Sizing boiler

[bmxer883]

Hey so this isn't really a question for a garage I was just unsure of another forum to post on.
So just re did my basement dug out old dirt floor and repoured walls and a floor. In the floor I put one loop in for in floor heat area is maybe 500 sqft.

I heat the house with a 100k furnace I really wanna switch to boiler system and install in floor heat or baseboard throughout house. Also want to get rid of old tank water heater.

So my question is I wanna do things in steps not all at once so my basement floor is ready for heat but no other rooms are. Can I buy a combination boiler big enough to run the whole house and just use it for the basement and water heat or will it not run right being over sized. I hope to install baseboard in each room but will take me years probably..

I'm also unsure of btu to replace furnace and water heater.

 

[engineer2]

I have a 500 sqft area of my finished basement with subfloor heat, but it is on top of the concrete floor with 1/4 fanfold insulation underneath. I have it connected the house's 50 gallon gas-fired water heater. Based on GPM and inlet/outlet temperatures I'm using 10,000 to 12,000 BTUs when it runs in the winter. It does not impact the capacity of the water heater nor did it have a notable impact on my gas bill. Not sure what BTUs an in-floor system will need, but there are calculators online.

Having radiant heat on the same system as your home's drinking water is against plumbing code, but mine still runs an hour or two per day in the summer, so no stagnant water issues. My highest recent usage was December 2021 when it ran about 17 hours a day. I've been using it for 19 years now without any issues. Keeps the basement at a comfy temp year round.

Someday when I finish the rest of my basement I will add another 400 sqft. At that point I will likely buy a small water heater and separate it from my domestic water system.

 

[Yankeefarmer]

All summer long, people whose houses use boilers (actually fired water heaters, since they produce hot water, not steam) typically have them operating only to maintain domestic hot water. Efficiency is lower than a water heater alone would likely be, but they run properly. So yes, you could do what you want, install a boiler sized for your projected needs, and run it as you put different zones into service.

 

[dogdog]

Is the op talking about indirect water heater off the boiler’s hydronic loop? I wouldn’t do it,I would just keep the water heater and the boiler system separate if you can. People can argue all they want how efficient the indirect fired water heater is /was, it’s not during summer months that you don’t need heat.

 

[yeldogt]

You need to size the new boiler for the finished house -- everything today is modulating. The problem you can run into with combination boilers (heat and domestic hot water) is the output needed to heat domestic water makes the boiler too large for many houses today .. especially with low temp radiant. Even with a 10/1 turndown they can be too big for many times of the year in many parts of the country.

How long is that loop?

What works for some people does not for others. I don't like on demand water heaters for my projects as they are too large and the wait for hot water drives me nuts. But -- just the ticket for my beach house outdoor shower.

With new houses and gas boilers I normally do an indirect water tank for the domestic hot water -- mostly because there is no vent needed for the water heater. Heating hot water if you have natural gas with a tank unit is not very expensive -- you will not save any money going tankless. Tankless is all about usage .... if you have 5 teenage girls in the morning taking showers. The typical house with a 40g tank is not using very much gas to heat the water.

The equipment comes into play today -- years ago tank heaters were so cheap ... today they are more expensive and replacing them with an indirect when you get a new boiler is common -- and the indirect will last 2x the tanks.

Bradford White did make a hot water (tank unit) with a larger burner and an internal coil for small radiant ... if you tank unit is going that would get you a new tank and the ability to heat the basement. I used that model in two projects --- one for a kitchen floor and another for a kitchen and bathroom.

Time line is important ... equipment is changing and baseboard hot water heat uses very hot water. With a condensing boiler you will only be able to use 180 degree water vs 180 or 200 for an old school cast boiler

 

[bmxer883]

Okay so sounds like I want to stay away from combination like I was thinking I just liked that it would take less room up..

I have a 40g hot tank that works but is in rough shape and rather than reinstalling it was thinking a tankless one to save room. You say you don't save much but my grandfather who doesn't use a ton of water saved a lot in his bill when he switched but his water tank was old and not very efficient.

The basement has one loop of 300ft. I'd like to have radiant floors in the house but not sure if they will work in my hous it's a very old house with 1/1/2 to 2 in wood floors so I thought I may have to install baseboard plus I think that may be cheaper. I'm not looking to spend 10k or more for all this

 

[yeldogt]

Okay so sounds like I want to stay away from combination like I was thinking I just liked that it would take less room up..

I have a 40g hot tank that works but is in rough shape and rather than reinstalling it was thinking a tankless one to save room. You say you don't save much but my grandfather who doesn't use a ton of water saved a lot in his bill when he switched but his water tank was old and not very efficient.

The basement has one loop of 300ft. I'd like to have radiant floors in the house but not sure if they will work in my hous it's a very old house with 1/1/2 to 2 in wood floors so I thought I may have to install baseboard plus I think that may be cheaper. I'm not looking to spend 10k or more for all this

It gets expensive quick ....

