Possible DIY boiler replacement questions

ZackR

I currently have a Dunkirk q90-100 natural gas boiler that was installed in 2007. It has recently developed a small condensate leak in the bottom of the HX where the 2 halves meet, that has lead me down the road for a replacement, as my understanding is that is not fixable and it's only a matter or time before it get worse and that's not something I want to deal with in the winter months.

I have had 3 people familiar with boilers in my area. Upon initial consult, all 3 have said they would replace my current boiler with a modulating condenser unit and redo the piping to primary/secondary loops with all new circulators. Only one got back to me with a bid and it was way higher than I anticipated. I currently do the annual service on the dunkirk and that is something I probably won't be able to do with a fancier unit, which would really eat into any potential fuel savings. All this has made me serious consider a DIY replacement with a non-modulating condensing unit. I believe I have above average mechanical abilities, have lots of soldering and electrical experience.

My current set up is:
Priority Zone- DHW (Circulator on supply side)
Zone 1- ~850 ft^2 (Circulator on return side) Cast base board radiators
Zone 2- ~500 ft^2 (Circulator on return side) "normal" radiators"

I have lots of short cycling, so I will need to do some calcs to get a better idea of what size I really need. I also have a big metal expansion tank between the floor joists.

-Is this generally a bad idea?
-Since the current pipe setup has worked for a while, would it be worth re-piping to a primary/secondary loop?
-I would like to do some pipe work to install isolation flanges on the circulators.

I am not trying to cheap out or cobble together a bad setup as we're not planning on moving. I am just trying to understand how much of what has been recommended to me is some "ideal" setup vs sticking closer to what I have.

I apologize for the long first post. Below are a couple pics of the current setup.

Thank you

EBEBRATT-Ed

@ZackR

I don't know the situation in your area about permits and inspections. If homeowner permits are available then your free to install it.

I am sure you could learn the operation of a mod con boiler by studying the boiler manual if you choose to go that route.

In my opinion primary secondary is my first choice for any boiler.

You should have all circulators on the supply side pumping away from the expansion tank.

Step one is to do a zone x zone heat loss of your house.

The right size boiler is important.

Once you do that come back and someone can help you with circ pump sizing and flow

ZackR

@EBEBRATT-Ed

Thanks for the response. What is the margin of error that I should be looking for in the heat loss calc? The house is old(1910), but has blown in cellulose in the walls, but have no accurate idea of what I have for R-value anywhere. Would an error of +/- 20% be too great? Is there anything useful that I can do with the gas usage data that I have from years past or is that not going to be too good of an indicator because I have a boiler that short cycles all the time? Sorry if this is a basic question, just trying to figure out what is good enough to have an accurate number vs spending too much time measuring every window when there is going to be some error.

hot_rod

It us worth the time to do an accurate load calc
In a building like that a blower door test may be worth the $$

Some of the fire tube mod cons do not need primary secondary if you can meet minimum flow requirements

Or a tank style mod con that also buffers itself

ZackR

I will try to do as accurate of a loss calc as I can, I was really only wondering about margin of error because if I go with a non-modulating unit, I will be somewhat limited in options as different models seem to make 15-25K BTU jumps between units.

I am really trying to figure out what I can get that is going to be the most reliable long term and easiest to keep properly maintained. Gaining some fuel savings wouldn't hurt, but I am reluctant to shell out extra money for "efficiency" when we use so little gas to begin with. Just to give some perspective, total gas usage from last year(after subtracting fixed connection fees) was $750, and that includes the summer months where usage is minimal. So, it would take a huge percentage increase in efficiency to pay off in terms of absolute dollars, especially if it comes at the cost of additional annual service.

EBEBRATT-Ed

@ZackR

Heat loss calculation is always the best way for hot water.

As a check # find the output of your baseboard at 170 or180 average water temp and use that as a general guide. There is no sense installing more boiler than the baseboard will output as long as the baseboard installed heats the house.

There are calculations for basing the new boiler off your gas usage but I am not familiar with those.

Perhaps someone else will chime in.

