New System---Sanity Check

OaklandNS · August 2018

Hi Everyone,

I have been reading the forums and it seems like folks here give excellent advice with a lot of patience. I'm designing a hydronic heating system using radiators for my house, and I want to run my plans through a sanity check. This is part of a larger remodel that has our floor joists exposed (our unit is over a second first-floor unit we are preparing to rent out), so it's all or nothing at this time.

Square Feet: 1500
Ceiling: Attic w/ R-38 Insulation
Floor: Conditioned Space with R-23 Insulation (primarily for noise attenuation)
Walls: Studs/siding, uninsulated
Windows: Double Pane
Design Temp: 34 degrees
Inside Target Temp: 68 Degrees
Heat Loss: 23,000 BTUs (used a Manual J calculator and Slantfin App, came out the same)

Heating System:

199,000 BTU DHW Tankless supplying DHW and Heat
DHW Temp: 140* F
Heat Exchanger to separate DHW and Heat, mixing valve to bring DHW to 120*F
Delta T supply/return on heating side: 20*F

Piping plan is to have a 3/4" Fostapex shared supply and returns with 1/2" branches to pipe in parallel 9 rundtal baseboard radiators (sized according to 140* F operating temp to hit my 23,000 BTU heat loss). Based on my calculations, a 3 GPM flow rate will easily provide 30,000 BTUs in 3/4" Fostapex and meet my heating requirements.

I do have a couple questions about designing an indirect system:

I think I understand the flow rate calculation on the heating side, but how do I make sure I size a heat exchanger and circulator on the water heater side to make sure it's delivering enough heat to the heat exchanger? The design I'm looking at has a very short loop of 1" pipe, so does that mean a relatively small circulator will work so long as it keeps a reasonable GPM flow rate?
I am slightly undersizing the radiators because we don't really use much heat compared to what I've experienced at other people's houses, and I figure since I'm designing for 140* system temp, I can increase the water heater temperature if I end up needing more BTUs at the radiators (the model I'm looking at is warrantied for space heating and runs up to 180 degrees), is this a reasonable assumption?
How do I size a pump on the heating size to make sure it has enough flow/head to run through the supply, return, and the radiators? Do I count each radiator in the head calculation, or just the total of the supply/returns and the furthest radiator (which is also the largest in my case)? What about head loss in the heat exchanger?
Would I benefit from a variable delta t pump from Taco that could change the flow rate on the heating side to account for relatively warm winter days (which are super common here)?
My house gets an awful lot of solar radiation from its south facing facade. I put in the south facing walls on the manual J calculator, but would I be wise to further undersize the radiator in that front room?

Thanks in advance, and I can provide any information you would like to see if it's helpful.

kcopp · August 2018

Do not use a tankless water heater as a boiler. It is the wrong use... If you need 1 unit to do both get a combi boiler. The tankless water heater will give you so many headaches w/ every other question you are asking...

Zman · August 2018

A couple flaws right out of the gate:
If your heat loss is 23k, why in the world would you want 199k?
Why the tankless? It is the wrong tool for the job. If you are keeping the house for any period of time, it makes no sense. It will cost more to install, have higher maintenance, and short cycle itself to death.

As for your questions:

Heat exchanger companies have calculators that will help you size the exchanger and will give you the head loss. The heat source will also have a head loss number. The tankless will have a very high head loss because it was not designed for the purpose.

Size the radiators for whatever temp water you intend to use. Yes, you can always turn up the temp if you need more heat.

You want to calculated every component in your longest run including rads and valves. You will need to figure out how to balance the loops. reverse return piping would be one way.

You need a control strategy which incorporates outdoor reset to handle the warmer days. Delta T control on it's own will not do that.

The room that gains more heat on sunny days will lose more heat on cold cloudy days. Runtals work well with TRV's Do not undersize the radiator.

OaklandNS · August 2018

Thanks for the quick replies!

