boiler suggestions, buffer tanks

jrkeat

I'm a DIYer looking to install a condensing boiler. I have an engineering background, and I've concluded that there are few things I'd enjoy more, and getting the average contractor to get it right is hard. One wanted to run an indirect tank off of the hot water connection of a combi boiler, and another didn't understand that the system need a buffer tank.

Based on a quote from one contractor I've become very familiar with the Laars Mascot FT, and plan to use a Laars MFTHW-199 with a Techtanium TT-79. (I've read the Laars install manual several times, and have gotten a bunch of other information from their fabulous customer support, in part to convince a contractor that they were proposing the wrong things, so it's the easiest brand for me to use at this point.) This is massively oversized for my heating needs, but I'll soon have 5 adults and two children taking showers, and it's only modestly oversized for HW demands. I'm not a strong believer in buying the smallest boiler possible, though I realize there are many advocates here. I can't buy one small enough to avoid a buffer tank, and the extra cost is minimal. (My smallest loop--which runs all the time--emits 7000 BTUs. It will emit even less if I modulate the temperature down.) Only the 199 has a 10:1 turndown, so my minimum firing rate will actually go up if I stick with Laars and choose a smaller boiler.

Questions:
1) The Lochinvar Knight WHB boilers seem like roughly the same thing but for twice the price. I don't want any of the extra features except for the programmable ramp-up. (As an answer to question 3 below.) Is there some important difference in quality and reliability, or is it mostly different features that explains the price difference?

2) Is the HTP UFT basically identical, except for the lack of a pump and 10x turndown for almost all of their models?

3) My plan is to have a buffer tank attached as a secnodary loop via a circulator. This will allow me to switch it on only when the loads are less than the minimum firing rate for the boiler.

It seems like every buffer configuration that I've seen--including mine--will cause the boiler to operate at max power. In the classic case, where it acts as a hyraulic separator between the primary and secondary loops, you have a bunch of cold water just sitting in the primary loop. The primary loop will pump it out at high speed into the boiler, and the boiler will work as hard as possible to bring it up to its target temperature. Once the boiler shuts down, the buffer tank it does it's work, letting it's heat out into the small load and preventing short cycling. This is bad in two ways: a) if the cold water could be parceled out just quickly enough to get the boiler to fire at it's minimum rate, you could get away with a tank about half the size and b) the boiler will fire at its max rate, lowering its efficiency. I was thinking about using a Taco 007-VDTF5 to make this happen. (Unlike other delta T pumps that I'm aware of, it can operate in an inverse mode, where the pumping rate increases as delta T goes down.) It could switch the pump on as required to maintain a delta T of 5 degrees across the boiler. The Laars pumps at a fixed speed, with a delta T of 20-25 degrees at max power. (I know this because of their excellent customer support.) This doesn't quite get the job done, as this will target about 20% power, not the 10% minimum firing rate.

Has anyone ever solved this problem?

I'll may punt entirely, and just use an HTP SSU-20B as a hydraulic separator like normal people. I think I'd have to switch away from the Laars, which has a built-in primary circulator, so that the buffer tank won't be used while the HW loop is operating.

Ironman

Since you're an engineer, I'm sure you realize that you should size the boiler to the actual heat loss of the house and up-size the indirect to accommodate the domestic load.

Are you saying that five adults will be showering simultaneously?

I have six daughters, plus my wife and son, and myself and we got by with 50 gal. gas water heater. But we didn't try to shower simultaneously.

I've been doing this a good while, and even with a large family, I've never seen where a 40 gal. indirect and an 80k btu boiler wouldn't do the job - assuming normal 2.5 gpm shower heads.

hot_rod

What is the heat load on a design day? You are correct that the boiler needs to size to the largest load

If you need or want large dhw capacity, that could be the bigger load?

Plenty of good options for buffer logic. One of my favorite topics😉

GroundUp

I think you're thinking too much like an engineer and not enough like a guy who wants a proper system ;-)

What is the heat load of the entire system, minus DHW? The Mascot is a nice boiler, I've got quite a few of them in. As Ironman mentioned, there are very few applications where DHW load should be prioritized over space heating load and the 199 is almost certainly a mistake. While it does have 10:1 TDR, the lower it fires the more chance for issues. A smaller boiler working at 100% will be less problematic than a larger boiler running at 10%, An 80k boiler with a 40 gallon indirect will provide ~180 gallons of 115 degree domestic water in the first hour, and sustain a constant draw of ~100 GPH. Unless you're running 3 showers at the same time for all 7 people back to back, you'd be hard pressed to starve that combination. If you are running that much, simply upsizing the indirect eliminates the problem.

