A boiler that is too small

Jon_blaney

Can a boiler which just meets your heat loss be too small to use an indirect water heater? Indirect WH seem to rely on significant amounts of boiler to do the job. If you are at design outside temperature, do you have to make a choice between hot showers or warm house. Is there some point were a standalone water heater is necessary and what is the effect on overall energy use. Oil is my fuel of choice if that matters, no gas.

ScottSecor

In certain circumstances the dhw load is,significantly higher than the heat loss. In those instances we may be forced to upsize the boiler and the indirect tank.

Robert O'Brien

Pretty rare, usually a small house with large DHW loads or a big house with even bigger DHW demand

Robert O'Brien

What is your heat loss and DHW requirement?

Jon_blaney

I need about 55 MBH. I was looking at a BIASI B-10/3 which is rated at 61MBH, 53 MBH net. There is only two of us in the house now but want to be able to cover the needs when we have guests.

GroundUp

Can you set the DHW as priority? We've done this a few times in radiant floor applications with large thermal mass where the indirect pulling heat for an hour during high DHW loads has little effect on the heating comfort

jumper

Storage? Enough height for thermo-siphon?

Steamhead

GroundUp said:

Can you set the DHW as priority? We've done this a few times in radiant floor applications with large thermal mass where the indirect pulling heat for an hour during high DHW loads has little effect on the heating comfort

That's what I'd do, if boiler capacity is an issue. But, most times it isn't.

The Biasi is a nice boiler. if it's installed and maintained properly, you should be happy with it.

Jon_blaney

I am going to connect it to two hydroair units. I will not have that thermal mass to keep us warm.

Alan (California Radiant) Forbes

Then go with a stand-alone water heater, either storage or on-demand.

Mark Eatherton

I'm heating my home with an 50 K Munchkin , and it does DHW (output = 35K btuH, @ 5,280' ASL) on a limited priority, never had a problem. Only one person at a time can shower, but theres only 2 of us here anyway.

ME

Robert O'Brien

What kind of DHW load?

Leonard

I've got a tankless coil in old CI boiler, I usually get all the steamy hot water I want for showers even when weather is a little cold.

Few months ago when was a rare minus10 deg (about coldest it ever gets here) shower wasn't as hot as I wanted it. So I just set room thermostat down a few degrees to shut off room heating while I showered. Got 100% of boiler output for nice steamy shower. Nudged thermostat back up after, no problem. But have forced hot water cast iron baseboards, they cool slowly. And even at -10 boiler only ran 1 hr on & 1hr off for heating, 1.1GPH nossel.

hydro_newbie

@Jon_blaney it might be worth trying to get an idea of how quickly the house temp would drop if you were at design day and had no heat output. Although your system may not have a lot of thermal mass, your house will still have some.

You can also get an idea of how much time it would take for the boiler output to bring the indirect back up to temp based on the amount of water used and your boiler's output. Idronics #17 (around page 15) has some info that might be helpful.

@Robert O'Brien has asked about the DHW load. Some more info (family size, long showers, hot tub?) would give folks a chance to provide better info.

Gordy

Look at it this way a 40 gal. conventional gas water heater is around 35-38k. Turning up storage temp, and using a mixing valve increases capacity. A mixing valve should be used regardless.

hot_rod

If there is such a thing as average DHW consumption, one rule of thumb is 20 gallons per day per person, or 20 for the first, 15 per each additional person.

But it really depends on YOUR DHW wants and needs. A 50 gallon tank, running at 140F and a mix valve should work fine with that sized boiler.
If you are an "average' DHW user

Homes with hi-flow dual shower heads or large soaking tubs would be an exception perhaps.

Mark Eatherton

And don't forget to cover the largest "dump" load, otherwise your wife may be unhappy in a partially filled soaking tub,,,

ME

Gordy

Must keep wifey happy

jumper

Mark Eatherton said:

And don't forget to cover the largest "dump" load, otherwise your wife may be unhappy in a partially filled soaking tub,,,

ME

which is why you need enough storage OR REALLY POWERFUL heater. Latter has issues which is why tankless is not ideal for houses.

Alan (California Radiant) Forbes

We have the same setup as Mark Eatherton; 50K Munchkin that heats the house (accelerator plates screwed to the subfloor) and a 50 gallon indirect, although we are low fliers; only 200 feet above sea level.
When we have stay-over guests, I have to start thinking about DHW usage and let everyone know there is a limited amount available although we only have one bathroom and by the time a person has finished showering and left the bathroom, the water heater has recharged and is ready for another shower.
Our design day is 36°, so DHW priority is not a huge problem; the indoor temperature only cools down a few degrees at peak DHW usage. If you're in a cooler climate, it will be more noticeable.
Two types of people: those that will not tolerate any interruption of heating or DHW and those that are willing to work around it or put up with a minor inconvenience. If you're the former and don't want to be a guinea pig, get a separate, stand-alone water heater.

kcopp

I have a Buderus 115/3 Oil boiler. nets out @ 64 k btu. Installed autumn of 2003... Tekmar 260 control to prioritize HW.
I have the heatloss for my home right on that line.
Indirect WH is 60 gallon Superstor Ultra. 3/4" mixing valve on it. Tank set to 140.
5 in the family. Including 3 teenagers w/ a daughter who loves her long shower...
Never 1 time have I run out of hot water.

SuperTech

> @Leonard said:
> I've got a tankless coil in old CI boiler, I usually get all the steamy hot water I want for showers even when weather is a little cold.
>
> Few months ago when was a rare minus10 deg (about coldest it ever gets here) shower wasn't as hot as I wanted it. So I just set room thermostat down a few degrees to shut off room heating while I showered. Got 100% of boiler output for nice steamy shower. Nudged thermostat back up after, no problem. But have forced hot water cast iron baseboards, they cool slowly. And even at -10 boiler only ran 1 hr on & 1hr off for heating, 1.1GPH nossle

There's no way I could stomach (or afford, lol) keeping my boiler at 160 degrees 24/7/365. Tankless coils are a huge waste of fuel in my opinion. I prefer an indirect tank, but I'd rather have an electric water heater with a setback timer, or even a direct fired water heater over the thankless coil.

But I really like your cast iron baseboards

Leonard

It's an old boiler. Dad installed it when he built the house some 60 years ago when oil was ~ 25cents /gallon. I do wonder if I installed a motorized exhaust damper if that would help with standby losses.