Need help with boiler replacement and DHW issue

[Deleted User]

I have a Monitor Products Inc. MZ25C, on Christmas day it decided to stop working :(

They say its the gas valve, but since the boiler is no longer manufactured (Monitor Products no longer makes any boilers), the gas valve is obsolete. Replacement is special order and pricey (vented gas valve). Since I don't want a repeat service issue, I am planning on replacing it. It is an older non-mod unit. The reason I bought it was that at the time Wirsbo was pushing it as the greatest thing since sliced bread. According the the Wirsbo heat lose program(1998 version) the heat loss for the house is 90K BTU for an inside temperature of 72 with and outside temperature of 0. Back in 1998 living in Illinois 70 mile SW of Chicago that seemed reasonable (who knew that global warming would actually make for colder winters) Anyway years of experience seems to agree with that finding. the MZ25C it rated for 90K BTU and on a 0 degree outside day it runs constantly and keeps the house at 72. if the outside temperature drops to -1, the house drops to 71 (-2 = 70, etc)The house is almost 6000 sq. ft. with a 2 car heated garage. Half log construction with R40 in the walls and ceiling. All in-floor radiant heat, basement and garage in concrete, rest of house in gyp-crete with wood or tile floors.

I also want to resolve a DHW issue I have, the master bath is nearly 100 ft. from the existing 80 gal. HWH, and when I remodeled it I added a big whirlpool tub. It takes forever to get hot water in the master bath and filling the whirlpool pretty much empties the current HWH. I am thinking of splitting the DHW in half. Using a new indirect tank for the master bath (and kitchen) and leaving the rest of the house on the existing HWH. Since the whirlpool is only used occasionally (and I am hoping an indirect tank has a better recovery), I was thinking of using an 80 gal. indirect tank.

Is that a reasonable size for the indirect HWH?

Is mixing radiant heat and DHW on the same boiler a bad idea?

Given that winter have gotten a bit colder and the additional load of DHW what would be a proper size for the boiler (planning on using a Lochinvar Knight wall mounted boiler)?

Any other comments welcome.

HomeOwner1

Combi Boiler is an option

Navien, Triangle Tube and others make them and would certainly be a very cost-effective and efficient option to consider.



We have a Navien CH-240 in our home with a similar heat loss to what you stated. (90KBTU seems a bit low for a home that large though). I believe many of them can be cascaded as well.



We service three full baths and almost 4k square feet and the thing never really has to full fire even on the cold days.



They have a newer model out right now that appears to have a 1/2" gas line connection and smaller PVC exhaust pipe and better efficiency.



The way to size the combi's from what we learned from people here and our installer is first by hot water demands, then heat. Example, on the coldest day of the year the unit does about 5 gpm of hot water and 8.5 during a summer day. Figure about 1.8 gpm per shower head on newer homes and gives you about 2 to 3 showers possibly at the same time at full pressure. Otherwise, you still get hot water but lose some pressure. We have a whirlpool bath, and this thing works fine for us. Also, hot water usually takes priority, so went you call for hot water, your heating of the home is temporarily stopped. It is really not an issue unless you plan on using how water continuously all the time, like a car wash business.



For the heating side, you then see if the output BTU's will fit your heating needs. Sounds like it may be an option based on your old boiler size. Another example, the turn down ratio is 10 to 1 on a mod con boiler. So, with the external temp sensor for the modulation feature, the unit can do a slower burn and be more efficient lowering the temp outputted to run longer, slower and cooler and somewhat coldish days. Then on cold days, it would run hotter. This modulation helps give a little more wiggle room in sizing as well, as the unit can dynamically figure out heating demands based on sensor and program region temperature curve inputs into the computer.



This would probably be your cheapest option.



Otherwise a mod con boiler and indirect tank may be a second option, but considerably more expensive.



You will see a lot of negative feedback on combis on this forum from some folks. But, I own one, know others than own one, and they work. If you go this route, just find someone that has done them prior, as they need to be installed correctly (primary/ secondary loops, pump placement/ direction, etc.)

Paul Pollets

Replacement

I'd be replacing with a mod-con boiler and an 80g indirect tank. You'll also want to consider a DHW recirculation pump so you get HW immediately to your fixtures. Mixing radiant and DHW is always iffy, and in your case, you have a large DHW load. Combi's produce 3-4gpm of HW. Not a good idea when a soaking tub is in the picture.