The WHN is nothing like the Mascot, but the UFT series from HTP uses the same heat exchanger and controls as well as the cabinet. Internal piping is completely different, burner and induction system is different, there are just as many differences as similarities.

If I were me, I'd figure out the actual heat load of the space and zone appropriately every zone that doesn't fall below 15k (if an 80k would serve the entire space) and add a small buffer tank for the zones that do fall below the threshold as another zone. Then draw each micro zone off the other side of that buffer tank.

If you can get us the heat load information, like loads of each zone, we can accurately assist you in designing a proper system.

hot_rod

Take a read through this journal. The 2 pipe method has some advantages over 4 pipe. An easier to source tank also.

https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf

hot_rod

Page 28 is where the formula for tank sizing with a modulating heat source

jrkeat

> Ground_up: A smaller boiler working at 100% will be less problematic than a larger boiler running at 10%

That's a really interesting! What bad things happen when you run a boiler at 10%? This is really the key bit of information. Otherwise, I don't see why I shouldn't stick with the 199. (Remember also that only the 199 Laars is 10:1, the rest are 5:1.)

> Ground_up: What is the heat load of the entire system, minus DHW?
My average gas consumption in the coldest month is 44k BTU/hr using a boiler rated at 80% efficiency. This will probably move up to the low 50's once our basement apartment is occupied. I did some heat loss calculations about a year ago, and don't quite remember the results--maybe 72k BTUs?

About 20% of all heating goes into a loop in the kitchen. I'm not up on radiant floor design, but the execution seems poor. I have to run the water at 140 to get the floor 8 degrees warmer than the room. At that temperature, it puts out about 7k BTUs.

The rest of the house are giant radiators, which will put out about 180k BTUs at 170 degrees.

There are also 3 radiant loops in the basement--20k, 20k and 10k BTUs I would guess, but I haven't measured them.

There are also some radiant loops connected to the bathrooms which we don't use at all. They release almost no heat.

Ironman, the 5 of us who live here currently routinely run out of water on Sunday night, and that's a 75 gal/75k BTU unit. Our oversized bathtubs may be the culprit. A 2.5GPM shower will use (65 degrees)*500*2.5/(.9 efficiency)=90k BTUs of gas or so. I do agree that we could design our water use around our hot water heater and never run out, but the extra capacity seems cheap and harmless. We will soon have at least two new adults, each with their own bathroom, living in the basement soon. At the very least, I'd like to get to the point where I can always run at least one shower head.

> hot_rod: Page 28 is where the formula for tank sizing with a modulating heat source

I haven't read anything but page 28, but it seems like formula 6-2 is kind of incorrect. This was the point I was bringing up in my point 3. It would be correct if there were some buffer tank configuration led to a boiler firing at its lowest rate. But it seems like they all cause the boiler to run at its highest rate. Perhaps if I read the rest of the document I'll find out how you can get the firing rate to be lower!

hot_rod

I think most mod cons fire at a high rate and quickly modulate down to the flame size required to maintain a save operating delta. I spent the week training along with Shane and Jason from NTI, I think some of their models fire at 30%, check flame and delta and modulate up or down to the load. The boilers fire at a rate close to the load on them these days if properly adjusted. Ramp delay and other features dial them in even more.

I also recall them indicting that running at lowest turndown for extended periods causes the burner cone to run hotter, shorter life expectancy.

But you still need an accurate heat load, you mentioned 44K on the coldest day, and a 72K load calc, quite a difference, which if either is accurate?

Also an accurate DHW load number, easily done with a water meter added to the coils feed of the water heater.

If you have tubs (dump loads) maybe larger storage could be the answer not larger BTU input?

If you want accurate answerers you need to tighten up and determine actual loads.

A radiant surface, if a consistent temperature, should give about 1.7- 2 BTU/square foot for every degree difference between surface and ambient. 80 floor surface with 68 ambient= 24 btu/ sq ft. Ceilings radiant are the lower number as you get some stratification at the ceiling.