As far as choosing between a mod con and a CI boiler there are two sides to that issue and they are both valid:

Mod con: more expensive to install and maintain. Generally considered to be a 15 year boiler. They can last longer with good service, and they require good service. We have had cases on here where sourcing parts can be a problem after 10-12 years as the manufacturers move on to newer designs. Parts are expensive. Replacing a blower and fan may cost $800 contractor's cost. Depending on your location some wholesalers do not stock parts. They are more finicky but there is no doubt the efficiency is better when they can be run in condensing mode and the modulating feature helps efficiency.

CI is cheaper to purchase and install. Parts are generally a lot cheaper (also less to go wrong) and parts availability is usually not an issue. Its easier to find someone who can service the boiler.


The other important consideration is the heat loss and radiation calculation. With that we can determine the water temp you need.

A mod con only gets 97% efficiency when the return water temp is below 125-130 deg so in your case you may get condensing in the spring and fall only.

With your low gas cost you may not recoup and extra savings for a long time

Hot_water_fan

It’s extremely easy to use your previous usage!

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

ZackR

@Hot_water_fan

That's a very interesting page you linked. I am going to need to read through it a few more times to make sure I understand it all. I did a couple calcs and based on those I need about half the output I currently have. Going to do some heat loss calcs and see if they match up.

Hot_water_fan

I did a couple calcs and based on those I need about half the output I currently have.

That’s pretty typical - my heat loss turned out to be 25% of what was installed. Three winters and many design days (and much colder) later, no issues. Lots of these contractors are just winging it. 

Ironman

A couple of thoughts on cast iron vs. mod/con:

1. CI Will obviously last longer
2. Since Covid, CI boilers have become very difficult to obtain.
3. CI boilers are no longer cheaper than most mod/cons; in many cases, they’re more expensive.
4. The complexity of new CI boilers can rival that of most mod/cons since new energy requirements have come into place. Many parts are not “off the self” or standard truck stock 
5. Since newer CI boilers are more efficient and have cooler flue gas temps, they cannot be directly vented into a masonry chimney unless a new liner is installed. If the chimney is on an exterior wall, the liner should be insulated. The added cost of this will probably drive the initial expense of a CI boiler well beyond that of a mod/con.
6. Since CI boilers are fixed input and don’t modulate, they are always oversized except at design temp under full load.
7. As hot_rod stated, most firetube mod/cons have a very low resistance to flow and many applications do not require p/s piping.
8. Mod/cons can require more maintenance, but I’ve seen fire tubes go 8-10 years without cleaning and still look good when taken apart  A lot depends on how well the installation was done.
9. You’ve got an indirect water heater and mod/cons come already setup for that; most CI boilers are not and require additional controls, etc.

Just some things to consider since a lot has changed in the last few years.

ZackR

Right now I am leaning towards just replacing the current boiler with a new q90 with a lower output. I understand the potential downsides to the aluminum HX, but our current one has lasted 16 years. I understand that my piping set up isn't ideal, but for the price I can get a new boiler that would essentially be plug and play, I am having a hard time justifying anything else. Going this route should allow me to have a trouble free winter and give me some time to decide if I want to re-pipe near the boiler next summer.

I know that posting prices is frowned upon, but there is about a $10k price difference between what I have been quoted for a mod-con with pipe work vs DIY q90 swap. I would be happy to post more specifics if it's allowed. Since the initial cost and future operating costs are major factors, I think it would go a long way towards the understanding of how I have come to my current plan.

ZackR

I know the current set up with the 2 heating zone circulators on the return and the DHW on the supply might be a little wonky, but is worth changing right now if it has worked for all these years? I believe I understand why the circulators should be pumping away from the expansion tank in a primary/secondary configuration, but my fill line and expansion tank are plumbed in to a different port on the top of the HX(similar to what is shown in the diagram below from the q90 manual. So, what I currently have seems to be mostly inline with the manual, or I am completely missing something?

hot_rod

those installation drawings are pretty dated. Its rare to see expansion tanks like that being installed on new jobs

Hydronics is very forgiving in regards to near boiler piping 

No question that pumping away assures a easier  to purge piping method. Air free systems allow for the highest efficiencies, less corrosion potential ,etc

As you noted the piping you have can and does work adequately

Your call on a major repipe is worth the time and $$

ZackR

@hot_rod

I was kind of surprised that the current manual still had 4-5 pages of the same piping setups that were shown in my manual from 16 years ago. They also have a bunch of pages of different primary/secondary setups now. I kind of figured what I have is ok enough, but not what you would want to do if one were starting over. I am going to do a little more thinking about it, but am leaning towards a "simple" boiler swap for now and think about messing with the near piping next summer. I've got a lot of other house projects that I should be working on and it would be nice to be able to take some more time to really plan a more ideal piping setup. Even though it was 100 degrees a couple days ago, winter is always just around the corner.

