Sizing for modulating condensing boiler in series with hot water tank

Piston_Pounder

I am in the process of learning to design a hydronic radiant heat system for my house. I live at 62 degrees North so it is a cold place. I have calculated my house to have a heat load of 140,000BTU/hr on a -40 day with 3.5 ACH.

Currently I have a 36,000btu water heater.

I am planning to install a modulating condensing boiler with two inputs and outputs. One for heat supply and one for domestic hot water. I have a basement suite that will run off the same system as the main house. First, I am looking for a bit of advice on how to apply my house’s heat load to determine boiler size. Since the hot water demands of two families in one house could exceed the GPM output of many hot water on demand boilers, I was thinking I could keep my existing Hot water tank and run it in series from the output supply of the domestic hot water from the boiler.

Currently, the hot water tank is just enough to maintain the needs of the hot water demand in the house. With that in mind, is a 150,000btu boiler a good size for my heating and hot water needs if it is run in conjunction with the 36,000btu hot water tank?

The heating system will mainly consist of radiant ceiling panels. One or two rooms will be run through in floor loops.

Hot_water_fan

You're more or less describing an indirect water heater, but need to sort out the details. Take a look at Caleffi's Idronics journals, you'll learn a lot!

Jamie Hall

You have the basic information you need there -- good show! That 140,000 BTUh load which you calculated is what you need. One of the outputs from the boiler would be for heating an indirect hot water storage tank, and there is no need to increase the size of the boiler to do that. However, what you cannot do safely is to run the domestic hot water through the heating boiler's piping. Either you will need a heat exchanger to separate the heating hot water from the domestic hot water, or you will need a new indirect storage tank, which has the heat exchanger built in. I would recommend doing the latter.

Your initial description, however, sounds a little like you might be thinking of using a "combi" boiler, which has completely independent an on demand hot water heater built in, which is completely independent of the heating side of the unit. You note, however, that the hot water demand might exceed the heating capacity of such a unit -- and I would say you are correct. While you could use your existing hot water tank as a storage tank, fed from the on demand side, that rarely is a good solution.

EdTheHeaterMan

There are often 3 ratings on boilers. Input, output, and NET The NET is the number you want to match to the heat loss calculation.

Back in the day, the engineers got together and decided to make a standard for this industry. The BTU or British Thermal Unit was established as the measuring unit for heat and the ruler to use for the machines that make and move heat. Over time a few rules were made so different machines could be compared to other machines of the same function.

When comparing boilers of one brand to another and the suitability for a particular size and construction of a building the Institute for Boiler Rating (I=B=R) was established. We still use their guidelines today. I=B=R determines that the amount of fuel that was PUT IN to the boiler to make a fire would be called the IN PUT or "Input". This is the number you would use to select the correct size for the gas pipe.

The amount of heat that is lost up the chimney is not able to be used to heat the house. This lost heat is accounted for in the amount of heat the boiler can PUT OUT into the radiators. This is called the OUT PUT or "Output"

Some of the Output heat is then lost in the pipes that connect the boiler to the radiators. the I=B=R people decided that on average this is about 15%. It actually changes from one house to another, but on average (100 years ago) that number was not much higher than 15% in any given home.

When energy became expensive in the 1970s the Department Of Energy decided to look closely at the efficiency of heating appliances and tested many heating appliances. As it turns out, the difference of the input and the output on the rating established by I=B=R many years ago closely matched the amount of heat lost up the chimney or out the exhaust vent. But they wanted the number to be more precise for comparison, so AFUE was added to the ratings of the heating appliances.

Over the years, the AHRI and the I=B=R were all consolidated into one entity. So now the rating system is still the same but with different names.

You can clearly see that a boiler rated at 85,000 BTUh input that is about 95% efficient will have an output of 80,000 BTUh. and the NET rating is 70,000.

Short version: I would use the 199,000 boiler in your home with the 140,000 BTU load calculation. The 150,000 Boiler rating may have a NET rating below 140,000. And as @Jamie Hall said, "sounds a little like you might be thinking of using a "combi" boiler". Those boilers modulate down to the actual input needed at any given moment of time.