7 btu might indicate 18- 24" spacing on your tube perhaps, wide spread of temperature from tube to tube. An infrared red camera or temperature gun could answer that question.

jrkeat

> But you still need an accurate heat load, you mentioned 44K on the coldest day, and a 72K load calc, quite a difference, which if either is accurate?

44K was the coldest MONTH of actual gas usage WTIHOUT heating the basement apartment, 72K was a coldest day heat loss calculation.

heathead

I don't know why or how, but is the 10-1 turn down not as efficient vs a 6-1 or 5-1 turn down. Is this because of excess air in the burning of the gas. I thought that 5-1 turn down or something was the best that could be configured. I may be off base, but am on this site to learn. Thanks

hot_rod

Then the boiler output for heating side needs to have a 72K output.

72K would not be adequate for DHW with a combi boiler, but with enough storage and indirect heat exchange you should be able to cover your expected DHW load.

You have the hydronic formula correct, now apply it to the projected DHW load and generate or store to that number.

Seems an 90- 100K mod con would be the right choice.

If you want to crunch tighter look up "hours of occurrence" data for your area. See what has been going on for the past 20 years or so Many areas are at or below design for a small % of the heating season.

Mid state NY for example.

GroundUp

As hot rod said, running at low fire for extended periods will run the cone hotter and reduce life expectancy. In many cases, it will also cause buildup in the heat exchanger due to excess fuel if the burner is not tuned perfectly. Remember the 199k at 10% fire is the same output as a 100k at 20% fire, the 80k will turn down further than the 199k despite the 5:1 ratio. If your total load is in fact 72k at design, an 80k would get you right there but personally I would go with the 100k for incidentals and quicker DHW recovery.

I have a very hard time believing any amount of radiators in a home with a 72k heat loss would put out anywhere near 180k at 170 degree water temps, perhaps that should be reassessed before a boiler choice is made. If you do actually have that much radiator, you may very well be able to reduce the water temps to 140 across the board and gather some condensing efficiency while still getting enough output from even a 100k boiler to adequately heat the space.

I'm having trouble understanding how the kitchen, with 7k output, can account for 20% of the heat used while the rads are using 180k.

As for the DHW tank, what temperature is it set at that you can run it cold? That may very well be as simple as turning up the tank temp to extend the usage as you'd be mixing in more cold with less of the stored hot to get the desired volume to the showerhead.

If the heat loss of the entire house is, in fact, 72k at design, the proper channel here would be to size a boiler to that load. If it were my job, I'd install a 100k Mascot with a 50 gallon Laars-Stor (with mixing valve) and a 30 gallon Lochinvar 4 pipe buffer tank. Pipe the indirect, buffer, and rads all as secondary loops off the boiler's primary. Then run all radiant off the other side of the buffer tank, mixed down if necessary. I will fight this unnecessary 199k tooth and nail but in the end, it's your house and your money.

Ironman

I would also recommend using an indirect tank over a Combi. You'll have less maintenance in the long run, particularly if you have hard water.

You mentioned large radiators which means the system has a lot of built in buffer. We do a lot of old homes with that scenario and we'll usually go up one size on the boiler in that case.

You know the formula, but it takes about 40k btus to heat 1 gpm at a 77* delta; so, a 100k btu boiler will continually produce 2.5 gpm for domestic. If you add 80-119 gallons of storage to that, I don't see how you'd ever run out of hot water.

As I said earlier, we do 80k mod/cons with 40 gal. indirects all the time for large families and they never run out of hot water. We also have done several frat houses with that setup and it works fine. I can't believe you'd have any heavier demand than six teenage girls wanting to shower one after the other.

jrkeat

I also asked Laars, which quickly responded that I shouldn't oversize, though they didn't give a good explanation of why.

Suppose I went with a 120. (I'm still having trouble making the mental shift--not long ago it was me trying to convince a contractor that a 199 wasn't too small!) Should I stick with Laars which has a 5:1 or switch to a UDP UFT, which will do 10:1?

> I have a very hard time believing any amount of radiators in a home with a 72k heat loss would put out anywhere near 180k at 170 degree water temps.

This is a home built in 1910, when it was still fashionable to leave your windows open at night to avoid "bad air". Additionally, all the appropriate upgrades have been made: many air gaps have been sealed, R 1 glass was replaced with R 5, the kitchen extension and basement have been reconstructed with modern insulation, and the attic has been thoroughly insulated.