ZackR

Dunkirk recommends water be PH 7-8, but I can't find anything in the manual that mentions water hardness. We have hard water and looking through some older threads it seems advisable to use a demineralizing filter for the fill water. Looks like caleffi has an all in one filter, pressure reg, back flow preventer that would be a good fit.

Since I am going to be messing with the piping a little bit, should I also add a mag filter on the return? We do have all cast radiators and I had been blissfully ignorant about possible issues between cast and the aluminum HX, but I want to make sure that I am taking the proper steps to ensure I can get a much life out of this new boiler as possible.

hot_rod

A mag separator for sure on any old system with iron and steel

This kit is a great price, available as press, sweat, or threaded

mattmia2

the magnetic separator is important in systems with cast iron emitters and wet rotor circulators. the magnets in the circultor will attract the iron particles and it will eventually damage the bearings.

why use a boiler with an aluminum hx? they aren't terrible but there are better options and other options will cost less than replacing it 5 or 10 years earlier.

EdTheHeaterMan

I did not see anyone mention that a good barometer of how much your boiler is oversized is to see how many minutes the burner operates during design conditions. Since this is probably not going to happen anytime soon and you will probably have a new boiler before you can test that, before you replace it, You will need to remember how short those short cycles were back when it was really cold. If the home was comfortable on the coldest day of the year, and the burner only operated for 30 minutes every hour, (5 min on and 5 min off) then that means your boiler is oversized by 200%. That means that if you have twice as much boiler as you need. You can use a boiler 1/2 smaller. If your burner operated for 10 minutes on and 5 minutes off, on the coldest day of the year, then your boiler is 150% oversized. You can use a boiler 1/3 smaller.

This may or may not help in determining your new boiler size. How good is your memory? The load calculation is still the best bet, even if you need to pay for it.

As far as doing a push pull replacement, I think that is a waste of a good opportunity. You do not need to do primary secondary based on the manufacturer instructions, as long as the heating zones are large enough to provide adequate flow thru the boiler if only the smallest zone calls for heat. Since your boiler is a condensing boiler, I do not believe that you need to worry about condensation of flue gasses causing damage to the boiler. You can also do the Pumping Away from the expansion tank in order to reduce any air problems you may have experienced that might have contributed to your existing boiler early failure.

The ceiling mounted expansion tank (Closed, Non-diaphragm) should never be used with a system that has an automatic air vent of any kind. Your system has both. You need to use a Diaphragm type tank, or lose the air separator.

This is an illustration of your existing system according to your photographs. It is not to scale but it is an acceptable rendering for this presentation.



Using the illustration , I believe there is room to remove the B&G Flow valves and install Taco 007F circulators with Internal Flo Check (IFC) valves. This will place the pumps in the correct location for the best air elimination. You can replace the Air Separator with a Magnetic Air Separator to remove metals from the system. the expansion tank and the water feed can be connected to a Tee fitting or a special expansion tank fitting to make service to the expansion tank and the other fill valve components easy.


The section of Iron fittings and steel pipe in BLUE are the only pipes that need to be replaced to accomodate the changes, and they can be done in 1" or 1-1/4" copper depending on the boiler size you select.

Finally you can purchase circulator flanges with valves to facilitate circulator replacement as needed.

I hope this will get you on the path to a better system design as I believe the components will fit as I have illustrated them.

