Undersized buffer tank

[flynmoose]

I have a Synergy3 Waterfurnace that is heating a buffer tank used to run a three-zone in-floor heating system (full system description here )

Here is a crudely drawn pic of the system:



(Hi-res here: here)


My concern is how often the compressor cycles due to radiant hot water demand from the buffer tank. I need to do some better timing over an hour - but I think that in the morning when I'm bringing the slab temps up, the compressor is cycling every 10 minutes with a 60-70% duty cycle. That is a lot of compressor starts.

Is this high? If so - could I add something to either:

• Put more hysteresis in the buffer tank thermostat (I had a Johnson Controls A419 running a water circ system in my last house where you could set on and off temps plus short cycle lockout)
• a simple short cycle circuit to lower the CPH on the hot water demand
• Put a second buffer tank in and have them in series (I was thinking about this anyway so I could add a DHW feedwater preheat)
• something else?

I hear the compressor start cycling at 4:30 in the morning and with the light flicker, notice it all morning long until I leave the house. Even if it isn't hurting the compressor, the cycle rate/noise is annoying.

Thoughts?

[palacegeo]

How big is the tank you have now?

[GeoXNE]

flynmoose

Generally the buffer tank should be 10 gal per ton. First try increasing the differential on the buffer tank if possible. Aquastats that are designed for DHW have a tight differential and may not be adjustable. A wider differential will help prevent short cycles.

If you do have to spend money consider this-
http://www.tekmarcontrols.com/literatur ... t/d256.pdf
The advantages are-
Auto differential or Manual adjustable
Outdoor reset
Warm Weather Shut Down
and others

[flynmoose]

The tank is a Hotpoint 40 gal electric heater and my Synergy is 6 tons so there is mismatch #1.

The controller is a "Therm-o-disc" 4000 - which appears to be a mechanical bi-metal thermocouple with a spring set point. So I won't be changing the differential on that device . . . The temp is set to 110 (which is as low as it goes)

Found that Tekmar D254 online for $155 - cheaper than a new tank and would probably want a different controller if I got a new tank anyway . . . will think about it.

It is probably cycling more than last year because the water temp was set higher when we moved in. When the installer came out for a "checkup" several months ago - he said someone had set up up and it should really be at 110. Higher set point would have given longer run times and fewer cycles. Or maybe I'm just more tuned in to hearing everything after he was here.

[teetech]

You should have at least 80 gal (well insulated) tank with a differential of 5 - 10 degrees. Outdoor reset is a good idea with slab heat. Water temp should be as low as possible for highest efficiency.

I would not recommend two tanks.

[flynmoose]

teetech,

Just for my education - why not a second tank (instead of removing the existing)?

Poor flow, cost,heat loss?

I guess if I'm buying a tank anyway - but a 40 gal will be substantially less than an 80 gal - no?

[Mark Custis]

I will skip the size debate and offer a control strategy, based on what I see in the drawing.

The plan I assume is to have the HP feed the buffering tank and then the tank feeds the floor. My question is how do you set the entering water temperatures for the floor loops? I would add a three way mixing valve to the tank to floor piping before the circulator. Now you can control the floors and allow the tank to go to a higher temperature with whatever control you chose and increase the differential to lengthen toe run cycles.

How is the tank stratified? Is there a coil in the tank?

[flynmoose]

Mark,

Regarding the tank - I don't know. Here is the data plate:

(Hires is here)

The tank is "fed" from the heat pump, on the side - with some very large diameter PEX. I believe the "hot" supply goes in the upper port on the side and the "cold" return comes from the bottom.

The zone manifold is fed from the top of the tank with what I assume is a traditional arrangement (hot drawn from the top and the cool return goes down the dip tube). But I don't know if there is dip tube installed.

Your control idea is interesting - what is the ideal temp for generating hot water from the Synergy heat pump? I thought I had read elsewhere on this site that you start getting pretty inefficient if you go too hot.

As a reminder - I do have access to the installer and could go back to him. But he installed the system and then came out and criticized some of its components. If anyone knows a good Geo person in the Oshkosh WI area - I'd be happy to get them in the mix rather than doing all this amateur sleuthing.

At this point anyway - I seem to be getting adequate heat in the floor. Though I indicated a desire to go to a DHW cold water tempering system at some point, I think that $150 on a controller may be the cheapest and quickest way to lower the compressor cycles.

Which is the other "first" question that is still out there? How often is "too often" from a compressor cycling perspective? It is possible I'm seeing 60-70 cycles a day on the heat pump - which seems like a lot to me - but maybe they are designed for that?

Thanks for all the good info.

[Mark Custis]

because I have it bookmarked. They show about 6* delta T per pass up to about 130* F. My point is the floors need nothing that warm to work. I would try and keep the tank as warm as possible then mix down for the floor temps. Short cycling will cause excessive wear and tear on the compressor and drive you crazy with dimming lights. So I would add a mixer and the adjustable control.

[GeoXNE]

I really have to disagree with raising the buffer set point and mixing back down to radiant temps. Raising the load ewt 10*F will lower COP by about .5.

The whole reason why w-w geo and radiant make sense is they are running around the same temp range. With outdoor reset controlling the buffer tank you never heat the buffer tank any more than required further realizing increased COPs on less than design days.