Also my measurement 180k (flow x delta T) was after about 10 minutes of warm up--it could be a bit smaller after it gets really warm.

> I'm having trouble understanding how the kitchen, with 7k output, can account for 20% of the heat used while the rads are using 180k.

The 180K almost never runs. The newer 7k will run constantly in cold weather, and can't keep up below about 25 F outside temp.
When it gets warm, the 180k doesn't run at all. And you're probably right: the real number is probably 15%.

> As for the DHW tank, what temperature is it set at that you can run it cold? That may very well be as simple as turning up the tank temp to extend the usage as you'd be mixing in more cold with less of the stored hot to get the desired volume to the showerhead.

It's at 123, and is currently unmixed. A was hoping to not go much higher so that the boiler gets to condense.

> Pipe the indirect, buffer, and rads all as secondary loops off the boiler's primary. Then run all radiant off the other side of the buffer tank, mixed down if necessary.

So you have a circulator for a buffer loop, and then another circulator for the small heat loads that comes off of that?

In my current design, I have a $400 20 gallon electric water heater that is connected to the primary. A motor switches on when the little loops are activated.

> I would also recommend using an indirect tank over a Combi. You'll have less maintenance in the long run, particularly if you have hard water.
I wasn't thinking about using a Combi.

> you may very well be able to reduce the water temps to 140 across the board

That's what I was hoping. The radiators at 170 are bananas--they're just ridiculously powerful.

jrkeat

A correction (thanks for calling me out GroundUp): Using the Stelrad radiation estimator, I have about 550 sq ft equivalent radiation. So it's "only" 83k BTUs at 170 F. So whatever I measured was based mostly on the thermal mass of the system.

I got the system up to full heat for an hour an re-measured the other way. Radiator temperature is 152, and output is 57k BTUs. So the area is about 500 st ft measured this way. (I have a few radiator covers, so it makes sense.) The current boiler--which does not run the HW--can consume 240k BTUs and deliver 200k in heat. That means if I ran everything continuously, I couldn't hit 50% of its capacity!

GroundUp

How are you measuring the flow to come up with these output numbers? I've installed more UFT than I have Mascots and they're both good boilers in their own ways, but if your small zone is 7k even 10:1 isn't going to save it from short cycling so 5:1 isn't going to be an issue- a buffer is still the right answer there IMO.

The DHW tank slightly above the temp being used at the faucet is the reason you're running out. Running a 119 gallon tank at 125 degrees is only going to gain you 3-5% efficiency and risks Legionella growth, compared to a 40 or 50 gallon tank at 150-160 degrees with zero possibility of Legionella while saving an incredible amount of space... I know what I'd do but again that's my opinion.

Assuming the rads are all on a single zone, then yes the "primary" loop aka boiler loop would use the internal circ in the Laars and 1-1/4" diameter piping while each of the three loads; rads, buffer, and DHW all have their own secondary loop off that primary so it's essentially a 3 zone system. Then the other side of the 4 pipe buffer tank (assuming all 4 radiant zones can utilize the same water temp) would then get a single mixing valve and either 4 circs or a single circ and 4 zone valves. Any and/or all radiant can call for heat and draw from the tank, without the boiler needing to cycle until the tank temp drops to the setpoint and signals the boiler to reheat it. That's my vision anyway

hot_rod

You will get much better performance from a 2 pipe buffer arrangement in your, and most systems. Boiler output goes directly to the load, immediately without any interaction with the tank. As then load decreases the tank starts charging. Tank charges to set point you determine. When any load calls the tank supplies until the tank drops to the lowest useable temperature.

Pull the loads from the tank on outdoor reset or weather station data, to enable the best use of the stored energy.

It will be a balance act, you want to keep the boiler in condensing mode as much as possible, return below 130. But that will limit the buffer tank usability. Knowing the SWT at design and optimize the system to use them lowest SWT is a big help.

A tank charged and stratified to 180F and drawn down to 100 would be ideal, but mostly only practical in wood fired boilers


Tanks stratify best with the least amount of flow from top to bottom, or across them 2 pipe minimizes the tank "stirring"

If you spend the time and money to pipe in a buffer, use it to the best of it's ability in your system.

jrkeat

Here's a scale drawing, 2" per square, I made prior to this discussion. When I drew it, for some reason I thought I needed more space next to the boiler, so stuff is really compressed on the left side. If I actually build it, it will be more spread out, with the boiler moved to the right.
There are two separate thermostatic valves--one for the the basement radiant floors which work well, and a second for the others, including the critical kitchen loop, which need to be at a higher temp. With a lower HW target, I won't need the Taco 0014, but most of the other things will stay the same. Two Taco 007's and the electronic valves (square boxes in the diagram) will be taken from the current system.

