Boiler and Indirect Sizing Help Needed Please

Amovida

Hello-

We are retrofitting a Veissmann CU3A boiler and Runtal baseboard and wall panels with Mitsubishi mini splits for cooling into a 1880 home in Boulder, CO. I have a hydronics guy helping me with design but at the moment he is pretty busy so I wanted to run this by you to see if I am on track. Any help would be greatly appreciated.

We have done a lot of air sealing, increased the attic insulation to R50 (code), added some wall insulation, and made 25 storm windows with double pane glass to help with the cold at the older single panes in the house. This has helped with the draftiness and heat loss a great deal….and made my wife and daughter very happy.

The house is 2900 sq ft including the unfinished basement of 800 sq ft which I plan on heating to a lower temp than the house but heating nonetheless. We had Energy Vanguard do a heat loss which came in at 75K. This seems possibly high due to an infiltration number of roughly 25K and an ACH of 7.0 We are doing some remodel work upstairs that might have increased the ACH number as there is possibly leaking to the attic where there is only insulation and no sheetrock in one room…not sure. There is R-23 Rockwool with 4 inches of polyiso that has been taped on top of that as well as plywood decking over the whole attic. This is less than ideal but it is what it is.

Veissmann makes the CU3A in 4 sizes 94, 125, 160, 199K. My thinking at the moment is to go with the 125K. The DOE rating on the 125K per the manual is 23.3-116K where for the 94K it is 17.7 to 83K. In the future we have plans to add a master bedroom off the back of the house which I am guessing would have a heat load of 10K-15K. I'd like to plan for that possibility and if I calculate the heat loss of 75K + the 15K would equal 90K which is more than the 83K BTU max of the Veissmann 94. The 125 at 116K would cover our heat load and likely provide for the master bedroom and bath addition. One of my next steps will be to size the Runtals and I was going to try to do that for a 160 degree temp….any lower and the radiator panel sizing becomes ridiculously large. I may have to go with 170 degrees as we have some rooms with really limited wall space.

I am also thinking to buy a Veissmann indirect tank and this is where I am really out of my element….The house only has 2 bathrooms at the moment but one has a larger Jacuzzi tub which is around 100 gallons. Veissmann makes a 42, 53, 79 and a 119 gallon indirect. I am thinking about the 79 as the first draw number is 76 which would largely satisfy the tub before recovery began. The 53 indirect has a first draw of 49 which would only fill the tub half way and less if anyone is doing laundry or dishes, etc. We are only a family of 3 but we frequently have family stay with us and plan to have more house guests if we ever finish the work on the house. This larger tank would make even more sense if we add the master bedroom and bath. The part I really don't understand is recovery on an indirect. Below is EdTheHeaterMan's post from 2022 (attached below)…where he suggests you would be waiting around 8 minutes for the recovery to begin. Pretty sure he is talking about an indirect as he mentions a circulator. If we partially fill the 100 gallon tub and exhaust the 49 gallon first draw if we went with the 53 G indirect are we then waiting roughly 8 minutes or so to fill up the rest of the tub?…. that seems less than ideal. If the tub has exhausted the hot water then is the rest of house without hot water during that time as well? Or maybe I am misunderstanding the recovery times?

At the moment I am thinking the 125K Veissmann CU3A and the 79 gallon indirect but I am not sure as I realize there is a long learning curve to hydronics…the more I learn…the more I realize how little I know. Any insight would be greatly appreciated.

Below is Ed's commentary on recovery:

There is a number applied to water heater ratings. It is called the First Hour Rating. When a tankless water heater is compared to a tank type water heater the tankless uses the Gallon per Minute at a given temperature rise and multiply it by 60 (because there are 60 minutes in one hour) Thank you Mr.Obvious

When you use a tank type water heater you use the recovery rate of the heater and add 80% of the storage capacity of the heater. So a 40 gallon water heater that has a 20 gallon per hour recovery rate (like maybe an electric) would have a first hour rating of 52 gallons.

The reason I bring this up is to reinforce the fact that you have about 30 gallons of hot water ready to go as it sits there in the tank waiting for use. At 5 GPM flow rate to fill the tub you only have 6 minutes of Hot water available until you start to run cold. The recovery rate of that water heater is relatively fast considering there is a 105,000 BTU burner connected to it. The thing is that there is a sequence of operation that takes time to get to the rated recovery rate.

