Help on New Boiler Size Please

Chainreaction

My wife and I are going to install a new boiler in our home. We have a couple of quotes with very different BTU ratings. To figure out what to do I started searching the net. Mostly what I found out was that this is a complicated topic and I don't know anything and that installation is key. Seems like a lot of experts are here and I hoping for your advice.

The House and Current System
This year we moved into a new house. This house was build around 1904. It is stone and stucco. About 4500 square feet over three floors and a full basement . There are high ceilings all the way up and lots of large windows. The current boiler is 225K and was installed in the 1980s. I think the house used to be on gravity feed but we now have a pump on the return line. The house has old fashioned hot water radiators. Some really big ones. There are 4 inch pipes in the cellar part of the system. Most of the house is on one zone however there is a single large room that has its own manual thermostat. The first cold week of the year we found that the house does not get over 62 degrees F, and most of the time it got no higher than 60 degree with boiler going full blast. The water temp is set at 140 degrees. When we questioned one of the contractors on this he said it could go higher, but that we "shouldn't push this old system". The single room mentioned above does better but we do not use it much, so no help. House is in southern PA so it gets cold but rarely below zero.

The Issue
We had several contractors in to bid on a new boiler. We have two that seem reasonable, however, the proposed systems seem really different. Both contractors measured radiators and took into consideration the windows etc. This is not a price issue. Neither is cheap and I would rather pay a little more than get the wrong system. One contractor suggests two Weil-McLain ECO110Ns (95% efficient) with four system circulators for a primary/secondary system and two thermostats. So 220 total BTU I guess. The other bid is for one Lockinvar Knight 210 and one Lochinvar Knight 150 (360 BTU total?). TACO zone valve Grundfoss circulators plus outdoor sensors.

The Question
Obviously there is a big difference in capacity here. Seems like one of these is pretty far off. Anybody have an opinion? Anybody have a suggestion? What questions should I be asking? Anything else you notice? Any help appreciated.

Harvey Ramer

What part of Southern PA are you located? I may be able to help you on site if you aren't to far away.

Harvey

KC_Jones

Did any of them do a room by room heat loss calculation on the house? If they didn't keep looking. Have you tried the find a contractor link on this site? There are plenty of good contractors here, where are you located specifically? We may be able to recommend someone.
https://heatinghelp.com/find-a-contractor/

Jamie Hall

If Harvey is willing to help, and you're willing to hire him, it would be hard to do better.

That said, you can "push" that system with higher temperatures, which will help with the heating issue. You may be pushing the boiler pretty hard, though, so that should be looked at. There are controls on the boiler for the temperature of the circulating water, and that's where I would start -- by raising that temperature. That may give you enough heat to be comfortable for enough time to do a proper job of choosing the boiler and the installer.

And a word on that: particularly on gravity systems, you aren't looking at plug and play. You need an installer who will take the time to understand the house and the system, and do it right the first time.

Such as, to go back to the beginning here, Harvey...

Gordy

Harvey is your man if he is close. Deffinetly someone you can trust to do this proper.

On a side note part of the issue is water temp to the rads.....maybe.

Could be air in the rads, or system.....maybe.

Could be a circulator sizing issue......maybe.

As you can see it really depends what can cause the lack of heating.

On a side note gravity systems can be really nice, and efficient IF PROPERLY done.

Has the envelope of the house been upgraded that you no of?

Chainreaction

Wow pretty fast answers. I'll try to address some questions.

House is in Philadelphia so Harvey you seem pretty far away

I am probably wrong but I thought the presence of a pump meant that the house was no longer gravity feed. Is this true? All new systems suggest a pump.

We've bled the radiators a lot and they seem to be getting hot.

Outside of house is original except for storm widows on the large windows. Small windows (2 ft by 2 ft or less) do not have storms. House is reasonably tight for its age. Rear entry is unheated but we are going to add small cast iron radiator there no matter what. I doubt that insulation has been added to walls but there is some in attic.

Harvey Ramer

Philly is a bit far.

Many thanks to all the folks who put in a good word for me!


A good question to ask your contractors is whether they are sizing the boiler based on the heat load of the house or whether they are sizing based on the amount of radiation. The proper way to do it is to size based on the heat load of the house. 100K BTU difference is to much. One, or both of them, did not do the calculations correctly. I know plenty of contractors around here size the boilers based on the radiation. Why I don't know. Maybe it's a left over product from the dead men doing steam systems?

As always, if you are getting a new high efficiency condensing boiler installed, make sure the boiler is protected from the debris in the old system. They make some nice dirt mags/seperators now to do the job.

