Evaluating the size of my current boiler

Lakefront

My house is about 10 years old, and right now it has an oil fired boiler (WM Ultra Oil UO-3) that feeds both a radiant slab on the first floor and hot water baseboards on two other floors. The radiant is fed by a Honeywell mixing control with outdoor reset. Everything else is at 180 degrees. The system performs very well. Recently, we've had natural gas run to our house so it's now time to plan the upgrade. My question is, what’s the best way to evaluate the size of my current boiler to find out if the new boiler should be the same size. I'd like to move to a mod/con. The house has 4 zones plus DHW tank. Zones are 31k btu, 13k btu, 29k btu, and 12k btu according to the calc that was done prior to building.

A few contractors have looked at it, and they’ve simply recommended that I go with something a little bigger than what I have (‘just to be safe’). My trouble with that is that I have a HVAC-calc workup from when the house was built and it came up with 84,000 BTU heat lose at 0 degrees (our low temp), and my current boiler is 107 BTU. They’re recommending 120-155 depending on the guy. Seems excessive but I don’t want to get the sizing wrong. So is there a way to evaluate how well my current boiler is doing with my heating load? I'd like to understand if it's correct or if I should be looking to stay around 100 or even lower.

Canucker

If your heatloss calculation is accurate, and because you say the system performs to your expectation, I don't see what their justification to upsize it would be. To be safe for what? Unless you're planning on keeping windows open all the time, there's no justification at all. With the fluff that is in most calculations, unless you're aggressive with the ACH, I'll bet you could drop to an 80000 btu/hr modcon and still be fine.

STEVEusaPA

How much oil did you burn in the last year, and what's your city/state

SuperTech

That sounds like a great boiler with a lot of life left to it. I certainly wouldn't want to replace it for a mod con.
Why do you feel like a mod con is such a better option? Not a whole lot to gain in efficiency, the ten year old cast iron boiler should outlast the mod con and will be easier to service and source replacement parts for. Unless you have natural gas available or really inexpensive LP I'd keep the Weil McLain.

Lakefront

Canucker said:

If your heatloss calculation is accurate, and because you say the system performs to your expectation, I don't see what their justification to upsize it would be. To be safe for what? Unless you're planning on keeping windows open all the time, there's no justification at all. With the fluff that is in most calculations, unless you're aggressive with the ACH, I'll bet you could drop to an 80000 btu/hr modcon and still be fine.

Canucker, I agree and that's what prompted my question. Seems like it should be possible to verify how well my current boiler fits the load, since it was sized based on an estimate, but I'm not sure how to go about that. Turn on all zones on a cold day and see how it does?

Lakefront

SuperTech said:

That sounds like a great boiler with a lot of life left to it. I certainly wouldn't want to replace it for a mod con.

Why do you feel like a mod con is such a better option? Not a whole lot to gain in efficiency, the ten year old cast iron boiler should outlast the mod con and will be easier to service and source replacement parts for. Unless you have natural gas available or really inexpensive LP I'd keep the Weil McLain.

We just had natural gas run down our street and the meter is sitting outside my house waiting for a boiler to be installed :-)

Lakefront

STEVEusaPA said:

How much oil did you burn in the last year, and what's your city/state

I'll check on that. I'm in northern NJ (07850) but I also have a wood burner that I use for heat most of the winter, so my oil usage is probably lower than average.

Jamie Hall

You really have two good options for evaluating your present boiler -- and by extension, a new gas fired one should you go that route (I wouldn't, for the same reasons that @SuperTech mentioned, but there may be other incentives). First, the HVAC estimate. Even if it's 10% low, that's still less than your present boiler, never mind what you are being quoted. Second, does your existing boiler keep the house warm on the coldest days when you are not using the wood burner? If so, there's no need to go bigger, is there?

Keep in mind that mod/cons, particularly, function best when they are very closely matched to the load. I'm a little concerned about your comment on 180 water for the rest of the house. That's fine with a cast iron boiler such as the one you have, but you would likely lose much of the advantage of a mod/con if you had to use that, unless you had some really good piping and control to use the hot water from upstairs in the radiant to try to get the return temperature down.

Robert O'Brien

https://hydronicshub.com/heat-loss-calculation-on-every-residential-boiler-replacement

Lakefront

Jamie Hall said:

You really have two good options for evaluating your present boiler -- and by extension, a new gas fired one should you go that route (I wouldn't, for the same reasons that @SuperTech mentioned, but there may be other incentives). First, the HVAC estimate. Even if it's 10% low, that's still less than your present boiler, never mind what you are being quoted. Second, does your existing boiler keep the house warm on the coldest days when you are not using the wood burner? If so, there's no need to go bigger, is there?

