Sizing boiler for both radiant & DHW

Tundra

It sounds as if it is well past time for a new boiler. I like the Biasi, but what ever you get it will be an improvement. Adding infloor radiant with other types of distribution systems is fine. Remember that each type of distribution system is designed for different water temperatures. This requires separate zones and mixing valves. Another way to do that is to run your radiant through a separate indirect.

If you use radiant in the basement floor remember to insulate under the new concrete. One of our local home owners installed his system by himself, neglected the insulation, and couldn't figure out why his heating bill went up.

It sounds like your basement ceiling might be unfinished. Have you considered staple up radiant for the first floor?

Jason Horner

Sizing boiler for both radiant & DHW

I'm in the market to update the heating system in my older (1915) home.

The house is 3 floors + basement (about 2400 sq. ft. + 1000 sq. ft. basement), with hydronic heating and the original cast iron rads in each room except for the basement. The basement has no rads and is just heated by radiant losses from the uninsulated iron pipes (1.25" OD) leading to each of the rads. These pipes hang below the basement ceiling joists and are in the way of using the basement space effectively.

The current boiler is a 25 year-old Raypack, 240,000 BTU input and this boiler continually cycles on/off the limit in the winter.

The house has had some insulation upgrades and new windows/door and the overall heat loss was calculated (estimated???) by a couple different contractors at between 135,000 and 185,000 BTU for sizing a replacement boiler. Some contractors proposed just replacing the current boiler with a boiler of the same BTU input.

As part of the heating upgrade I was also planning to rip out the basement floor concrete and have in-floor heating installed (the floor needs replacing anyway - it's only about 1-2" of uneven concrete right now).

I wanted a very high efficiency boiler and a couple of the contractors recommended the Viessmann Vitodens and the Weil McLain Ultra.

I have several questions and would like your considered opinions.

1) If I decide to use either the Vitodens or Ultra and I decide to opt for indirect-fired DHW, does that change which model of boiler I purchase? eg. Weil McLain Ultra comes in 160,000 and 230,000 BTU models. Let's say that the heat loss calc says that the house needs 140,000 + 10% margin for really cold days, that effectively uses the full capacity of the Ultra 160. Would I need to step-up to the Ultra 230 if I do DHW in that case?

In other words, how much extra BTU capacity does the new boiler need to do DHW? When I look at my existing traditional natural gas HW tank, it's a 40,000 btu device running at 50-60% efficiency as a guess. So in the example above, saying that the house on the worst day needs 160,000 and DHW needs 25,000 btu, that means 185,000/0.93 = 199,000 BTU input (at 93% efficiency - if I'm thinking about this correctly). Does this make sense?

Here's a twist on this...my teenage daughter loves to take 30-40 minute long showers (2.5 gal./min. showerhead), and my wife is arthritic and very sensitive to changes in heating temperature. If the Ultra 160 is chosen would I'd be better off to install a small 25 gal. indirect tank as a buffer, rather than doing DHW on-demand??? I just can't see having DHW priority for 30-40 minutes in the dead of winter - the drift in temperature in the house would begin to make my wife miserable. Or is going with a bigger boiler in the first place a better idea?


2) When the heating controls give priority to DHW production, is it an 'all-or-nothing' proposition? ie. 100% of the boiler capacity is diverted to DHW, or can the controls be set to say 75/25 bias in favor of DHW vs. radiant heating for the rest of the house? If it can be adjusted, what kinds of controls should I be looking for in the contractor quotes?


3) I want to get rid of the iron supply/return pipes flying all over the basement. I'd like to have all the supply/return pipes relocated into the basement ceiling joist cavities. I would keep the steel risers to each rad, so the only place piping would be replaced would be in the basement. Would using Ipex 1" dia. Pex-AL-Pex pipe be an ok choice to repipe the supply/returns in the basement? or would you recommend "L" copper or iron pipe instead?


4) What diameter should the manifold supplying the cast iron rads be? The current manifolds are 2" dia. but each of the nipples leading to the radiator supply/returns is only 3/4", which in turn connects to 1.25" runs to the rads.


5) Given that both the Viessman and Weil McLain boilers are expensive condensing boilers, would it be a good idea to isolate the cast iron rads from the boiler by installing a heat exchanger between the boiler and the supply/return manifold for the cast iron rads to prevent rust particles getting into the boiler? I'd rather sacrifice a $200 heat exchanger every 4-5 years than ruin and expensive boiler.