At my old weekend place in PA I had an oil fired water heater (tank). It was in an old stone basement that was cold in the winter when the house was not occupied. We would turn off the unit when we left for the week. Why have it trying to heat the water all week. When we arrived it would heat the tank in no time ... also, it shared a space with two oil furnaces. When the house was occupied the space was much warmer all around. Your grandfather could have had an old unit in a cold basement ..... In the off season my gas bill is under $30 w/ hot water/ dryer and stove. Newer gas tanks are a little better insulated and not pilot light.

Electric is another ball game ... much more expensive to run and you don't want a cheaper one with thin insulation in a cold space.

At my new place in PA I did an indirect off the propane boiler -- most indirects have heavier insulation vs a regular tank. Also -- many boilers have settings for the hot water. The german boilers have always had settings were you stop the hot water heating for a period of days and/or night setbacks. Mine are set to stop heating the tank around 8:30pm and not reheat until 5:30 am weekdays ...... the tank has enough hot water to take a couple of showers .... no point in reheating and having it sit all night. It's all ready to go in the morning ... this made more of a difference with a big cast boiler in the summer when it was only going to fire for the hot water. If you set it correctly for morning use the boiler was still full of hot water when the tank was in need of a recharge after use in the morning. In Germany where fuel is expensive this all mattered more ..... if you have natural gas the cost difference in the USA is very small. With propane you use all of the items available to save some fuel.

 

[Yankeefarmer]

Seventeen years ago, I ripped out the 125K forced air furnace in our house and replaced it with a Weil-McLain Goldseries boiler of the same rating. I used John Seigenthaler ‘s Modern Hydronic Heating as a textbook and guide to design my system. I had hoped to do at least a couple rooms with floor radiant heat, but with winter approaching I elected to install baseboard in every room. The change to hydronic heat with 5 zones completely transformed our comfort in the wintertime. Previously, the whole house had been one zone, and I had been unable to adjust airflow to avoid drastic changes in temperature and comfort during cycles. After the first winter, I no longer felt I needed radiant floor heat to be comfortable.

The other thing I learned from this project was that I could have gotten away with a smaller boiler. When I did my heat loss calculations, they indicated that I should use a 125k boiler, so that’s what I went with. However, the way we operate our system, with living areas set back at night, and sleeping areas set back during the day, I didn‘t need the full calculated capacity. On top of that, the newer areas of the house were much more energy efficient than the old. I have addressed this partially by using a smaller nozzle in the burner.

 

[u3b3rg33k]

@bmxer883 have you looked at the Rinnai i120CN? it's a modulating condensing CH+DHW (simultaneous) combi heater. 15-120kBTU for heating, 199k for hot water.

no need to worry about oversizing it (all the models minimum fire is 15kBTU), provided the smallest zone can handle min fire or you have some thermal mass for it to heat.

 

[yeldogt]

@bmxer883 have you looked at the Rinnai i120CN? it's a modulating condensing CH+DHW (simultaneous) combi heater. 15-120kBTU for heating, 199k for hot water.

no need to worry about oversizing it (all the models minimum fire is 15kBTU), provided the smallest zone can handle min fire or you have some thermal mass for it to heat.

Sizing has so much to do with application and what he finally wants to do with the radiation. His 10k is going to be a problem as is the single radiant zone if that's all he wants.

 

[yeldogt]

Seventeen years ago, I ripped out the 125K forced air furnace in our house and replaced it with a Weil-McLain Goldseries boiler of the same rating. I used John Seigenthaler ‘s Modern Hydronic Heating as a textbook and guide to design my system. I had hoped to do at least a couple rooms with floor radiant heat, but with winter approaching I elected to install baseboard in every room. The change to hydronic heat with 5 zones completely transformed our comfort in the wintertime. Previously, the whole house had been one zone, and I had been unable to adjust airflow to avoid drastic changes in temperature and comfort during cycles. After the first winter, I no longer felt I needed radiant floor heat to be comfortable.

The other thing I learned from this project was that I could have gotten away with a smaller boiler. When I did my heat loss calculations, they indicated that I should use a 125k boiler, so that’s what I went with. However, the way we operate our system, with living areas set back at night, and sleeping areas set back during the day, I didn‘t need the full calculated capacity. On top of that, the newer areas of the house were much more energy efficient than the old. I have addressed this partially by using a smaller nozzle in the burner.

In cold climates unless you have a nice modern house -- hot water heat is going to be more comfortable. That's why the big old radiators w/ steam or hot water made those leaky house comfy -- the radiators got hot and stayed hot sending out radiant heat. Baseboard is not quite the same unless it's the cast type -- but you get that hot temp air from the 200+ degree water going through the pipes.

