How much additional BTU/hr is required for an indirect DHW tank?

Shahrdad

I had posted a few weeks back about considering replacing my 1970s WC cast iron boiler (rated 245K input, 196K output) with a high efficiency Viessmann Vitocrossal CU3A. I am even considering replacing my current conventional water heater with an indirect Vitocell DHW tank connected to the CU3A.
Several contractors have looked at the house and given bids. Only one contractor went around the house and did an actual heat loss calculation. The bids range wildly in price, for pretty much the same job.
The contractor who did a heat loss calculation came up with 186,000 BTU/hr on a zero degree day with the house at 70. Another one estimated 200,000 by just looking at the house (house is close to 6,000 sq feet). I myself watched my gas consumption on a zero degree day when the boiler ran continuously. It consumed 5 cubic feet every 88 seconds, which put the actual boiler input at 210,000, and assuming it's still 80% efficient, the output was only 168,000 BTU/hr. The house stayed at a perfect 70 degrees.
One of the contractors is STRONGLY discouraging me from pairing the boiler with an indirect DHW tank, saying that the boiler doesn't have enough capacity to heat the house and the water at the same time on a zero degree day. The boiler I am looking at has 199 BTU/hr maximum input and is supposed to be 95% efficient, and it gives priority to DHW production. There are only two people in the house, and we don't have any of those car-wash type showers in the house.
What do you all think? From my own observation of my current gas consumption, it appears that the boiler will be plenty big even on a design day, but the contractor seems to think otherwise. He even recommended using two boilers, which seems like an overkill for my relatively modest heat loss.
Thanks!

Ctoilman

How many showers, bathtubs, sinks does the hot water tank need to satisfy.  I'd assume with that size home, at least a single 100 gallon capacity heater would be necessary.  A better option would be to have direct fired multiple water heaters closer to the point of use.  That way the hot water will be quicker to the point of use (assumes no recirc line) and the big home heating boiler won't be cycling unnecessarily heating just an indirect tank in the summer.  That's been my experience.

Shahrdad

It's a 125 year old house, so it has two full and two half baths. It's done perfectly well with a conventional 75 gallon tank so far.

Jamie Hall

Well... it won't be 95% efficient on a design day! It will ramp up to meet the load, and it's unlikely that the return water will be cool enough to get to 95%.

However, that aside, there should be ample capacity for an indirect -- unless someone in the house is addicted to two hour long hot showers. Think about it for a moment. Let's say you take a 10 minute shower. The DHW comes on and the boiler heats the water in the water heater back up. That 10 minute shower, unless you have deluge shower heads, will take 25 gallons of water, or about 200 pounds of water. Going from say 50 to 110 that will take a whopping 12,000 BTU. This is not a problem. The boiler will be running to hot water for all o 6 minutes or so, and you'll never notice the loss of heat to the rest of the house.

So... go or it. But I would use a big indirect.

Shahrdad

Jamie, on a zero degree day, my supply temp was 132 and return temp was 120.

Jamie Hall

Then you're in great shape! Condense happily!

flat_twin

Most modcons give priority to DHW so there's no need to upsize the boiler. The supply and return temps you mention make me think you have high mass emitters, a great combination to have with an indirect. I doubt you notice any change in space heating while the boiler is making DHW.

Alan (California Radiant) Forbes

The large CU3A sounds like the perfect fit and I wouldn't have (almost) any problem coupling it together with a 79 or 119-gallon indirect.
However, some of my customers are nervous about the fact that one heat source is responsible for both heating and DHW. If something goes wrong with the boiler, you have a cold house and cold water. They either want an on-demand or storage type heater. The additional benefit of an atmospheric, storage-type water heater is that it requires no electricity to operate. When the power goes out, you still have hot water.

EBEBRATT-Ed

With only two people in the house I wouldn't put in anything larger than a 50 gallon indirect

Shahrdad

Alan (California Radiant) Forbes said:

The large CU3A sounds like the perfect fit and I wouldn't have (almost) any problem coupling it together with a 79 or 119-gallon indirect.
However, some of my customers are nervous about the fact that one heat source is responsible for both heating and DHW. If something goes wrong with the boiler, you have a cold house and cold water. They either want an on-demand or storage type heater. The additional benefit of an atmospheric, storage-type water heater is that it requires no electricity to operate. When the power goes out, you still have hot water.