5) Should the basement in-floor pex tubing be isolated from the boiler with a heat exchanger? A related question is: what is the efficiency of heat exchangers from one side to the other? 95%? Do heat exchanger manufacturers post this information?


6) Given that I will wind up with a mixed heating system - cast iron rads, pex in-floor, and either a stainless steel (Viessmann) or cast aluminum (Weil McLain) heat exchanger in the boiler, should any corrosion inhibitors be added to the water, and if so, what type?


7) My current boiler runs the circulator (Taco) 24 hrs./day. Should I expect that with the new boiler this will be the case as well? Should I be looking to get a variable speed circulator?


8) Finally - any opinions on the Veissman Vitodens vs. the Weil McLain Ultra - price considerations aside?


Sorry for asking all these questions. I'm just trying to understand some of the options which are available.

Thanks for your opinions and help.

Mike T., Swampeast MO

3 & 4

3) I want to get rid of the iron supply/return pipes flying all over the basement. I'd like to have all the supply/return pipes relocated into the basement ceiling joist cavities. I would keep the steel risers to each rad, so the only place piping would be replaced would be in the basement. Would using Ipex 1" dia. Pex-AL-Pex pipe be an ok choice to repipe the supply/returns in the basement? or would you recommend "L" copper or iron pipe instead?

4) What diameter should the manifold supplying the cast iron rads be? The current manifolds are 2" dia. but each of the nipples leading to the radiator supply/returns is only 3/4", which in turn connects to 1.25" runs to the rads.

-------------------------------

If the system is original to the house (1915) it would have been a gravity system (no circulating pump).

The piping arrangement you describe (2" mains with ¾" taps that size UP to 1¼" branches to the radiators) DOES NOT SOUND ORIGINAL--to ANY type of system--at least that I've studied.

A gravity system generally has two pairs of mains that lead away from the boiler in different directions. If all three floors are about 800 square feet each it might have been done with a single pair of mains. Regardless, these mains start BIG and gradually reduce in size as they proceed around the basement.

It almost sounds to me as if the mains were ALREADY reduced in size--probably when the system was converted to forced flow with a circulating pump. And possibly because flow problems (particularly to the 3rd floor) were ALREADY encountered.

If this were a later forced system, I cannot understand any possible reason for increasing the size of a line to a radiator after its smaller connection to the main.

Please have your system examined by someone familiar with OLD hot water systems!!!!! If your description is accurate further reducing the size of the mains will be almost certain disaster!!!

Mike T., Swampeast MO

After reading over and over...

...is this something like what you have? (simplified of course)

Is this why pipes "are flying all over the place"? Because all of the radiator lines have been brought to a centralized location?

Why is your circulator running 24-7? Do you have TRVs?

Jason Horner

From what I can tell, the house was extensively renovated around 1955 (I found a newspaper in one wall we opened up from back then), so I'd imagine that the original heating system was modified around that time to the configuration it's in now (plus the boiler change back around 1978-79).

The house is just over 1100 ft. on each of 1st/2nd floors, balance on 3rd floor.

When the heat is on, the house is never cold (except the basement and in a few areas where we have not yet upgraded the insulation). So I'm guessing (amateur speculation) that the changes in pipe sizes between the manifold, its nipples, and the supply/returns to the rads is not causing any grief - at least as the system is currently configured. Its as efficient as it can be and certainly isn't pretty.

Due to the design of the house (very large stairwell between 1st/2nd floors, the 2nd floor is always hot when the 1st floor is comfortable, and the 3rd floor is bathing suit territory even in the dead of winter.

We have no zoning and no TRV's. The circulator runs 24/7 because it isn't wired to anything except 120v AC - only way to turn it off is to flip the main cutoff switch to the AC.

Your diagram captures the essence of my installation.

Oh, I just noticed that I can attach pictures. I'll take a couple shots with my digital camera and post them here later today.

JH

Jason Horner

Here are some pictures. All pipe sizes are approximate o.d.
Looks like I mis-spoke earlier... the distribution piping is larger than I remembered it to be, but I measured all the risers to each rad and they are mostly 1.25".

View1.jpg shows boiler from the side.
Boiler is a Raypack 250-T. The boiler inlet/outlet pipes are 1.625".

Taco circulator (Model 007-2F, 1/25 hp) and cold water feed are on return side of piping. Manifolds are silver painted, and manifold pipes are 1.875" o.d.

