Is this Buderus DHW tank piped backwards?

Bob Bona_4

Not that I see. Op is blowing his T and P on an alleged 140 setting. What is his tank temp, really? Or is it a weak XL100 or whatever. We're on the same page.

6006 could be way out of whack as well. Got to put it in the right place, make the best thermal contact and make sure the bulb is bottomed out in the well.

Jim R.

Success! So far.

Moved the 006 to the correct well and it's responding much nicer. DHW temp is about 5-10deg hotter than the aquastat setting, which is good enough for me.

I just bent a peice of metal to act like a spring/wedge and pushed it along with the bulb into the well. Couldn't find thermal compound so I used never seize-- not ideal but I needed to get it together.

I will see how it works over the next few days, we'll find out tomorrrow if the shower situation is any different.

Next step is to get a new mag rod for the tank. Probably get a new gasket for the top just in case. Of course their literature doesn't have a part#'s. I'm not going to reverse the supply/return pipes. With the faster pump speed the temperature difference on those isn't that much, I don't think it will make a big diffrence.

Fingers crossed, hopefully this thing keeps on ticking. Looks great inside and out!

Bob Bona_4

Great! You always have the option to upgrade to the Buderus 2107 control for more precise operation and features.

icesailor

The best part about this forum is that it makes you think. And remember forgotten things. After a night of sleeping under it, and another crash or two (I think it is being caused by "Cookies", I realize that Mark Eatherton hit the problem on the head. Mostly. Its a form of "Stacking" but not in the way thought.

It all gets back to my utter disdain for cold start boilers.

That tank is more than large enough to satisfy all demands for hot water. It is big enough to have good internal circulation. It shouldn't be running out of hot water. But it is. When the boiler has been off for a long period, and it gets cold, the heavily insulated tank should be completely hot through internal thermal circulation with all the heavy insulation around the tank. The top of the tank is the hottest. When you turn on the hot water tap, the tank has to empty the hot water and replace it with cold, until the cold gets to the top well. Then, the signal is given to start the circulator and burner. Which although the burner may start, the cold water is trying to rob all the hot water out of the storage tank to heat the boiler. Heat doesn't care where it goes, as long as it is going to the cold. Cold water in the tank. If the circulator comes on and you feel it, both supply and return may be almost instantly hot, because the boiler is robbing heat from the water heater. The boiler gets hot around the same time as a shower is done, and the hot boiler water continues to give off really hot heated water which stacks at the top. No new cold into the bottom.

Some of us used to see this with oil boilers and tankless heaters. We noticed that if you turned up a small heating zone, a few minutes before you needed really hot water for a shower, you got more hot water because you by-pass the thermal lag. Its the same reason you need buffer tanks on instantaneous gas water heaters with short draws.

You don't get this with warm start boilers.

If the OP has a small zone in the building, I'll bet the performance will improve if the zone is actuated before the DHW is and the boiler starts to get over 130 degrees.

Then, what is the size of the input into the boiler. If oil, what size nozzle? This can also happen to boilers that are "Just Right" for the perceived heating load of the building, but woefully too small for the DHW load.

ME pointed it out that it happens with high mass boilers. I've seen it far too many times. Converting oil boilers to warm start solved any issues so it becomes a non issue.

One more thing.

Any possibility of a bad check valve allowing gravity flow from the water heater to the boiler when the boiler is off? It seems to stop at the top boiler fitting. Regardless of the fact that it is piped backwards. The coils are all below the well.

Make that boiler run with a circulator. When the boiler gets running and hot, make the water heater circulator run. Take two long showers. See if it is better.

Oil fired boilers with tankless coils ALWAYS had the boiler control stuck into the middle of the coil to sense the colder water as quickly as possible.

Jim R.

Interesting. So, when the indirect calls for heat, there could be an initial dip in temperature due to the boiler circulating cold water until it's up to temp. This robs heat from the DHW. In essence, the tank is acting as a "heater" for the boiler during this time.

If you figure maybe 6 gals of water total (3 from the boiler and maybe the same amount in the 1" loop) -- that's roughly 10% of the tank volume. That could end up being negligble if the water is circulating quickly and the heat gain from the boiler is higher than the heat loss from the tank (which it is). Just my wild speculation.

But point taken. It would be better if the circulator didn't start until the boiler water was a certain temp. I know it's not wired that way.

Humor me on cold start for a minute:

When I filled my indirect the boiler was cold (switched off for several hours). As soon as I turned it on, DHW priority took over and immediately it started circulating. This means that very cold water in the coils got sent through a "cool" boiler (not ice cold but more like room temperature). I assume this was the correct thing to do? Alternative would be to start the circulation of cold coils into a hot boiler and the larger differential could cause a shock? Was I overthinking?

Also... way back when I was looking into the Logmatic 2107 Buderus boiler control... I vaguely remember it maintaining a 100deg temp or something like that, bascially becoming a warm-start boiler. I couldn't understand how that isn't hugely wasteful especially in the summer months. Tank heats maybe 1-2x a day in the summer (morning and dinner time), why do I want a hot boiler all the time? Winter months... still questionable if you are going to heat the boiler several times w/o circulating it.

As for the other questions... we're on gas. 1/2" supply to boiler, 74MBtu input, 61.4Mbtu output. Baseboard hot water heat, DHW priority. Single story ranch (1600sqft), one zone, some leaky windows. We'll be adding another baseboard zone to the walk-out basement soon (1600 sqft). Supposedly we have enough capacity.

I'm pretty sure the check value is working in the circulator. The pipes going to the tank are cool (comparatively) for heating calls and vice-versa.

icesailor

In my minority opinion your boiler is ideal for heating a house but sucks at heating hot water.

Its like buying a Ford F250 with a six cylinder engine, when you should have bought a Ford F250 HD with the bigger engine. The total GVW and tow weight is higher on the HD. If you are towing a horse trailer with a horse aboard. the F-250 can't pull itself and the trailer safely. Where the F250 HD can. Barely. A bigger motor really helps.

You want hot water, you need NUTS!!!

If your wife is complaining about lukewarm showers, tell her about all the money you are saving by having a under fired and underperforming boiler for your hot water needs. When you only need what your boiler is capable of to put out for a few days a year.

Some Heater and Heater "experts" are clueless as to how much energy it takes to heat hot water. No one is promoting Mod/Con boilers (low temperatures for "efficiency") on a steam boiler replacement.

Your biggest problem is that when the boiler has been off for a long time, and the HW tank is off on the control, internal circulation will finally mix the water to some equal level of hot. When you get into the shower and it starts to run, the cold has to fill the tank (from the bottom up) until it gets to the control at the top. Then, when the 006 closes, it starts the circulator pump and starts the burner. Add whatever time there is for pre-purge and the time the burner takes to light and get the boiler water hot. The whole time the boiler is running, and the circulator is running, it is transferring the heated water temperature out of the storage tank and in to the boiler. Where it helps heat the water, but if the input of the boiler is too low, you get a cold shower.

No one here has ever suggested sizing a boiler for a steam heated building by the heat loss. The smart guys all say to size the boiler to the EDR of the installed radiation. If you don't, and make the boiler too small, you don't get enough steam to do the job. Think of hot water like the EDR of a steam house. If you want DHW, size to the hot water load. Or else, people either learn to live with cold water in their new high performance systems, or they come and ask complaining questions about why they can't get the soap out of their hair. .

SWEI

The majority of gas-fired tank-type water heaters have a net output of less than 40k. There is exactly one mod/con boiler available in North America with a net output of less than 40k. Everything else will outperform the burner on that standard water heater. Size your indirect carefully.