I have thought about also having an old-fashioned water-heater with a pilot light as a backup. Some years ago, we had a few days without power, and it was so wonderful still having hot water.

Shahrdad

flat_twin said:

Most modcons give priority to DHW so there's no need to upsize the boiler. The supply and return temps you mention make me think you have high mass emitters, a great combination to have with an indirect. I doubt you notice any change in space heating while the boiler is making DHW.

Yes, I have large cast iron radiators. They don't seem humongous, but apparently they're large enough to be able to keep the house at 70 on a zero-degree day while being at only 130 degrees. What I told the installer was that with the high heat retention of cast iron radiators, we shouldn't notice any difference while the boiler switches for a few minutes to heating the indirect tank. And there are only a couple of zero degree days a year in St. Louis, so it shouldn't pose much of a problem.
What's interesting is how wildly divergent the quoted prices for the system are, and I mean by almost 15 grand.

Alan (California Radiant) Forbes

What's interesting is how wildly divergent the quoted prices for the system are, and I mean by almost 15 grand.

If they are all quoting the same work, then it comes down to reputation. Check references and I would even go so far as visiting a job to see their workmanship. If they have multiple crews, tell them you want the most experienced installer.

EBEBRATT-Ed

I would lean towards the contractor that actually did a heat loss. That shows me something.

Some would say throw out the low pric and the high pric and pick one in the middle. But you get what you pay for sometimes.

The ones that beat their chest and tell you how they have been doing it for 40 years usually suck.

We used to say "Do your work and your work will speak for itself"

Shahrdad

EBEBRATT-Ed said:

I would lean towards the contractor that actually did a heat loss. That shows me something."

I am tending that way too, and some friends just had their boiler replaced by them, so I can check out their work. One reservation is that they install mainly Lochinvar boilers, and I rather have my heart set on a Vitocrossal CU3A, and they don't have much experience with Viessmann. But I also suppose a good installer can learn about any boiler rather quickly.

EdTheHeaterMan

The original question is How much additional BTU/hr is required for an indirect DHW tank? andthe correct answer is NONE

You need to do 2 load calculations. The manual J for the space heating and then a hot water use load calculation. Size the boiler based on the larger of the two. If your space heating requirement exceeds the DHW requirements then use the space heating (manual J) to select the size. If your hot water usage requires a larger BTU capacity then size for that.

As long as you cover the largest load, the smaller load will be covered. Based on your further explanation(s), I believe your selection of the 199,000 BTU ModCom boiler is plenty.

Paul Pollets

The Viessmann tanks would take 10-15 minutes to recover, depending upon incoming CW temperature. A 21/2 bath house really only needs a 53g tank, but 79g will obviously have greater capacity. The more experienced contractors will have installed more than 1 Viessmann boiler and probably taken a factory seminar

Alan (California Radiant) Forbes

The Viessmann tanks would take 10-15 minutes to recover, depending upon incoming CW temperature. A 21/2 bath house really only needs a 53g tank, but 79g will obviously have greater capacity. The more experienced contractors will have installed more than 1 Viessmann boiler and probably taken a factory seminar

We were installing a Viessmann 53-gallon tank for a couple and noticed they were installing a huge soaking tub, well over 100 gallons. This after they told me their hot water usage was minimal.

Shahrdad

Paul Pollets said:

The Viessmann tanks would take 10-15 minutes to recover, depending upon incoming CW temperature. A 21/2 bath house really only needs a 53g tank, but 79g will obviously have greater capacity. The more experienced contractors will have installed more than 1 Viessmann boiler and probably taken a factory seminar

Unfortunately, the company which says they have installed Viessmanns before also gave the outrageously high bid, and they did not do a heat loss calculation. The company that did the heat loss calculation (and has a good reputation) installs mainly Lochinvar (my neighbor who has a Knight boiler complains of how loud it is). And to me, the CU3A seems like a much better design than the Knight. I did speak to the distributor to find out who's done the most training on Viessmann, and I'm waiting to hear from that installer.