Keep in mind that mod/cons, particularly, function best when they are very closely matched to the load. I'm a little concerned about your comment on 180 water for the rest of the house. That's fine with a cast iron boiler such as the one you have, but you would likely lose much of the advantage of a mod/con if you had to use that, unless you had some really good piping and control to use the hot water from upstairs in the radiant to try to get the return temperature down.

Great info, thanks. I think I'm started to get the picture.

I want to replace the boiler for 2 reasons:

1. The Weil has been somewhat unreliable due to the fact that it's direct vented and I live in a windy location. It has a tendency to dirty the nozzle and then lock out at the worst possible times (like when I'm away on vacation). I've had it checked and cleaned by multiple companies with experienced techs, and nobody can get it right. They've even worked directly with Weil and failed. I've learned how to replace the nozzle every 3 months to keep myself out of trouble :-) I won't miss this thing at any price.

2. Natural gas is just way cheaper than oil around here. Less than half the cost per year plus $1000 in rebates if I go 90+ eff.

When I say 180 degrees, I just mean that the system runs that now, and there are no heating issues. Everything works well. According to the HVAC-calc report, my main floor for example has 23k heat loss at 0 degrees. I just measured the baseboard and I have 77 feet of Slantfin 30 and a kick heater - which comes to 48k of output capacity at 180. So at least in that case, I don't think 180 is needed very often :-) or ever. This conversation has helped be get down the road of figuring this stuff out. And I went through the house and inventoried all the heating elements, and the total capacity comes to 105k, so it looks like my current boiler is a good match and I definitely don't need anything larger. Could probably get away with the 80k as mentioned above.

This doesn't factor in the 50G DHW tank, though. I do have one of those soaker tubs, but it doesn't get used much.

Also, I'm not totally sold on the mod/con. The local tech that works on my current oil boiler recommended a Burnham cast iron ESC5. He says it's not as efficient but more reliable. So I'm still seriously considering that type of boiler, but then I lose the rebates and the cost of the boiler is similar.

JUGHNE

The nozzle size and pressure of the oil burner could tell you the input BTUH.

40 years ago when I serviced oil burners (probably badly), the few I worked on would get a nozzle downsize with every servicing. I would go down to the lowest size allowed by the nameplate of the furnace.
Then as the customer never complained about too little heat, I had that basis for sizing the gas or heat pump (yuk!) replacement.

Have you considered a gas replacement power burner for your existing boiler?

Lakefront

JUGHNE said:

The nozzle size and pressure of the oil burner could tell you the input BTUH.

40 years ago when I serviced oil burners (probably badly), the few I worked on would get a nozzle downsize with every servicing. I would go down to the lowest size allowed by the nameplate of the furnace.
Then as the customer never complained about too little heat, I had that basis for sizing the gas or heat pump (yuk!) replacement.

Have you considered a gas replacement power burner for your existing boiler?

I have considered the conversion, even got a price from a pro for the work who had confirmed that Beckett had the right unit. Then I checked with Beckett myself and found out that they do in fact support the UO-3, just not in my direct vent application. This was one of the reasons that I decided to confirm all this myself :-(

SuperTech

Is the Weil McLain direct vented or power vented? I'm not a fan of direct venting that unit. Power vented with enough pre and post purge shouldn't have any negative effects on the nozzle. Perhaps a power vent installation (or stainless chimney) would provide you with another twenty years of operation with your current boiler.

But I understand considering gas if it's available. I'd look into Energy Kinetics. Can use oil or gas, reliable, efficient and non condensing. Can be sidewall power vented as well.

PerryHolzman

I understand your concerns with your current boiler. I suggest that installing a power vent on that boiler would solve your problems at the lowest cost.

Several other factors to consider:

1) The cost of installing any replacement boiler is likely to be significant. The payback on potential lower fuel bills may be many years - even over a decade. Other than the venting issue you likely have a very solid boiler already.

2) Not sure if there is one available: But, what about a gas conversion burner for your existing boiler. Those were very popular decades ago and retrofitted into many boilers. Worth checking into.

2) You need to find a contractor that is willing to work from a good heat loss calc - or just closely match your existing boiler size. The concept that they need to increase the boiler size for conservatism over your existing boiler does not in my book indicate that they are actual professionals, whatever their technical abilities.

