can handle a very wide flow range down to very, very low flows. You can therefore most likely use them on a variable flow primary and add TRV's to the run the pups based on pressure, then use outdoor reset... which is also integrated in the units. The UFT are a medium mass boiler and have a wide turndown. With 2 boilers, you'd have a 20:1 turndown.

Are 2 boilers more effective, yes, you gain about 1-2% because 2 boilers at low fire, is more efficient than 1 at a medium fire rate. On mild days, you could achieve 97% inefficiency.

SeymourCates

@nystateofmind

The decision to utilize twin boilers is not made on the basis of efficiency. The single large boiler will likely come close to the efficiency of the twins.

The decision is made on the basis of redundancy. These units have some intricate internals that are not easily replaced in the dark of night when the outdoor temperature is 10F. The general state of affairs today is that no servicing company can stock all of the necessary parts for all of the various boiler makes and models out there. So, it's always several days before your system is returned to operation.

The decision can also be made on the basis of function. If your home is divided into 5-6 heating zones, it is very likely that a single zone will require less than 20K BTUH in the warmer months. The large boiler cannot get down below 20K, and, depending on which manufacturer you select, the large boiler might not get down below 40K. This now requires the boiler to cycle which is undesirable. The twins can get down to 10K which is most likely the level that is required for your smallest zone on a warmer day.

Finally, the decision can also be made on the basis of cost. The purchase price of the larger boiler is going to be 1.7X the cost of one smaller boiler. So, there is an increase in installation cost but it is not excessive considering the aforementioned benefits.

bob eck

> @Dennis said:
> Perfect application for a Lochinvar Noble 199, covers your domestic and heating, great stainless fire tube system. 20,000 to 199,000 btu range

Yes I work for F W Webb and we sell the Lochinvar Noble combi boiler. The 199 delivers 4.8 GPM of domestic hot water and on the heating side it actually goes up to 199,000 btu input.
I you look at the Navien combi boiler there largest goes up to 199,000 for domestic hot water but only to 120,000 btu on the heating side.

A true heat loss will let you know what size boiler to go with. Plus with the Noble you could put in two boiler together.

If you find that your actual heat loss today is 200,000 and you install the Noble 199,000 and as you put new insulated windows and doors and then add wall and roof insulation and say the btu heat loss goes down to 175,000 your heating contractor can come back in the future and the heating btu side of this boiler it can be lowered.

If the home needs say 300,000 btu I would look at installing two Noble 150,000 combi boilers.
On the domestic side you would have 7.2 GPM of domestic hot water.

There are any options check them out.

nystateofmind

Update:

Heat load
Slant-Fin gave me 190,000 load. Backing in from oil consumption gave me 169,000. So I just have to decide on safety factor and if we might add additional load later (considering an addition) but I have a good starting point. Contractors recommended 399,000 and 405,000 BTU, so just a little bit high

Modcon applicability
Based on radiator EDR (as best I can estimate it), by the numbers I would only be condensing when the outdoor temp is above 45F. Using degree day data, about 44% of the heating needed throughout the year would be in condensing mode. I'd prefer that number to be a lot higher obviously. I'm basing that on 140 degree AWT as the threshold. Anything colder than 45F would need hotter water, per my calcs. Is 44% reasonable? Does any amount of condensing justify the cost?

Contractor buy-in
I've talked to three additional contractors since first posting and all three sing from the same sheet: those old cast iron radiators have to run on hot water so you never condense, and modcons are nothing but trouble. My calcs show they will condense (I'd love to be convinced that they would condense even more than 44% of the time) so there's some truth, but I don't want someone installing a modcon that thinks it's not going to work and give me I told you so's when I need service.

Timing
We still have the issue of scheduling the gas company and hoping the leaking oil boiler lasts that long (probably needs to last a few months). Basically will it survive this cold season or do we need to settle for an oil boiler and look at an upgrade in a couple years after tightening up the house, completing any addition projects, etc. It's actually tempting to go that route, get out of the woods with the leaking boiler, and take our time to tighten up and re-assess our heat load then move forward with a modcon system. I can probably justify that cost with the fuel savings we get just from the new standard boiler and conversion to gas in a few months.

Thanks for all the great input and guidance. I went from being completely clueless to knowing just enough to be dangerous in no time!

nicholas bonham-carter

Some cast iron boilers can be ordered with an oil burner, which can be switched out for a gas burner. At the time you do the changeover, you could add the necessary parts to have outdoor reset.
A pair of boilers would not only give you redundancy, but also would be a kinder job for the lead boiler, with longer run-times leading to better combustion.—NBC

NY_Rob

nystateofmind said:

I've talked to three additional contractors since first posting and all three sing from the same sheet: those old cast iron radiators have to run on hot water so you never condense...

Complete BS from the contractors.

The old cast iron rads were fed 180F water for 30min then they sat dormant for an hour before being fed 180F water again... and so on and so on.

The mod com will supply them with 140F water for 60+ min at a time without pause till setpoint is met. Less cycling more even heat.

Mod-cons love cast iron radiators, all the heat output is literally going into the radiators- no waste! As long as you have enough of it you can feed cast iron 120F water all day long and condense all day long... and the house will be warm.

nystateofmind said:

I've talked to three additional contractors .... modcons are nothing but trouble.

Yeah, if you size and install them improperly they are troublesome.

njtommy

Even if you have to run your swt at 160 on design day you can try slow your water down and drive your delta T up to say 30-40f. Giveing 130-120 return water temps. That would help you stay in condensing range. I would go with two Mod con boilers and find a different contractor.