I am curious about your current boiler? Is it a cast iron boiler? Perhaps a converted coal boiler? If it is cast iron, you need to run the system at a higher temp. If your only at 140° on the supply, your return water is likely cool enough to condense the flue gasses inside the boiler. Not only does this rust away the heat exchanger, it also creates an evaporative wall between the hot gasses and the heat exchanger, thus preventing a lot of needed BTU's from transferring into the system water in the normal fashion. I would set the system temp at 160° and see if that helps.

Paul48

Is the current boiler oil-fired? If so, when was the last time it was serviced?

Chainreaction

Current boiler is gas fired. I doubt that it is converted from coal. Don't know about cast iron. I would look but I'm currently in another state. (long story)

Thanks for all the help I'm getting here. It is really appreciated.

Gordy

That's a shame, use find a contractor at the heading on the home page here. Don't know your zip, but Jim Pompetti, and Gary Davis I think are close. Some good people.

You have a converted gravity. Agree with Harvey you need to get the temps back up around 180 to get the return temp up.

Bottom line you need a room by room heat loss. The btu output of the present radiators needs to be calculated to see how well they are matched to each room. If each room is over radiated that is good news for a condensing boiler. What it means is you can run lower water temps most of the heating seasons which helps up the efficiency with that type of boiler.

It would seem both contractors want to stage boilers that's good IF the heat loss dictates the need. I would favor the Lochinvar WHN Fire tube boiler over the WM Eco fire tube. The loch has a few more bells and whistles. In either case outdoor reset should be a part of the package

Rich_49

You will get good advice here . You are already being smart about contractors . You'll certainly end up with a good product with help and a good ear .

Chainreaction

I want to thank everyone who sent comments. I feel a lot more educated now. But I know I'm still pretty ignorant. I will check on how calculations were done. I'm beginning to suspect that the lower BTU system is the closer estimate. At least contractor says all the right stuff.
Should I / Can I ask for a print out of how the estimate was calculated?

Chainreaction

Thanks Hatterasguy. If by design day you mean a day with the lowest temperature the system was designed to handle, I have no idea what that is. This system was put in decades ago. We are having trouble on 20 degree days. While Philadelphia is not really in a cold region, 20 isn't that unusual. I'd be surprised if this boiler has seen a serviceman in the last 10 years so I'm sure its not running at full efficiency.

Paul48

My house is from the mid 20's, about 1800 sq/ft. It has new windows, but no insulation, and the heat loss is about 100k. His house is 4500 sq/ft. With his boiler putting out 180 or so, Houston, we have a problem.

Chainreaction

yes we do!

Paul48

Make sure the installer you choose has done an accurate heat loss. You also want to see some of his installations.

Chainreaction

Thanks Paul. That has been my most important take home lesson from this discussion. The last thing I want to do is spend a bundle and still be cold. I can freeze for nothing.

Paul48

Believe me.......Anyone that can do your job correctly, will take great pride in their work.

Chainreaction

Thanks again Hatterasguy. Good advice.

Gordy

Last I saw the boiler was set at 140. No post since has indicated the aquastat was raised to 180 to see if this helps. Pump running constantly, and boiler firing constantly are two different indicators. It needs to be clarified what boiler running flat out is.

Is the burner firing the whole time?

If so is it reaching the high limit of 140 then shutting down, and relighting?

Or are you saying the boiler is running because the pump,is running?

Is the present boiler 225k input, or DOE?
To say the present boiler is to small If the high limit is still set to 140 is incorrect. Without more details on what is actually happening with the boiler.

Edit: we don't even know if the circ is the right size.

You have to ask has this always been this way. I would hardly think past owners would have settled for 62* is it..... Maybe. Usually boilers are predominantly oversized to begin with. No one ever gets a call for a heating system that heats.

Paul48

I only meant to suggest that it was a possibility. It's one we can't answer accurately, with the information we have.

Gordy

You understand that, I understand that, but does the OP.

hot_rod

boiler must be condensing it's brains out also. 1980 install, must be cast or copper?

Gordy

One other burning question is system pressure? OP discription 3 stories with high ceilings all the way up. Is 15 psi enough? Is it set to that?

Are all radiators getting warm? All the way across the rad?

Paul48

If it's cast, the back sections will be plugged solid with rust. 180k just became 120k. I know........WAG

Gordy

I agree, and for how long? Don't push the system was the reasoning??? 15psi is 15 psi at 140, or 180. If she's going to blow the piping then more than a new boiler is in order. I hardly think that's the case. Someone does not understand hydronics completely . The HX is probably fouled to no tomorrow condensing with all that high volume system water at 62* curious on the near boiler piping.