3) The best results from a mod-con occur if the house can be appropriately heated with lower temperatures than 180 F. My mod-con has a maximum temperature of 167 F hard set into it. My house was designed for a large radiant boiler heating the basement well and 140 F water heating the rest of the house via cast iron baseboard in the mid 1950's. Removal of the old large radiant heating boiler in the basement drove my maximum heat temperature to 155 F for the house (and the basement is cool - even after I put up 2" polystyrene insulation on the concrete basement walls (and framed the basement so that it can be finished).

If your house actually needs above about 160 F water most routinely I'm not sure a mod-con will provide any meaningful efficiency benefit. The outlet water temperature must be low enough to allow easy condensation of the water vapor in the flue gases within the short distance of the heat exchanger surface (a physically larger heat exchanger would work at higher temperatures - just not aware of any being produced as that would be a more expensive boiler as well).

Your best results might actually be with a cast iron natural gas boiler.

4) Be extremely careful of installing a mod-con with any contractor unless that contractor has experience with the brand and model, and can make a promise to always service it down the road.

I have a one of a kind mod-con in my area (Viesmann Vitodens 200) that I had a contractor install about 13 years ago. They don't know anything about it and experience has shown me that they have no idea how to service it. When I talked to them about a problem I was having a year ago all they wanted to do was replace it with a new boiler (and I believe it is now ready for another 10-12 years without issues with the parts I replaced). I had to learn to do my own service and trouble-shooting. Admittedly, since I'm a power plant engineer with a lot of hands on experience with controls and rebuilding equipment I have the technical expertise do so. Few homeowners do.


I wish you the best,

Perry

PerryHolzman

Rereading above I notice that there is a gas burner conversion available, just not for a direct vent system.

I suggest that you look at combining a power vent with that gas conversion burner. I suspect that will be the best answer. You may need to pay someone to do some "engineering" paperwork for approvals; and Beckett may be willing to do that for you (Does Beckett sell the power vent option for your boiler?). Still likely the cheapest option.

Perry

Lakefront

PerryHolzman said:

Rereading above I notice that there is a gas burner conversion available, just not for a direct vent system.

I suggest that you look at combining a power vent with that gas conversion burner. I suspect that will be the best answer. You may need to pay someone to do some "engineering" paperwork for approvals; and Beckett may be willing to do that for you (Does Beckett sell the power vent option for your boiler?). Still likely the cheapest option.

Perry

I checked on that, but they only support conversions with chimney vent.

EBEBRATT-Ed

@Lakefront

Your original heat loss is likely correct.

If you want to check it get a cheap timer motor and wire it with (in parallel) with the oil burner motor.

This will record the hours the burner runs x the burner firing rate in gallons/hour x 140,000 will give you the btu input for whatever period of time the burner runs. The longer you leave the timer on the more accurate the reading.

Steamhead

A power venter will make the boiler think it's exhausting into a chimney. Then you can have a Beckett or Carlin gas burner installed in that UO-3.

EzzyT

@Lakefront if your really thinking of converting to gas along with a new boiler and don’t want to go the modcon route then I would suggest you look into the Energy Kinetics boilers. Great products and fantastic support. I see your not that far from our neck of the woods. If you want us to take a look at your system we would gladly present you a quote on the best possible set up for your situation.

PerryHolzman

@Lakefront
A powered vent is a special fan that is installed after the boiler on the exhaust (stack/vent) ducting. It creates an "induced" draft on the boiler and pushes the exhaust out a stack that does not meet "normal self venting" considerations.

From an engineering standpoint that is the technical solution you need to solve your current wind effect venting issues without major modification of you existing vent.

They are often wired so that they only run when the burner does.

This would allow you to install a gas conversion burner that would work properly.

Code and/or insurance compliance may require you to have an independent certification that your powered vent is the correct size for your application (or may not). Sometimes the Boiler Mfr's engineering department will provide this (and you need to talk to the engineering or Technical options department - not the sales department who only looks at the normal designed vertical stack drawings in the preexisting literature); which is the best solution as it comes straight from the Boiler Mfr. Sometimes you may need an independent consultant for it. In both cases expect to pay for this certification that the powered vent chosen is the correct sizing. For some places this certification will not be required at all.

You need a heating contractor who is willing to really look at the options (and you need to likely pay them for their extra time). I suggest that a little money spent up front with someone willing to work with you on this may provide a solution that no one has seriously considered (or presented to you) before.

Please note that very large industrial and power plant boilers (small house size and larger) all have powered vents (and call them "induced draft fans").

I hope that helps,

DZoro

Seems like your contractors don't believe in heat loss calculations. I'd keep searching for a true contractor, who performs his own heat loss calc. and goes by it. I'll bet your calc. is spot on and probably has plenty of cushion built in.
Stick to your guns you are on the right path.
D