Parts you may want to consider.
https://www.supplyhouse.com/Raven-1129-Single-Shutoff-Expansion-Tank-Valve

https://www.supplyhouse.com/Taco-007-F5-7IFC-007-Cast-Iron-Circulator-with-Integral-Flow-Check-1-25-HP-3647000-p
These are available in thread, sweat, or press connections. https://www.supplyhouse.com/Webstone-41404-1-IPS-Isolator-Flange-w-Rotating-Flange
also available in 1-1/4"https://www.supplyhouse.com/Caleffi-546116A-1-NPT-5461-Series-DIRTMAG-Dirt-Separator-with-Magnet
https://www.supplyhouse.com/Amtrol-102-1-30-Extrol-EX-30-Expansion-Tank-4-4-Gallon-Volume

ZackR

@mattmia2 The original "logic" behind replacing my current boiler with another aluminum HX was the overall cost and ease of installation as I would not have to do much pipe work as the inlet and outlet are in the same place on the newer model. Our current boiler has lasted for 16 years, but maybe we just got lucky?

@EdTheHeaterMan As far as boiler sizing, the only thing that I have done so far is use my past 2 years of gas usage data from my gas utility to calculate average monthly BTU/Hour. With a outdoor design temp of -3 and using base heating degree day data of 65 degrees, the largest number I was getting was 34K BTU/Hr. With a 1.4x sizing factor that gets me around 53K BTU/Hr. That data averages out the really cold days we had and I need to see if I can get daily use data from my utility. I also did some calcs where I subtracted my average summer usage, which is DHW and a dryer. The DHW is a priority zone, so won't be competing with heating zones. With those numbers and a 1.4x sizing factor I was getting right around 50K BTU/Hr.

If I stick with another Dunkirk q90/utica UB90, my options are 50K and 75K, each with 90% efficiency. Seeing as the 50K is right on the edge(assuming my calcs are good) that might be cutting it close, but since we have a small, fairly well insulated house and we keep it cool, maybe it would be ok? Or maybe I need to look for a completely different boiler?

I am going to have to read that post the couple more times to completely digest everything, but I really appreciate the detailed response. Everyone's responses have been very thoughtful and helpful.

WMno57

How heavy of a boiler can you move into your basement? If you have the equipment and ability (does your basement have external stairs?), go heavy. Get the heaviest old school cast iron boiler you can buy. You might be able to buy a sectional boiler and assemble it in your basement.
It's not worth getting hurt (weight capacity of stairs?). Mod-cons are much lighter, so easier to get into the basement. But a good install of a mod-con should have pumping away piping, primary secondary piping, dirt mag, new bladder tank, new circulators, new zone valves, etc. Even then, it won't last as long as an old school cast iron boiler.
My boiler is 75 years old.

• No dirt mag
  
• No pumping away
  
• No primary-secondary
  
• No zones
  
• No bladder tank (air over water tank like yours)
  
• No wet rotor pump

NO PROBLEMS!

WMno57

The Dunkirk has the lifespan of a mod-con. And like a mod-con, it will be a high maintenance drama queen. I don't think the minimal savings in re-piping is worth the cost of limiting yourself to Dunkirk.
Buy a propress tool, or do the whole thing in oxygen barrier PEX.

ZackR

We do have concrete steps from the outside to the basement, so weight isn't too much of a concern, within reason. The other factor that somewhat limits my options are we do not have a chimney. It was knocked down below the roof a few years ago.

As far as the Dunkirk, in 16 years we've replaced the blower, 2 ignitors and 1 gas orifice. I would think that is pretty good considering it wasn't until an ignitor dies 2 years ago that any of this boiler stuff really came on my radar. Maybe I could have got more life out of it with more regular maintenance? I haven't been able to find too much first hand info on peoples experiences with the dunkirk/utica models. Most of the info I did find was old complaints about short ignitor lifespan, but that issue appears to have been solved with a part change. The lack of info is one of the things that makes me wonder if I just got lucky with mine or people who don't have problems with them don't post online?

WMno57

Doh. Missed the part about the chimney.
DIY is more enjoyable with the right tools. Look into PEX and/or ProPress.
Since you are in Wisconsin, check out Menards offerings. I would definitely replace the air over water compression tank with a bladder tank. Bladder tanks keep more oxygen out of the water, which is important with all the modern high drama, boiler in name only appliances.

Hot_water_fan

With those numbers and a 1.4x sizing factor I was getting right around 50K BTU/Hr.

Nice! 

EdTheHeaterMan

The lack of info is one of the things that makes me wonder if I just got lucky with mine or people who don't have problems with them don't post online?