Same with old houses and oil heat forced air -- the hotter temps of the oil made for more comfort. Todays modern modulating and variable speed forced air are game changers as they don't go one and off.

17 years ago even if you did a heat load I'm not sure you could have purchased a smaller traditional boiler -- Buderus used to make a 112k and that was one of the smallest at the time.

 

[u3b3rg33k]

Sizing has so much to do with application and what he finally wants to do with the radiation. His 10k is going to be a problem as is the single radiant zone if that's all he wants.

I thought that was someone else's numbers but maybe I read it wrong.

If I didn't need to heat my whole house, and I wanted a new hot water heater as well, I looked into the Sanden CO2 heat pump unit. get the stainless tank, or maybe an indirect (and run it backwards, use the lower HX for the floor loop) , and you have plenty of hot water on tap (makes 160F water). wouldn't do to heat the whole building but it could be enough for hot water and some ancillary zones. 15kBTU/4kW worth of heat.

 

[yeldogt]

I thought that was someone else's numbers but maybe I read it wrong.

If I didn't need to heat my whole house, and I wanted a new hot water heater as well, I looked into the Sanden CO2 heat pump unit. get the stainless tank, or maybe an indirect (and run it backwards, use the lower HX for the floor loop) , and you have plenty of hot water on tap (makes 160F water). wouldn't do to heat the whole building but it could be enough for hot water and some ancillary zones. 15kBTU/4kW worth of heat.

Sorry .... the 10k was Dollars not BTU. OP did not want to spend more.

There are lots of possible solutions .... typically a house with reasonable ductwork can be made to work with a modulating furnace

 

[u3b3rg33k]

Sorry .... the 10k was Dollars not BTU. OP did not want to spend more.

There are lots of possible solutions .... typically a house with reasonable ductwork can be made to work with a modulating furnace

it's definitely possible to stay within budget for a pump panel and boiler, if you're doing the install. the Rinnai is $3k, a 4 pump primary/secondary zone panel is about $4k.

 

[fitter30]

Your furnace 100k thats probably input not output. Output 75k to 80ksize a hi efficiency boiler on furnace output if water temp is below 130@° return temp. Did it cycle on the coldest days? If it did probably could go smaller. Hi efficiency boilers water temp 130° or below return temp for the 95+% efficient. Above 130° 87% doesn't matter what brand. Have never seen a combi boiler with 80k for heat and 199k for domestic hot water. Properly designed floor radiate water temp runs 5-10° over room setpoint. The 500 sq ft basement have 300' loop 1/2" pex? Baseboard heaters run 140°+ water temp. Boilers can run at different temps have to piped correctly theres several different ways. With a boiler each room heat load needs to be known for the correct size of heat emitters.

 

[u3b3rg33k]

Your furnace 100k thats probably input not output. Output 75k to 80ksize a hi efficiency boiler on furnace output if water temp is below 130@° return temp. Did it cycle on the coldest days? If it did probably could go smaller. Hi efficiency boilers water temp 130° or below return temp for the 95+% efficient. Above 130° 87% doesn't matter what brand. Have never seen a combi boiler with 80k for heat and 199k for domestic hot water. Properly designed floor radiate water temp runs 5-10° over room setpoint. The 500 sq ft basement have 300' loop 1/2" pex? Baseboard heaters run 140°+ water temp. Boilers can run at different temps have to piped correctly theres several different ways. With a boiler each room heat load needs to be known for the correct size of heat emitters.

all 3 models have the same minimum fire so I ordered the big dog. And yes, it's Simultaneous DHW and heating.


if you're looking for ideas menards sells them prefab, or you can call someone like blueridge and have them make you one to spec.

https://www.menards.com/main/plumbing/hydronic-radiant-heat/radiant-heat-control-panels/c-8521.htm

there's also pump panels that can mix down to a lower temp, for example:

 

[bmxer883]

Your furnace 100k thats probably input not output. Output 75k to 80ksize a hi efficiency boiler on furnace output if water temp is below 130@° return temp. Did it cycle on the coldest days? If it did probably could go smaller. Hi efficiency boilers water temp 130° or below return temp for the 95+% efficient. Above 130° 87% doesn't matter what brand. Have never seen a combi boiler with 80k for heat and 199k for domestic hot water. Properly designed floor radiate water temp runs 5-10° over room setpoint. The 500 sq ft basement have 300' loop 1/2" pex? Baseboard heaters run 140°+ water temp. Boilers can run at different temps have to piped correctly theres several different ways. With a boiler each room heat load needs to be known for the correct size of heat emitters.

Yes 300' of one loop 1/2 pex. And my heat load will change over the years we are redoing one room at a time probably half of the house has no insulation rest I have been putting spray foam in.

So beginning of post made it sound like I should stay away from the combo boiler