Gordy

Paul48 said:

If it's cast, the back sections will be plugged solid with rust. 180k just became 120k. I know........WAG

And could be a reason it does not reach 140 high limit setting......if that is the case.

Gordy

Hatterasguy said:

Point.

He never confirmed whether it was running 100% of the time during his observation at 20°F.

Actually, I have a hard time believing that it was at 100% because an SWT of less than 130F. cannot send but a fraction of the necessary BTU's to the radiation.

Pondering if the radiation is so large that it can deliver 180K to the building with 135F SWT................???? That would be quite a feat.

Well at 225k you would need 1500 SF of radiation at 180* Don't know what the house load actually is

nicholas bonham-carter

ChainReaction, you can spend a few interesting hours with the SlanFin heatloss program/app, available from their website. This will enable you to double check the boiler sizing. The room by room feature will catch any undersized radiators.
I also doubt that an underperforming boiler would have been in service for so long, so I would assume that it worked at some point. Maybe such a large circulator had been put in that the water races around unable to pick up the heat.--NBC

Chainreaction

Wow, reading this discussion I realize how little I understand about this. I can see my imprecise description has lead to some questions. I'll try to do the best I can to clarify. Just remember I'm no expert. For example I have no idea what SWT is.

First, the boiler is a 1980 A. O. Smith, cooper. 225 input I think.

By going full blast what I meant was this. During cold days (10F) we can't get the house temperature above 60F no matter what the thermostat is set at. The system does seem to cycle in that the radiators get hot and then cold, then hot etc over time. I hope this clarifies things. If not I'll be happy to investigate if someone can tell me what to do.

All radiators get warm all the way across during heating. One radiator occasionally does not heat all the way across. We can fix this by bleeding it. We have had to do this maybe once a week. I don't know if this matters but, this radiator is the one highest in the house by about two feet.

I have a couple of specific questions.

We would like to try to increase the water temperature. Does anyone have an idea how this is done on this boiler? Just a rough idea would help. I'll do it myself but I want to be sure I'm not going to break something.

Nicholas B-C: My wife and I are just science nerds enough to try to do the heat loss calculations ourselves. Is the free version sufficient? Any suggestions for use?

Finally I would like to, again, express my thanks to this group for all the help.

Gordy

There is an aquastat on the boiler with a temperature dial, and an arrow indicator pointing to the number setting. Turn the dial to 180. It should be at 140 now if the tech set it to that as you have been told.

Your boiler is cycling off the present High limit of 140 so the water is not warm enough to sufficiently heat the structure. By being at 140 on the aquastat the boiler reaches that temp the burner shuts off, and then relights at about 130.

Check system pressure on the boilers Tridictator. Should be a gauge that shows pressure, and temperature. Pressure should be 15 psi.

See how that works.

Chainreaction

We will do the experiment tonight. If no is the answer we'll try to raise temp. If the controls are not obvious pictures will appear.

Thanks

PS I've never dreamed of 70F. We'd be all giddy with 65F

Harvey Ramer

Prepare to be giddy

Gordy

By the Fact the radiators get hot then cold tells me the boiler is cycling. If the boiler was running flat out the emitters would be a constant temperature never achieving set point. never getting above 140 for that matter. Another thing is if the HX is fouled.

Chainreaction

HX is? (I feel like I should go back to school and get another degree)

Harvey Ramer

Heat exchanger. The sections inside the boiler where the heat produced by the flame is transferred to the water inside the boiler. Typically smaller studded passages ways inside cast iron boilers.

Paul48

Heat Exchanger

Paul48

Not sure with that boiler, but the aquastat may say Honeywell on it. With a picture, we could steer you.

Gordy

Let's see if the 180 works. Give it some time if you set the tstat to 70. Your heating up a lot of mass. rads, water, and structure, furnishings etc.

If it does not work ( I think it will) then the heat exchanger condition would be the next step.

The unknown is how long, and or how much the boiler has been condensing with the low aquastat setting. We will talk you through that one if your game.

Paul48

The radiators getting hot and cold bothers me. Cold enough to be noticable, to me, means t-stat satisfied. I hope he gets the heat.

Gordy

I think it's cycling. Depends on frequency. 140 to low limit will happen pretty quickly with all the gravity system mass once things get rolling. 60* return water.

If it does not work then the next thing would be checking how plugged the HX is.

What have learned is that over radiation is not there. Unless we have some other unknowns like faulty aquastat, temp gauge etc. maybe 140 is not even happening. Maybe over pumping.