The expansion tank is the green beast wedged up between the ceiling joists.

The brown/black pipes are the distribution runs. These are rather large pipes, ranging from 1.875" to 2.375" diameter.


View2.jpg show boiler and manifolds from an angle.
Supply/return manifolds are clearly visible. Note that the manifold nipples vary between 1" and 1.25".


View 3.jpg shows a typical run to a rad. The risers to the rad are 1.25", though 2 of the larger rads on the main floor have 1.625" risers.

View5 and View6 show typical rads in the house. (View 4 not posted).


When the house was redone in the '50's, the rad pipes were hung below the plaster ceiling in the basement and in most places are just about 6 feet from the floor. I'm 6'-0" without shoes on, so needless to say when I'm in the basement with shoes on it is a challenge to not keep smacking my head on the pipes.

As to the sizes of the pipes, I can understand that given a manifold with only 4 nipples that the installer decided to use larger diameter distribution runs and then do a 'take-off' to each rad along the way. However this meant that some 1st floor rads were on the same 'zone' as 2nd floor rads, and on different compass exposures, and that means uneven and uncontrollable heat in different areas of the house.


Hope this gives you enough info to help answer my original questions.

Thanks again.

Jason Horner

I have 1" thick red oak floors on the main floor of the house so I'm reluctant to do staple-up unless the water temperature for that zone is 85F or less. Somebody pointed out this product to me -
http://www.ultra-fin.com/c_home2.htm

It looks interesting because the Pex tubing and the radiator fin are suspended in the joist cavity rather than stapled directly to the subfloor. I think that being suspended would help avoid 'hot spots' on the floor right over the tubing.

The supply/return for the rad in the main bathroom on the 2nd floor just happen to run under the tile floor of the bathroom. This makes the bathroom floor comfortable in the winter by accident rather than design. The cats, my daughter, and my wife all love walking on that floor in the winter. That's how I was first exposed to radiant floor heating. If I ever build a house from scratch, I'll have in-floor radiant heating everywhere.

Mike T., Swampeast MO

That's not a piping arrangement that I've seen in my limited experience. While I've seen thousands of iron radiators I haven't seen ones with those dual-tapped end sections.

My guess is that the two (piping & radiators) are linked--likely the mfgr of those radiators also published information on piping. Judging by your statement that the system is poorly balanced, either the original installer did not follow instructions well or the system did not work well in all situations.

Indiscriminate tinkering with the piping may well make a poor situation intolerable. I'd certainly find someone familiar with that type of system--hopefully it was common in your area and you won't have too much difficulty.

The constantly running circulator in such a system is unfamiliar to me as well. Older systems generally run the circulator only on a call for heat (ones with old, large water content boilers) or both the circulator and burner (newer, lower water content boilers). The logic of this is that once the radiators are hot they continue to liberate heat LONG after the circulator stops--keeping the circulator running without a "call" for heat would just waste electricity and allow more heat to be given off by the distribution piping.

I'm guessing here but the circulator may be running constantly to try to compensate for flow imbalances by "moving" heat throughout the system during the "off" portion of the cycle.

Perhaps someone will enlighten you (and the rest of us unfamiliar with this system) as to its theory of operation and even some references if we're lucky.

Mike T., Swampeast MO

IF

The radiation was layed out so that each branch of those engineered manifolds was supposed to receive equal flow...

...my understanding of flow/balance/pressure tells me that such is NOT the case here. The flow likely favors the final branch that has been attached directly to the end of the manifolds via an elbow.

You can make the pressure equal AT POINTS OF CONNECTION TO THE MANIFOLD, by replacing those end elbows with tees and piping back to the other side of the manifold creating a pressure-balance loop.

Pressure-balance loops are the "trick" used in irrigation systems to keep pressure equal at all outlets. I'm not certain why they are not used more commonly in other applications as well.

Jason Horner

In your messages you wrote:
"That's not a piping arrangement that I've seen in my limited experience. While I've seen thousands of iron radiators I haven't seen ones with those dual-tapped end sections."


In my neck-of-the woods this style of cast iron rad is the norm, ie. dual inlet/outlet at the bottom on one end section - I've seen them in hundreds of houses - whole tracts of the city were built with rads like these from the 1890's to the early 1950's.