I’m planning to use the tankless for a few reasons. It can supply DHW and heat in one unit, it has a lower minimum btu than the combi boiler so will hopefully avoid short cycling, it is warrantied for space heating applications, since I am separating water systems between my two units I need to install a water heater anyway (i.e. install cost is the same either way), it has lots of flow for DHW (I have three showers) and the model can be installled outside. Similarly, I decided against the combi boiler because the units that can supply the most hot water for three bathrooms are oversized for my heating needs but still have less DHW flow capacity than a tankless water heater, there is no outdoor model, and the units are about $1,000 more.

If using a tankless is really a bad idea I can definitely switch course and go with a boiler, but I guess I have trouble seeing why a tankless would be a bad move when I’m using it as the manufacturer intends and within the manufacturers warranty.

Thanks for the answers.

To balance the loops, I can envision a reverse return setup that would work. Would using TRVs on each radiator be an alternative?

I saw that Taco makes an outdoor reset for its relays. Would that unit provide outdoor reset capability assuming my heat source does not?

Thanks again!

hot_rod · August 2018

A typical tankless is designed to heat small gpm flow rates across a large delta the, say for example 3 gpm at a 70° temperature rise. That is opposite of how a heating system operates, being higher flow rates, small delta maybe 15- 20°. Much different heat exchanger design.
A tankless will require a high head circulator, generally, to overcome the flow resistance in a HX designed to instantly heat water at high delta conditions.

A boiler will have a user friendly control with all the features needed for heating. Many include ODR, electronic derating, ramp delay, etc. you can customize the control logic. No need to add on additional controls to make it do what you want.

OaklandNS · August 2018

Is there a boiler model that would work for my situation and provide hot water for three showers and a relatively low heat load? My design heat loss is only 22,0000 btus and will be much less on most days.

Thanks again,
Nathan

Rich_49 · August 2018

http://www.htproducts.com/literature/UFTC-Brochure.pdf

OaklandNS · August 2018

The uftc-199 looks like it has comparable hot water supply to our existing tankless, but it’s min btu input on the heating side is 19,000 Btu. Won’t I be short cycling all the time with a 22,000 design heat load?

Jon_blaney · August 2018

Look at doing it with a hot water tank heater. I use oil fired BOCK tank to heat part of my house and provide DWH.

hot_rod · August 2018

OaklandNS said:
Is there a boiler model that would work for my situation and provide hot water for three showers and a relatively low heat load? My design heat loss is only 22,0000 btus and will be much less on most days.

Thanks again,

Nathan

Better define 3 showers

the challenge for tankless or instant "on demand" DHW is capacity.
If you have 2- 2.5 gpm shower heads running at the same time that would require a unit designed to supply the required GPM at the temperature rise common in your area. I suspect water in your area doesn't drop to 34F in winter? That works in your favor.

120K is probably the low end of combis, providing around 2 gpm DHW. 199K or larger for those needing or wanting more dhw capacity.

Often these days the DHW load is bigger, sometimes much bigger than the heating load. You need to size to the larger load.

I think many tankless and Combi owners learn to live within their units ability as they will not supply DHW the same way a tank type water heater will. Some are fine with the compromise, others not so much.

The product Rich mentioned has systems like your in their sights, other brands offer similar products, look for what brand is common and well supported in your area. I think JTG Muir reps HTP in your area, they have since almost day one.

A combi is a compromise design by default, when it tries to cover so many wide system and owner expectations. Decide if the trade offs are worth it for you. I don't yet see a perfect product for low load systems like yours, all will have pros and cons.

I like a tank style boiler for those low loads, but DHW function adds complexity and cost if you want it in one "box"

OaklandNS · August 2018

Thanks hot rod that is super helpful. Space is the problem in that I don’t think I really have room for a tank style boiler (not to mention the separate hot water system).

I'm not at all adverse to using a combi-unit, but I'm concerned about short cycling with mininum btu inputs in the 17,000 range and my design load of 22,000 btus. Would it make sense to install the unit, and then if it seems to be short cycling under normal use, to add a buffer tank?

hot_rod · August 2018

Maybe download the manuals from the brands and models you are considering, I have the small Lochinvar Nobel so I can offer my experience.