Interesting observation. I wonder how many times you said something nice about the tires on your car. Im sure they have lasted you many miles. I know I have not had a flat tire in the 3 years I have driven my pickup truck, and This is the first time that I have told anyone about it!


Page 45 and 46 of the manual has a number of items that you should do for maintenance on a regular basis.

A sure way to find out if the boiler is too small is to install the small one, then when you dont get enough heat on the coldest day of the year, you will know. If it is too big, then is will short cycle on the coldest day of the year. After the third choice you will get very close to the correct size.

Or you could do a load calculation for your home. Even if you need to pay someone.

ZackR

Well, I pulled the trigger and ordered a new HX. Apparently the Q90 has been out of production for a few years, so the few sites that claimed to have new units were not accurate. Now that I am committed to this route, it frees up some mental energy to focus on the other details.

I still plan to do a more thorough heat loss calc. To change the output between 50/75/100K BTU is minimal and not expensive, I will just need to figure out if I want to change to 50 vs 75.

As far as piping, I am leaning towards moving the circulators and getting a bladder expansion tank.

@EdTheHeaterMan I appreciate the drawings you put together, they are very helpful. For fill water, can I keep it going into the current location or does it need to do in the space between the expansion tank and air separator? It wouldn't be a big deal to move it, but if I can just keep the current piping, it'll be once less thing to mess with.

ZackR

So, I was looking more into why your not supposed to use an air separator with an open expansion tank like I have. Eventually all the air will be pulled out of the tank and the system can over pressure and release water through the over pressure valve? Do I have that right?

Is the whole point of pumping away just to make it easier to start with an air free system?

mattmia2

The air from the tank will eventually migrate out in to the rest of the system. If you remove it with an automatic vent of some sort then you will remove the air pocket from the tank that allows the water to expand as it heats. With a conventional compression tank the air separator should direct the air back in to the compression tank.

ZackR

@mattmia2 Thanks for the clarification. So, the air separator with bladder expansion tank is inherently a better system because it gets the air out and assuming there are no leaks you end up with an air free system, where as with an open tank you're always fighting some air infiltration which needs to be bled from the radiators?

So, if I were to do the bare minimum of pipe work to do this HX swap and plan to do some more extensive re-piping next summer, would I be better off keeping the air separator(hoping it keeps the air out) and keep an eye on the level of the expansion tank. Or would it be better to remove the air separator and just bleed air out of the radiators as needed?

ZackR

Never mind the question on pumping away. After reading these articles, I have a much better understanding. Making it easier to purge is only part of the story.

https://www.fiainc.com/sites/default/files/The Point of No Pressure Change.pdf
https://heatinghelp.com/systems-help-center/pumping-away-piping/

EdTheHeaterMan

So you need to understand the Physics of Air and Water. The easiest way I used to explain it is that air can be dissolved in water like salt or sugar. The stuff is in there but you can't actually see it. But you can taste it, so you know it is in there. The amount of air that is dissolved in the water depends on the water pressure and the temperature. Think of it like relative humidity in the air. You can't see it until you get close to the dew point. Then you see fog. The relative amount of air in the water depends on the water. If you change the pressure you change the amount of air that can be held in suspension invisibly. The lower the water pressure the less air in suspension, the higher the temperature the lower the amount of air. When you get close to the saturation point, the air is released as micro-bubbles that look like fog in the water.

So if you place the lowest pressure water (the circulator inlet) and the highest temperature water (the boiler supply) in the same place, that is where you will find the most amount of air leaving the water and turning into micro.bubbles. Now take those micro-bubbles and run them thru a micro bubble air vent, then all the air will leave the system. That is the reason we design the air eliminator to be on the supply side of the boiler. If we place the expansion tank under the air eliminator, that will make the point of no pressure change happen at the air separator. Next if we place all the pumps for all the zones pumping away from the expansion tank, that will increase the water pressure in the rest of the system. As the higher pressure water goes to the radiators, that water gets cooler. Now that cooler higher pressure water can absorb more air bubbles in suspension like the relative air amount mentioned above. You circulate that water thru the system over and over again, eventually all the air bubbles will get absorbed and leave the system as air bubbles thru the micro-bubble air vent.