Without ever having seen the inside of one of these rads, I'd guess the dual-tapped rad end sections are cast with a small dividing wall between the taps, thus forcing water flow forward and up through each section and forcing return water flow down the end section, all in a series of loops, sort of like the following crude illustration

<--------<---<---
| ^ ^ ^
V | | |
outlet <-- | | |
inlet --------->|-->|-->|


and you wrote:

"You can make the pressure equal AT POINTS OF CONNECTION TO THE MANIFOLD, by replacing those end elbows with tees and piping back to the other side of the manifold creating a pressure-balance loop."

That make a lot of sense. In any event I want to get rid of as much iron pipe as possible (including these manifolds) and have modern brass/copper manifolds with each room on its own zone and zone isolation valves and aquastats on each.

Getting rid of the iron pipe means copper or some variety of Pex as supply/returns.

If I wind up with a large diameter modern manifold and say 1" Pex as supply/returns to the rads (not much different than the current 1.25" o.d. iron), it seems to me that all I have to do is have a properly sized circulator fro the amount of water in the system and flow balancing valve/acquastat on each zone to tailor the BTU's to each rad to match the design heat loss. Is that correct?

Mike T., Swampeast MO

Please investigate...

...thermostatic radiator valves (TRVs).

"tailor the BTU's to each rad to match the design heat loss"

That is EXACTLY what TRVs do!!! They do it VERY well and can even compensate for poor initial system balance.

If you really investigate you will likely find that they are less expensive, more reliable and more comfortable than any other method you could use in your situation. CERTAINLY better than digital zone valves on every radiator!!!!!!

"...properly sized circulator for the amount of water in the system and flow balancing valve/acquastat on each zone"

Again, you ARE describing the action of TRVs--yet changing one AUTOMATICALLY changes ALL THE REST to compensate as they "communicate" through flowing water itself without wires connecting them.

Sorry for my ignorance regarding your radiators and system piping. It's just not something I've seen or read about.

Mike T., Swampeast MO

If you do use TRVs you might encounter a bit of a problem that most don't have. Generally the valve bodies of TRVs directly replace the old without any piping modifications.

While I can't say for certain, it appears such will not be be the case with your valves as the union tailpiece appears much longer than standard.

Tony_8

double tapped

ends probably have a dip tube type assembly attached inside which takes the water to the opposite end directly.
You can have staple-up under hardwood w/ WATER temps above 85F. Keep the FLOOR at or below 80F. You won't get striping from staple-up through 1" hardwood. The transfer in that application is achieved primarily through convection to and from the joist bay. Plates are a better way to keep water temps down.
Constant circulation most likely was used to balance the system. Can cause condensation in the boiler in low load weather unless a bypass is piped in. It does eliminate swings in room temp caused by excessive run times and resultant dissipation of built up heat in the water during off cycle.

Tony_8

also

DHW... won't run the entire time your kid is showering. The boiler (Ultra anyways) fires up to 100 % and heats the domestic then switches back to space heating. It's SOOO much faster than conventional that you won't notice it in the house temp. Speed also negates the need for large storage capacity like a conventional tank.

Jason Horner

You wrote:
"DHW... won't run the entire time your kid is showering. The boiler (Ultra anyways) fires up to 100 % and heats the domestic then switches back to space heating. It's SOOO much faster than conventional that you won't notice it in the house temp. Speed also negates the need for large storage capacity like a conventional tank."

Thanks for the reply. A couple more questions based on how we use DHW ....

We currently have a 50 gal. natural gas DHW tank. When my daughter takes her 40 minute showers, it is not unusual for us to also be doing laundry and running the dish washer. We've never run out of h/w in these circumstances. Our tank is a GE/Rheem and I have the temperature set at about 127F (I know it's on the hot side, but that's where we like it). Delta T is about 75F.

So under these circumstances, would you say that a small (20-30 gal. buffer tank is sufficient, or would you go so far as to say that the DHW should/could be done as an instantaneous on-demand application without having a buffer indirect tank? I'm just worried about the days when it's -20F to -40F outside.

Thanks

Tony_8

DHW

is not a tankless coil type for the Ultra. A matched tank that is piped as a zone on "priority" serves as DHW. W-M supplies tanks labeled "Gold" and "Ultra" w/ a number designating gallonage (storage). 40 is usually adequate. Your 50 gal gas probably is rated about 80-90 gals/ 1st hr. A 40 indirect will out-do that with ease.