The small Nobel Combi turns down to 11K the 150 to 13K. If you program ODR properly, it takes a few tries, and set the space heat limiting, you should be able to keep the cycling to a reasonable level.

The decision would be based on the DHW, once again. We two are fine with the 110, your family may need the 150K, with a low turndown of 13K.

I also like that it has a setting to maintain a hot boiler so DHW is near instant. Minus piping length of course.

A great rep in Osborne Sales out of Livermore, if you are in Oakland, CA?

http://www.lochinvar.com/_linefiles/100275091_2000533998_Rev M (23992).pdf

OaklandNS · August 2018

My current tankless will do about 5.0gpm at 77* temperature rise, so it looks like for a 1-1 replacement I would go with the 199,000 btu Nobel or something like the NoritzCB199, also 199,000 btu. The former has a min input of 19,900 and the Noritz has a min input of 18,000.

Would I see a big difference in cycling on one versus the other? And would they be ok on a day where I only needed to draw 10,000 btu/hour to maintain temp? My plumber is generally more familiar with Noritz units.

hot_rod · August 2018

Well, if you want that 5 gpm and the current tankless does that, then the 199 is the correct size.
But as the numbers show the turndown is not as low and it will cycle more often on low heat load conditions. How much more depends on the loads across the heating season.

If you like to crunch numbers you can use past weather data and compile a graph showing how many days are at what temperature and come up with % of days at various required heat input.

I'm not familiar with the Noritz, is it a Combi? Does it have the same features as relates to limiting output, ODR, quick responding DHW etc as the Loch?

Here is an example of a "hours of occurrence" graph we used in a webinar a few years back on maximizing condensing technology.

I suspect your weather differs from Boston.

OaklandNS · August 2018

The Noritz is a combi. They primarily make tankless units, but have a couple of combis available, now as well. According to its specs/manual, it has similar features in terms of ODR and actually has a lower possible output on the heating side than the HTP or Loch units of comparable DHW capacity. I don't believe it has the holding tank for DHW like the Loch, but neither does our current unit and the Noritz is about $1,000 cheaper.

I'll try to crunch numbers and see what I come up with in terms of cycling.

EBEBRATT-Ed · August 2018

Pick the combi you like and use a buffer tank to reduce short cycling

hot_rod · August 2018

OaklandNS said:
The Noritz is a combi. They primarily make tankless units, but have a couple of combis available, now as well. According to its specs/manual, it has similar features in terms of ODR and actually has a lower possible output on the heating side than the HTP or Loch units of comparable DHW capacity. I don't believe it has the holding tank for DHW like the Loch, but neither does our current unit and the Noritz is about $1,000 cheaper.

I'll try to crunch numbers and see what I come up with in terms of cycling.

The Nobel doesn't have a holding tank, it just maintains temperature in the fire tube boiler block to quicken DHW production. Of course it does consume some additional fuel in doing so.
It can be enabled or disabled, depends on how long you want to wait for DHW. The energy code in CA for examples requires DHW recirc if the fixture is more than 50 feet from the heater, to eliminate running water down the drain, waiting for it to get hot. The enabled function also lessens water waste, as the boiler is always up to temperature.

I tried both ways with mine, with that function disabled it took over a minute to get hot water even at the closest fixture. The boiler cold started every time you turned on a faucet.

I think the critical control logic when the loads are so lopsided is a ramp delay and/or the function to electronically derate, in addition to ODR. It basically locks the output to a lower number on any heat call. So on DHW call you get about 179K, (199 x 90% efficiency) about 17K on heat call, in a 10-1 turndown boiler.

Typically with mod cons the higher priced units have more control options, compared to price point models. In your case that could be money well spent. Those are the questions I would ask.

As you and others mentioned if cycling drives you crazy add 10 or so gallons of insulated buffer later. Turn the low mass boiler into a mid or high mass basically.

That is what Viessmann is offering with some of their current models, more mass and water content inside the "box".

https://www.viessmann-us.com/en/residential/gas-boilers/condensing-boilers/vitocrossal-300-cu3a.html

New System---Sanity Check

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