Taco 007e's will be used on the two loops that run much of the time. As I mentioned in my original post, I'm considering putting a 007-VDTF5 rather than a 007e on the buffer tank loop so that I can regulate speed (using the temperature differential across the boiler) so the boiler will always be at a 20% firing rate, rather than 100%, when it comes into play.

jrkeat

Hot_rod, I'm confused about how to implement a stratified tank. Imagine I'm trying to implement Figure 3.5 from the idronics document you sent, where the heat source is the Laars and the "heat source circulator" is its internal pump.

When the flame is on, everything is good--the thermocline gradually moves down. When the flame is off, we have a problem: the "heat source circulator" keeps running. (Otherwise the heat source would not know the temperature of the system.) That means it's taking cold water from the bottom of the tank and putting it in the top to mix with the hot water and to flow into the load.

Can I construct this system with Laars? I could imagine switching the heat source pump off and using a temperature sensor in the tank, but it gets more complicated with all the other stuff the system has to do.

jrkeat

Ground_up, you recommend a LS-SW-2-50-L tank? Any reason to close that over the Techtanium TT, which is cheaper?

hot_rod

try and get to two SWT if possible, add more plates to the high load radiant areas perhaps?

Here is a simple 2 temperature, a 3rd could be added.
1" is fine from the boiler for 100K or less. Increase at the tank and build a 1-1/4 or 1-1/2 short header/ separator.

DHW could be from combi direct, or add an electric tank for some dump capacity with a recirc pump between tank and combi. a lot of the commercial tankless are now attached to storage tanks, go figure

I used a manual set 3 way valve on my latest, it floats with ODR temperature of the boiler high temperature 140F?

If you use that AO Smith tank, remove side nipples, get a 3/4- 1-1/4 or 3/4- 1-1/2 copper to male adapter to increase the port sizes. Or a specific hydronic buffer with large side ports.

Let the boiler run to whatever output is required for the tank recovery and design load. If it has ramp delay that too helps eliminate short cycles.

I think you can run low turndown, just not 24/7 or the boiler is oversized.

The boiler is most efficient when oversized at low turndown as you have a large HX surface, small flame, lots of condensing potential. But there is some concern of burner cone overheat, not sure it has been a huge issue?

jrkeat

Thanks for the diagram, hot rod! I don't see how it resolves the mixing problem. When the boiler flame is off but we still are heating one of the zones, won't the primary pump keep running? Won't that re-mix the buffer every couple of minutes, destroying the stratification we would like to get from the cold water returning from the zone?

GroundUp

The Laars-Stor is something I've used and am comfortable with, as I know they make a quality product and stand behind it. The TT is a no-name with no support, like a Westinghouse but worse. For 5% more money, I'd rather have something of quality

hot_rod

jrkeat said:

Thanks for the diagram, hot rod! I don't see how it resolves the mixing problem. When the boiler flame is off but we still are heating one of the zones, won't the primary pump keep running? Won't that re-mix the buffer every couple of minutes, destroying the stratification we would like to get from the cold water returning from the zone?

The circulator in the boiler? It should only run on a call for heat in a zone or the tank. Once all the zone calls stop, the boiler will charge the tank, at this point it will stratify, stacking hotter at top. assuming the sensor is located properly I've seen tall tanks stack 20° hotter up top.

Then on the next heat call, of whatever zone or combinations, the tank supplies the energy only until it runs out of exergy.
Certainly when any circs are running there will be some movement in the tank, with the exception of boiler running directly to the loads, that may happen on design days. The tank has no interaction, doesn't need to on a design call if the boiler output matches the load accurately. Constant circulation, the holy grail of hydronics

There are special stratification tanks I have seen in Europe, numerous ports up the side and different temperature loads pull from that thermocline to reduce "blending"

My buffer drawing I don't really consider primary secondary. The boiler is piped with its own circ as a parallel input, the loads also.

