Playing with different dandoss thermic elements

Considering a 140-100 temp in storage, you may get a better result by running your boiler pump constantly from the very beginning. The Danfoss will keep the boiler hot as long as the circulator is running. What is happening in your case as it is now, the circulator comes on at the set point and sends hot water out through the Danfoss. The hot water out heats the element and opens the flow from your storage and loads the boiler with cooler water which cools the entire boiler. The boiler cools and the circulator shuts down until the boiler comes back to set point. Up, down, up, down, up down, drastic cycling of the circulator and Danfoss element. Imagine how those 2 lines over time would look on a graph (1 for the element and 1 for the boiler temp).

I was not satisfied with the 140F element and am totally pleased with a 158F. I have set up the boiler circulator to be on, off, or aquastat controlled. At fire beginning the switch is set to 'on' for most of the burn and then set to aquastat @ 185-190F during the burndown. This is not a one lap storage loading but for the little cost in running a 15-58, I am ok with that. Previously in an attempt to get one lap storage, the result was similar to yours with the boiler circ and combustion blower cross cycling.

hartkem,
Did you change the pump launch temp. or did you leave it the same? Some times I wonder if having my pump launch at 170 is the right thing especially when the storage temp is low.

How often does your storage drop below "acceptable" levels? I'm wondering if your charging challenges are more of a symptom and not the problem.

I never had a problem with the 140 stat. Yes, the first burn of the year took a very long time but that was to be expected I guess. My suggestion would be to review your burn/load plan before you make too many more changes to your t-stat/pumping strategy.

hartkem said:

So the question is do I install the larger pump or lower the element to 140 without the holes or maybe a 150 stat if I can find one.

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Considering the pump, the 3-speed you have might be right for you. The Grundfos 15-58 through 40 feet of 1" copper will flow around 5 gpm on low and around 9-1/2 gpm on high. During the early stages I will run the pump on low and switch to high midway in the burn. My reason for swapping from 140F to 158F element is that the 140 was actually keeping the temp nearer to 130. That has been the experience here and I imagine each element may differ in accuracy.

stee6043 said:

How often does your storage drop below "acceptable" levels? I'm wondering if your charging challenges are more of a symptom and not the problem.

I never had a problem with the 140 stat. Yes, the first burn of the year took a very long time but that was to be expected I guess. My suggestion would be to review your burn/load plan before you make too many more changes to your t-stat/pumping strategy.

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What temp did you set your pump launch at Stee?

huffdawg said:

What temp did you set your pump launch at Stee?

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I'd almost always run at the max which is 160 on my controller. I'd only lower it on the first burn of the year and even then I think it's more perception than reality when it comes to the speed at which we charge cold tanks. The thermic valve shouldn't really be "wasting" heat. It's sending it back to the boiler and then back out again until the boiler is able to keep up with the loads (which takes a long time with cold tanks, no disputing that).

As for my comment on symptom vs problem - I also wonder why the OP would have 140 at the top of the tank and 100 at the bottom. Perhaps he's pumping too slowly and not getting water through his WAHX fast enough? Does he know the temp in/out on the HX? I guess tank temps isn't always a sure indicator of delta T on the HX though. I did some playing around back in the day on flow vs mixing vs stratification

Speed up the flow (maybe) on the load pump and see what happens? My two cents only, of course. If the OP is fighting a load that's slightly over sized for 500 gallons of storage it won't be curable with pumps or t-stats, however. Keep that gem from dropping to 100 on the bottom and you'll have solved your problem!

Just for kicks I found a pic from a first burn a few years back. This was after I had 3+ hours already in on the burn.

Cold tanks are pure pain...pure pain.

I think supplyhouse.com sells them (used to be pex supply?).

hartkem said:

Thanks for the good dialog guys. Just to clarify my system worked pretty well with the 140 element and 170 pump launch. I tried the 160 element in hopes of getting more out of my storage by letting it run down cooler before firing again. With the 140 element I would go fire up again when top of storage dropped to 150 to prevent the long delay before it really gets going again. I'm thinking a 150 element would be the best of both worlds. Anyone know where these elements can be purchased?

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Are you sure you are going the right way with the replacement sensor?

My ThermoBloc came with a 130F sensor, it has an 18° differential so 130+18= 148° before the bypass closed completely. So my boiler would hit the 170 limit while there was still a heat demand.

I replaced the 130 with a 115. so now 115 +18=133° when the bypass closes completely. so I get full flow going to my storage tank faster and i don't hit high limit before the bypass is closed.

The graphic below shows and explains proper operation of a boiler return protection block. A valve without the integral pump would work the same, same guts really.

I would guess the Danfoss works about the same, not sure what differential they use?

A properly designed boiler bypass valve, or pump block really should not need a valve to adjust or throttle that bypass, it's just "forcing" the valve to do something it wasn't properly built to do. The temperature sensor in the valve should do all the work just as it does in you car or truck cooling system

I had my Danfoss figured out back nine years ago when I installed it. Don't know if any technological changes have occurred since. My unit has an automotive (or similar) thermostat and my thoughts are that it is not an "off - on" mechanism but more a variable valve that fluctuates with varying return temps. On a cold start the thermostat bulb is activated by the bypass flow and when the flow approaches the temp value of the element it starts to open allowing the system water to flow thus mixing the system water with the bypass water. I can imagine the element fluctuating as temps change.

When the return water reaches the temp value of the thermostat it will remain open. This is where the throttle becomes important. If you don't equalize the bypass resistance with the system resistance you will get more flow through the bypass and not place the heat where it is useful such as storage or emitters. Boiler could reach it's high limit and go to idle sooner or other inefficiencies that you can imagine if the heat is not scavenged from the boiler.

It doesn't take much flow through the bypass to open the element since the Danfoss is designed in such a way that directs the bypass flow directly on to the sensing bulb so I believe my resistor "sweet spot" is more restrictive than my system with all it's elbows, heat exchangers etc.

Actually, Bob, I'm just an old senior citizen now, but back in the fifties and sixties when I was doing some grease monkey work I recall that there was a thermostat bypass on those cars. It was usually a short one inch hose that went from the pump to the thermostat bell that nobody paid attention to until it sprung a leak. Don't know what they do now.