Expansion tank question

1daveman

Kaos,That does seem pretty simple- as you mention, as long as the pump has enough umph. I have an Alpha 2 now that is giving me acceptable flow - does the mixing valve increase the head much?

I will review both designs with my plumber. I mentioned earlier I am in a remote area where plumbers are few and far between. The one I am using is pretty young - but I don't think he would have an issue doing either of these designs.

Really appreciate the help.

hot_rod

one of the goals with solar is to always use that first and use it to the lowest possible temperature.

You don’t want the tankless to ever heat the tank if possible, of if you do, never above the minimum temperature the floor needs. Call it 110.

That way the tank is always at the coldest possible temperature to accept the solar input.

So with a two pipe buffer piping you have hydraulic separation. When the tankless runs and radiant is calling you have direct to load, no energy goes into the tank.

When the load is satisfied the tankless will only warm the tank to say 110. That keeps the tankless efficiency up, and leaves a nice load in the tank for the solar to cover. Some days the solar may meet the load entirely? The tankless now has a 70 gallon buffer to work with and lessen on off cycling. The tankless is now a tank heater

IMG_9320.jpeg

IMG_9321.jpeg

IMG_9322.jpeg

So one of 3 conditions is always in play.
1) there is no energy, hot water,in the buffer, tankless feeds directly to the floor, no need to engage the tank.

2) heat call has stopped, sunny day house is warm. The tankless will take the tank to only 115, tankless pump shuts off. solar will continue to contribute to 160- 180. Or if needed the tankless can kick back on, if a heat call comes in and the solar cannot keep up.

3) evening rolls around, temperature drops, heat call comes on. Tank was heated all day by the sun to 160. Flow is direct from tank to the radiant, it doesn’t flow through the tankless, which would pull heat out of the storage.

Once the tank is depleted, condition 1 kicks back in by the setpoint control.

The mix valves lessens over- temperature in the slab, the flywheel effect.

. How many kw is the element? Kw x 3.41 gives you btu/ hr of the element

Any idea on the house load?

Remember you only have 70 gallons of stored energy, it may not cover many hours of load.

Currently I think the tankless fires whenever flow goes through it up to 150?

Kaos

@1daveman The mixing valve does increase the head a bit but you can get one with a high CV to reduce that. Something with a CV of around 5 should be good enough for about 3GPM.

@hot_rod If you look at my diagram, all the controls you mention happen on their own.

Lets say floor heat needs 110F water. Set mixing valve to say 120F and tankless to 110F.

If there is sun and tank is being heated, the outlet of the mix valve will be above the tankless trigger temp and it will never run.

Once the tank gets down to about 110F, the tankless will start to fire but only enough to raise the temp a couple of degrees to meet the setpoint. At that point you are still using the bit of residual heat in tank.

Eventually the tank settles to the return water temp of the floor at which point the tankless is providing all the heat.

In the morning, the tank is as cold as it can get, so it is ready for a recharge from PV.

1daveman

Thanks @hot_rod

I have been studying your drawings. I think I understand it - it seems pretty elegant. I am going to try to repeat it in my words to make sure I understand it. Please let me know if I have this correct.

1. The tankless pump will turn on when the tank temp is say below 115 - and heat the tank to 115. When the tank is at 115, that pump turns off.
2. If solar is available, solar will continue to heat the tank to whatever is max.

This happens separately from what happens to the floor.

1. when the floor pump turns on, it will take the water from the mixing valve, that will bring in hot from the tank water. However, if the tank water is less than 115, the tankless pump will be running so the hot water will come from the tankless. The cold input for the mixing valve comes from the floor return?

Advantages -

1. Not feeding hot water into the tankless, causing it to cool down

2. When the tankless is feeding the floor directly, it is not heating the water in the tank, which is good because we want solar to heat the tank.

Did I get this correct?

Dave

1daveman

@Kaos and @hot_rod

Thanks to both of you. You have given me a lot to think about and I will review those two concepts with my plumber. This has been a great learning experience and I sincerely appreciate it. I am excited to get my expansion tank set up correctly, and the mixing valve in place!

hot_rod

doesn’t it always come down to how much money and technology you want to spend to harvest free energy😎

1daveman

@hot_rod

Ha that is really funny. You got that in my case!

1daveman

Thanks hot_rod

I have been studying your drawings. I think I understand it - it seems pretty elegant. I am going to try to repeat it in my words to make sure I understand it. Please let me know if I have this correct.

1. The tankless pump will turn on when the tank temp is say below 115 - and heat the tank to 115. When the tank is at 115, that pump turns off.
2. If solar is available, solar will continue to heat the tank to whatever is max.

This happens separately from what happens to the floor.

1. when the floor pump turns on, it will take the water from the mixing valve, that will bring in hot from the tank water. However, if the tank water is less than 115, the tankless pump will be running so the hot water will come from the tankless. The cold input for the mixing valve comes from the floor return?

Advantages -

1. Not feeding hot water into the tankless, causing it to cool down

2. When the tankless is feeding the floor directly, it is not heating the water in the tank, which is good because we want solar to heat the tank.

Did I get this correct?

Dave

hot_rod

https://forum.heatinghelp.com/discussion/comment/1830984#Comment_1830984

there are a number of ways to make this work. What he has currently seems to work except for the expansion pressures.

I was trying to check as many boxes as possible with a repipe, maximize efficiency of both components, maximize storage, seamless switch over, hydraulic separation.

He didn’t get what he wanted from the get go😙

Im not a fan of thankless wh for radiant. They are built for the exact opposite condition. Low flow high delta, high pressure to overcome the small passages, no listings, filters plug constantly, etc, etc

When you upsize mix valves you need to keep an eye on required minimum flows that the valve needs. The Caleffi hi-flow thermostatics need 4.4 minimum flows to mix properly.
I find a motorized mix valve is a better, more accurate option for higher flow lower pressure drop, and ODR option.

IMG_9323.png

1daveman

@hot_rod

I know the person advising me on the initial design considered a boiler, but I do not remember why we ended up going with the tankless. I am starting to understand the limitations.

The mixing valve min flow rate is a problem - so I am off to the Caleffi site to study motorized mixing valves.

I can't afford to lose much of my flow rate, and would prefer to not have to get another circulator, but that might be preferable over the complexity of a motorized mixing valve…

Thanks again