The wide piping at the tank serves as the hydraulic separator. Mainly to separate pumps hydraulically speaking so any combination can run without bothering one another.

Really no reason to primary secondary pipe it, very little pressure drop in the piping from boiler to tank, the boiler circulator should be able to handle that additional few feet of head.
The boiler will be most efficient with the coldest return. When you primary secondary pipe you may "blend up" the return to the boiler reducing efficiency. Generslkly there is always some mixing in closely spaced tees, unless, rarely both flow rates are exact.

A reverse indirect as a buffer could be a nice upgrade, it would pre-heat the supply to the DHW side of the combi, possibly doubling your available gpm when the system is in heating mode.

Summer months it could bring incoming water up to room temperature to help the combi DHW performance.

Not a lot of strain or stress on a buffer tank, low pressure, reasonable temperature, dead water. The boiler buddies are thin gauge, plain steel air tanks., Basically a 75psi rated tanks, they seem to be holding up just fine. Same with expansion tanks, sheet metal gauge metals! In an O2 free system, properly sized, exp should last 20 years, not unlike panel rads.

Water heaters are glass lined, 300 psi tested tanks, should last 50 years in a low pressure, sealed hydronic system. Leave the anode rod in for additional electrolysis protection.

Lots of ways to skin this cat.

jrkeat

It's clear we're thinking of different things, and I can't quite tell what your thinking. I'll try to carefully explain what I think you've drawn, and hopefully you can explain where I've gone wrong.

The big box is a Laars Mascot. When there is a call from the little zone, the TT line is activated. This turns on the internal pump until the TT line is deactivated. This pumps water out through a primary-like loop which has the tank at the other end. (You've drawn a box over the outgoing hot water line-- I'm not sure what that is.) The burner turns on when the temperature at its inlet is about 20 degrees lower than its target, and switches off when it can't run without exceeding its target--maybe target-4 degrees.

Lets say the zone pumps 1 gpm at 20 degrees. This is also always running during the zone call.

While the burner is running, things are great. The zone is getting heat and any excess (9 gpm) flows into the buffer. At some point the buffer gets full and hot water flows out the bottom, switching off the burner.

If we switched off the internal pump at this point, the running zone would strategy the tank nicely. This would be nice, because the loop would only pull in the hottest water.

But the internal pump continues to run at 10 gpm. 1 of that goes through the little heating loop, but 9 comes from water getting sucked out of the bottom of the tank and put in the top of the tank, mixing the cold with the hot.

So the tank never gets stratified. The loop, rather than running at an almost constant temperature, will fall 20 degrees.

Did I get something wrong about how the Laars will operate? Are you thinking of a different setup--maybe you've put a temperature sensor in the buffer tank and are treating that as the zone, doing your own ODR calculation, and the Laars knows nothing about the zone calls?

jrkeat

Thank you, this has made me think much more clearly about stratification. I'll change my system so that water gets pushed in through the hot water port at the very least. That will keep the boiler intake temperature lower while it is firing.

GroundUp

The boiler circ stops when the heat call stops. The TT signal both fires the burner and runs the boiler circ. When the load is satisfied, the TT switch opens, turning off the burner, followed by the circ. The Mascot has a programmable post-purge for the onboard circ to clear the rest of the heat from the boiler, I think factory setting is only 1 minute

jrkeat

OK, so that agrees with my analysis. Maybe hot-rod has his configured differently. (Or I made a mistake somewhere else.)

hot_rod

jrkeat said:

OK, so that agrees with my analysis. Maybe hot-rod has his configured differently. (Or I made a mistake somewhere else.)

Pretty much all boilers run as Ground mentioned, when the call for heat stops so does the circulator.

You could take control of that circulator externally, and run its anyway you want. Just make sure when the burner is on your have flow or the boiler will go to high temperature lock out.

I prefer a delta T control myself. It reads two temperature points and has a wide range of adjustment.

Most of the schematics in the Ironic 17 show a delta T control in the wiring drawing that accompanies the piping schematics.

jrkeat

Here's a proposal to make your diagram work without de-stratifying once the burner turns off: let the motor run normally when the heat is on. When the burner is off, turn it on for a few seconds each minute or two so that the boiler can an updated water temperature without doing enough circulating to mix the buffer.

You can also add a single external motor and do the same thing with the low-load circuits.

I'm not sure if circulators are designed for that much cycling.