The reason you don't want the closed expansion tank, as you found out, is that a micro-bubble air vent will take any expansion tank air that is absorbed into the water (because it is in direct contact with the air in the expansion tank, and the expansion tank water is cooler) and move it to the system over time as the water expands and contracts in and out of the tank.

Any water from the expansion tank that absorbed any air from the tank will release that air when it gets hotter from the boiler flame.

ZackR

Little bit of an update.

I received the new heat exchanger, so will be going forward with that route. I will also be changing the parts necessary to go from 100k down to 75k BTU. This is a legitimate change that I spoke with Dunkirk about. I do not plan to move the circulators or install a bladder expansion tank at this time. I am going to make a few changes to make that swap as painless as possible next summer without having to drain the system again.

I am going install a drain valve and shutoff before the air separator, as shown in the pumping away diagram. I am going to leave the air separator in place, but cap the output. This way I will only have break open the piping from where the zones tee off from the DHW when I want to install new circulators.

On the return side I am going to add a mag/dirt separator. That will be able to stay unchanged when I remove the circulators, as I will just be replacing some pipes above that.

I hope that makes sense? Now I need to draw everything out and get a parts list together.

Once again, I appreciate all the advice given to me, even if I am only taking bits and pieces at this time.

WMno57

When you have the system down for HX swapping, drain the air over water tank, then close the drain valve, before refilling the system. That will leave you a nice air cushion to handle the expansion of heated water.

john123

@ZackR: what parts are you changing to bring the boiler down to 75K? not the model numbers but the description.

ZackR

@john123 For the q90-50, 75, and 100, the parts that are needed to change between outputs are

Gas orifice, restrictor plate for the burner, baffle for supply air, and air pressure switch.

ZackR

Just got finished firing up the boiler after the HX swap and changing the parts needed to bring the output from 100kBTU down to 75K. Everything seems to have gone well.

Exhaust analyzer gave me these results:
CO2- 7.1% (factory spec 6.5-7.5)
O2- 8.5% (factory spec 8-9)
CO- 45ppm (factory spec <100)
Manifold pressure 2.45ish "WC (factory spec 2-3)

Since the results are all within spec, do I gain anything by messing the gas pressure at all or should I just leave it alone and consider it a success?

EBEBRATT-Ed

Looks good. The only improvement I would look for is the Co. Generally, it is considered less than 100 is safe to run and less than 50 is preferred.

You hit both those #s so you're ok. But in most cases, you can usually get the Co below 30ish to give you more of a safe margin.

You might get this by adjusting the gas pressure.

Did you have the boiler hot and up to temp while testing? A cold boiler will give you higher than normal Co

ZackR

Those readings were from the end of a run, so the boiler was hot. I did notice that the CO started considerably higher and quickly trended down as it warmed up. I will be running the heat zones tomorrow to make sure everything is good, so I might mess with the gas as things will be hot.

ZackR

Well, I gave it one more go will trying to get the CO down and now I am at 24ppm while everything is nice and warm. Going to leave it at that and call this project a success.

With the new zone controller the boiler doesn't power cycle when the priority zone calls and ends a call, and with the new HX there is no more leaking condensate out and unmetered air in.

I would like to thank everyone who offered advice, even if I didn't take it. I kept the circulators where they are since I really didn't want to mess with anything else. I did get all the crude out of the expansion tank. So that, along with the dirt separator, and no leaks, I will hopefully be good to go for a while. Both heat zones have been run and everything is nice and quiet and all the radiators get hot.

This is a great forum. As much as it has made me sad to realize that I had spent a bunch of money on 2 different techs to try to figure out issues that I was having that were very obvious(although not to them), at least now I know that I can keep this boiler running myself.

ZackR

To really close out this thread…

After an entire winter running with the newly replaced heat exchanger and swapping parts to make this boiler a 75kBTU I can say that this was a success. No issues since I did the work and I have used about 13% less gas in terms of Therms/HDD day than last year and even though it was a mild winter we did have 1 week that was below 10 degrees with a couple overnights down to the design temp. The boiler easily kept up on the coldest nights while still not running continually, which was expected as my calculations said I was still oversized.