1. The water tank temperature needs to drop to activate the water heater thermostat. 30 seconds

2. The boiler gets a call for DHW ----------------------------------------------0 seconds

3. the boiler goes thru the pre-purge and start up cycle-----------------------------60 seconds

4. The burner lights and starts to heat the cold water in the boiler ------------------- 10 seconds

5. The circulator is circulating the cold boiler water for those 10 seconds and an additional 110 seconds

6. The water in the boiler is now up to temperature and and heat is being transferred to the indirect

A total of almost 4 minutes (3.666 munutes) has passed and you are finally at the point where the tank recovery will take over and heat the water at its specified recovery rate. Now since you may be using the water at a rate faster than the recovery rate of the tank/boiler combination, you will start to feel luke warm or even cool water within 8 minutes.

Hot_water_fan

1. How much gas do you currently use per year?
2. with any hot water heater, you can raise the set point high then mix the temp down to a safe exit temp. This gets more initial capacity from the same gallons.
3. This particular boiler model is special: it’s a high water content boiler. Think boiler + buffer tank all in one. So there will be 13 G of water already heated (and likely to a higher temp than the indirect) in the boiler in addition to the indirect. As this water is already heated, it can almost instantly be pumped to the indirect. Next, if the boiler is sized well, which it seems like it will be, it will likely be running already when DHW is needed in the winter, which is when DHW loads are highest due to lowest incoming water temps.

Amovida

• @Hot_water_fan Thanks for your response…
  1. Unknown….not sure the last gas bills are going to be super meaningful. We have been doing a bunch of construction and during a part of the year we had the upstairs partially open to a vented attic….and even when we closed it up….it was almost entirely uninsulated. We also did not have a lot of the double pane storm windows put up as well. Finally, my family huddled in a couple rooms as the rest of the house was pretty cold as we directed heat to the few rooms we were in. At one point before all the fixes the upstairs was a cool 37 degrees. And no, before you ask, my family did not try to murder me….as far as I know ;)

• 2. If I use the mixing valve to bring the indirect DHW water temp to 120-140 and the radiant panels are either 160 or 170….when the boiler fires to feed the indirect….can you set the temp at which that happens at or are you beholden to the 160 or 170? My guess would be as the indirect is pulling off the boiler supply piping you are getting the panel temp of 160 or 170? Guess this is another thing I don't understand. ;)

• 3.I had forgotten to add in the 13 gallons that will already be in the boiler…but are those 13 gallons always at the ready? Or have they occasionally been sent out for a heating call and replaced by water that has yet to be heated when the indirect comes calling? If not is this enough to downsize the indirect to the 53g? I am attaching the data sheets on the 53g and the 79g if they would be helpful.

Screen Shot 2024-08-06 at 7.32.49 PM.png

Screen Shot 2024-08-06 at 7.33.06 PM.png

Hot_water_fan

The 13G will be hot and ready - the boiler just pumps them to the indirect instead of the panel radiators. Because of the modulation, it should be almost always on and ready when it matters so it’s just a matter of the pump turning on. Should be nearly instant. That said: it’s pretty cheap to install the biggest indirect so you can absolutely pay a little more for peace of mind.

Amovida

Coolio…thanks for the comment. Guessing you can choose on the Viessmann interface if you want DHW priority and for how long. Other than the extra cost for the larger indirect (and I do like that peace of mind) is there any other downside? Looks like the loss on the 79 is rated at .47 and the 53 is .7

I'm sure boiler and water tank sizing is probably the most talked about subject on the site…It's gotten to the point in my house if I mention anything about heating or cooling my wife and daughter immediately make a snoring sound to drown me out. ;)

Really just wanted to double check I'm headed in the right direction….if anyone has anything else to add I would love to hear it…thnxs.

Hot_water_fan

DHW priority is generally the default for all boilers. Not much downside besides space and cost. If you’re installing a Viessmann, saving $5 a year on standby losses isn’t a priority I assume.

Amovida

Thanks for the help @Hot_water_fan I just ordered the boiler and indirect and while $5 on standby losses isn't a big deal that purchase was…really appreciate your experience and insight and being willing to help out. 🤩