If you do get a temperature colder than the design temperature and you also want to use hot water t the same time. You will probably like that little extra capacity. The savings of operating the 150,000 BTU boiler at the lower modulated inputs, compared to the 199,000 BTU boiler at the lower modulated rate will not be a significant difference. There will be savings, but not significant savings.

Piston_Pounder

Jamie Hall said:

You have the basic information you need there -- good show! That 140,000 BTUh load which you calculated is what you need. One of the outputs from the boiler would be for heating an indirect hot water storage tank, and there is no need to increase the size of the boiler to do that. However, what you cannot do safely is to run the domestic hot water through the heating boiler's piping. Either you will need a heat exchanger to separate the heating hot water from the domestic hot water, or you will need a new indirect storage tank, which has the heat exchanger built in. I would recommend doing the latter.

Your initial description, however, sounds a little like you might be thinking of using a "combi" boiler, which has completely independent an on demand hot water heater built in, which is completely independent of the heating side of the unit. You note, however, that the hot water demand might exceed the heating capacity of such a unit -- and I would say you are correct. While you could use your existing hot water tank as a storage tank, fed from the on demand side, that rarely is a good solution.

Thanks for the input. I am still learning of all the different boiler types. Why is it not recommended to send the domestic supply of a combi boiler into a separate hot water tank? A separate storage tank with heat exchanger integrated with a modulating condenser boiler is a better way to go?

ScottSecor

Sending the boiler water into a separate tank is typically done with a indirect tank. The tank would typically hold say sixty gallons of dhw with a coil of some sort that passes hot boiler water to and from the boiler. These are common and a great way to take advantage of the high efficiency boiler your considering. You might want to consider a larger indirect tank as it will provide ample storage and allow you to run the boiler in condensing mode most of the time.

Here is an example chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/viewer.html?pdfurl=https%3A%2F%2Fwww.lochinvar.com%2Flit%2F366398LOC_T_SIT_BR_1004_2020.pdf&clen=5557850&chunk=true

hot_rod

You have 3 options for the DHW. A boiler and separate indirect tank, maybe a 60 or 80 gallon.

A 199,000 combi boiler which would give you 4 gpm or more of DHW, continuously.

If you want a lot of DHW all at once or run multiple high gpm loads at the same time, you could runthe tank you have in series with the combi boiler. giving you that extra "dump" capacity.
It would take an additional stainless pump top do that.

Rinnai and a few other tankless DHW manufacturers are adding tanks to their commercial tankless heaters for that dump capacity.

It really comes down to how you use the hot water, how much and how quickly you want it.

A tankless or combi with a 4 gpm capacity is plenty for most households, able to keep up with two showers running at once.

For an exact output, select a product and use their sizing charts. It asks what the incoming water temperature is in your area, to give you a more accurate gpm output.

Some show a 70° temperature rise, others a 77° so read the spec sheet carefully.

Piston_Pounder

Anybody have any experience with IBC boilers? Their SL30-199 G3 model looks like it could be an option. I can't find any BTU ratings on it but it has two outputs. One output can go to the DHW indirect storage tank and the other to the radiant heat. The flow runs through the radiant heat side of the boiler then when there is demand to fill the indirect tank, it temporarily switches the ouput to the DHW supply line until the demand is met then reverts back to the radiant heat side. 

hot_rod

Most all mod cons go to high fire on a demand for indirect or HX domestic call.

So if the boiler is running on outdoor reset, low temperatures, the DHW call will take priority and go to 180- 190 until the call is satisfied.

On some brands those temperatures and time frames are adjustable. So if there is an extended call for DHW the boiler will go back to space heat after an elapsed time. This prevents potential freeze ups in the heating side.

With a generously sized HX or indirect coil you can sometimes keep the boiler in condensing mode while DHW is covered. HeatFlo and others have those high performance tanks, usually a dual coil concept.