Brewbeer

I have a 4 pipe buffer, the tank doesn't stratify because the boiler pump moves more water than the system pump.

GroundUp

I don't think you're grasping what hot rod is saying. The boiler doesn't read tank temp, it does not circulate unless the sensor in the tank calls for heat- which also fires the burner. The tank will stratify better that way, which is what you want with a 2 pipe buffer.

jrkeat

You could do what you're suggesting--add a thermostat to the tank. You'd have to add your own ODR sensor and circuitry, though, if you wanted to be able to use ODR in your system. And you probably have to add a second sensor outside. Then you have to set that second ODR so that is always cooler than the one in the boiler. My suggestion--making the boiler pump run REALLY slowly when the burner is off--was to try to avoid that duplication.

Are there any boilers that will integrate an extra sensor so that you don't have to replicate the ODR? I see the Knight has an extra sensor called a "System Sensor", but that seemed to be for coordinating multiple boilers.

hot_rod

Not sure how complicated you want to make the control logic for what % of gain? Very easy to use proven piping and control logic with onboard controls or other off the shelf controls.

Often the controls become more complicated to figure out, compared to piping.

I alway suggest you think about your control logic before you start any piping.

Also a list of goals and priorities for the system.

Buffer tanks can:

Reduce cycling
Moderate over-sized boiler(s)
Allow for long burner off cycles
Store energy from multiple sources
Provide a DHW function
Provide hydro separation
Leverage off peak rates
Add electrical element for dual fuel function

jrkeat

> Not sure how complicated you want to make the control logic for what % of gain?

Yes, I'm inclined to stick with my current design, and make your adjustments so that it stratifies while the boiler is firing. The gains from getting the stratification of the buffer during discharge right are small.

I wish I could fix my stupid kitchen floor. I think the PEX is sandwiched between plywood and the tile kitchen floor, so I'd have to rip up the whole kitchen.

hot_rod

Stratification will happen when the boiler is firing and the boiler pump running. The hotter water will be fed into the top and the bottom sensor will turn everything off when it reaches set point.

What will bust up the strat is running the boiler pump continuously and blending the tank, after the burner is off.

If no heat is being added or removed, no circulator should be running.

If the boiler circ stops, post purges, you should be fine.

I put the sys supply sensor in the tank, let the boiler control run the buffer, if your boiler has that sensor option.

Some brands have supply and return sensors to watch delta t across the boiler, those need to remain in place.

I've seen others remove the boiler operating sensor and put it in the bottom of the buffer, but you need to leave any safeties or high limits in the boiler itself.

jrkeat

> Some brands have supply and return sensors to watch delta t across the boiler, those need to remain in place.

The Laars doesn't. The Lochinvars, which do, are tempting. However, the radiant part of my system just isn't that important, so I'll certainly never make up the cost.

>I've seen others remove the boiler operating sensor and put it in the bottom of the buffer, but you need to leave any safeties or high limits in the boiler itself.

That's really interesting! When you say "boiler operating sensor" are you talking about the CH return sensor?

hot_rod

Yeah I am most familiar with Lochinvar and the various models I have do allow optional sensors. It varies from brand to brand and model to model. The more expensive models usually have more control options for customizing.

The manual should identify which sensor is the control one. if it is removable it could be moved to the tank. All boilers have an operating sensor. Check with a rep or factory tech support.

I know the boys at Rathe Associates on L.I. know that boiler well.

Some sensors slip into a dry well, others thread right into the boiler block.

hot_rod

They talk about control options at 5:22
https://www.laars.com/videos/mascot-ll-high-efficiency-boiler-and-water-heater-combi-boiler

jrkeat

Questions:
1) If I wanted to create control logic for my buffer circuit motor, is there a standard thing that plumbers do? Let's say I wanted the motor to run when either of the two radiant floor motors are running, but not when the the motor for the big radiator load is running. This could be accomplished with 3 relays like the RIBU1C which I could mount on a junction box and wire them together with wire nuts inside the box. Would that be the normal thing to do, or is there something better?

2) Roughly what would I expect the return temperature be for the DHW loop? Tank target temperature minus the delta T of that loop? Or does it have to enter the tank at a temperature that's significantly higher than the tank target temperature?

hot_rod

Sometimes easiest to draw the piping and below the wiring schematic

Most switching relays allow a selection of relay interface