My preference is Viessmann equipment, but there might not be a trained installer in your area.

HomeOwner1

There are combis that put out more than 3 GPM

There are larger combis out there that sound like they would meet his hot water demands just fine.



Why add the expense and complications if one unit can handle it all with nearly the same efficiency?

Paul Pollets

Flow rates for DHW

I 'm not aware of any combi appliance that can produce more than 3.5gpm of 120 degree water when incoming CW is 35-40 degrees. Soaking tubs typically require 5-6gpm. Some tub fillers fill at 7gpm.

[Deleted User]

That what I was planning, but

How to I size the boiler?

current 90K con non-mod boiler only good to 0 degrees outside

how much BTU do I need to allow for the DHW

Steamfitter66

I'm in the Fox Valley area

When sizing boilers for both radiant and DHW you don't need to add for the water heater in most applications. A restaurant or other large demand would need to be added. I would recommend a Triangle Tube Solo 110 if you have a 90k load. I would double check against gas consumption.

Triangle tube has very good support in the greater chicagoland area and are extremely reliable. The Knight is a good boiler as well.

I would put it all on 1 80 gal indirect with recirc. No need for both if done correctly.

Henry

Capacities

One of the biggest problems we encounter in our cold weather climates, is the guy that does not take into consideration the hot water load! The mod-con boilers have domestic hot water priority. Depending how it is programmed, the boiler can provide only DHW for over an hour while providing no heat. In large buildings this is a major problem as building temperature starts to drop specifically in very cold weather (below 0F). Your tub can need easily over 150 plus gallons of hot water since your 80 does not suffice. A 80 gallon indirect can provide 330 GPH or 5.5 GPM of water at 140F. You need a minimum of 140 F in a storage tank to combat legionnella! No instant water heater can give you 5.5 GPM at 140! So, it will take about 20 minutes to fill your tub unless you have 3/4 hot water pipes. So, the DHW indirect needs 280,000 BTU! Before the advent of mod-con and condensing HWT, we used to install a G85-360 in luxury homes that had large tubs or multiple shower heads.

The temperature in your area does drop below 0F so your existing has no safety factor. Here your house would need 162,000 BTU. 50% of the DHW requirement of the indirect extra would be needed for the DHW, so 140,000 BTU. With 5 to 1 modulation your boiler would match the heat loss while being able to fully supply you with DHW. A HTP Elite 301 plus a SSU-80 would work correctly. In 37 years of doing this, I have never made an error in sizing. BTW, we refuse to install combo units with instant hot water since 2004. Why? They don't work in cold climates and our large DHW requirements!

HomeOwner1

Henry, There are larger combi options now coming to the market

You know your stuff.



We live in a moderately cold climate in Philadelphia area. Not subarctic conditions, but get a good cold winter with about a 4 degree design day.



We spent a lot of time deciding our unit to make sure about the hot water demands.



You are correct, we have a 3/4 line for our larger soaker tub, otherwise it would take an eon to fill it. Any good plumber would probably make sure the lines are sized correctly if possible.



We have everything you mentioned and the larger combis these days can do it. We ruled out the triangle tube Prestige for that very reason, its flow rates on hot water were too low. Although it has an excellent reputation in the industry. We have about the same heat loss as this person's home.



We deliver enough flow in the dead of winter to satisfy a few showers at 130 degrees in our climate. Take a look at the large combi options, you may be surprised how far they have come in the last few years. Navien is just one, but there are others now coming to market in the same larger sizes with 10 to 1 turn down ratios.



We are very happy we decided our combi. Many of the installers coming out for quotes told us it was not possible. They can indeed do it now. Otherwise, if one unit can't deliver enough hot water, many of these units these days can be cascaded to work jointly with each other. Still cheaper and simpler to install two of them in many cases than other options.

HomeOwner1

240,000 BTU

Henry, we are pretty close to your stated number with our unit. 240 KTBU is the size of our unit. The large output was needed for hot water, like you said.



Otherwise when running heat, much lower. Impressive stuff with these mod cons these days, compared to our old cast iron unit. Fuel bills are drastically lower.



Also, hot water taking priority really has not been a big deal in real life in our home. We don't even really notice it to be honest.



Hope this helps. Interesting and cool stuff to look into nonetheless. It is nice to have options for people with tight budgets, like ourselves.

Robert O'Brien

Interesting use

Interesting use of the possessive pronouns,our and we!