tankless options

[hydroflow]

I am attempting to install a tankless system downstream from a geothermal heat pump. Water output temp on the geothermal unit is anywhere from ground temp water to 130 degrees (depending on the heat pump activity). The problem: all the tankless that I have investigated need a delta T of at least 20 degrees or damage to the tankless could occur. The only "authorized" solution came from Rinnai who says to install a Honeywell AM1 mixingvalve and introduce cold water to lower the input temp to 100 degrees. Does anyone know of a tankless unit that does not require the delta T requirement?

[Dana]

I am attempting to install a tankless system downstream from a geothermal heat pump. Water output temp on the geothermal unit is anywhere from ground temp water to 130 degrees (depending on the heat pump activity). The problem: all the tankless that I have investigated need a delta T of at least 20 degrees or damage to the tankless could occur. The only "authorized" solution came from Rinnai who says to install a Honeywell AM1 mixingvalve and introduce cold water to lower the input temp to 100 degrees. Does anyone know of a tankless unit that does not require the delta T requirement?

Takagis of an earlier generation merely inhibit the burner from igniting when the incoming water is over ~130-135F (not sure about current generation stuff.) I suspect they'll be fine in all instances from a damage point of view, but will short-cycle and have difficulty regulating the output temp at low delta-Ts.

Programming the tankless to output ~150F and putting a tempering valve on the tankless output should be able to protect any units that can actually be damaged with low delta-Ts on the heat exchangers. When the incoming temps are high the hot water flow through the heater will often be lower than the min required for ignition (typically 0.5-1gpm) for most draws, and higher flow draws can take the heat of the burner activity. But this will also lead to short-cycling & temperature fluctuation issues at low flow.

The Rinnai-authorized solution will work better at low flows and have better output temp regulation overall. What don't you like about it? And what sort of "damage" are they talking about? (If it's wear & tear from short-cycles, there are no low-delta-T solutions that will avoid that.)

Another solution (and perhaps the right one) is to install a small buffer tank (8-10 gallons- a small well-insulated electric HW tank would do) plumbed as flow-through from the geothermal, but with a circulation pump on the tankless using the tank's aquastat to turn on the pump on the tankless loop. Program the output of the tankless to at least 20F above the setpoint, and with sufficient pump flow that when the tank temp is 15F below the setpoint the tankless modulates up to at least 50% of full output. That way, the tankless just maintains the temp of the tank with big delta-Ts under all conditions, and can be set up to NEVER short-cycle (since the tankless output temp, pump flow, hysteresis of of the aquastat, and the volume of the tank determine a minimum burn length- this may be tweaked using multi-speed pumps and playing with the output temp of the tankless.)

When the flow from the geothermal is over the setpoint the tank the tankless never fires- it's slaved only to the setpoint of the tank, not the hot-water-use flows. This fixes forever the cold-water-sandwich and low-flow flameout issues as well. Even though some tankless manufacturers void the warranty on tank-temp-maintenance applications, properly set up it actually SAVES the tankless and increases the operating efficiency (for a very small penalty in tank standby loss) by reducting the total number of ignition/burn/flue-purge cycles. This also increases the peak flow potential, since the hot water runs through the big-bore low-head plumbing of the tank, not the high-head of a tankless heat exchanger.

Let the pump handle the head of the heat exchanger. Setting the tankless outputs for a high delta-T the pump flow doesn't have to be all that high either- 4gpm @ 50F rise is ~100KBTU/h. If you set the tank to 125F and the output of the tankless to 175F and the pump flow to 4gpm a tank temp recovery burn is a healthy~100KBTU/hour out by the time the tank has chilled to 110F from high-rate hot water flows the output of the tankless is roaring along at 130KBTU/h out. (Most 199KBTU tankless outputs max out at ~155-158KBTU at full fire.) You can tweak the flows and output temps to find what works, but try to set it up with the recovery/maintenance burns long as possible to maximize low-flow/low-use efficiency. Lower fire maintenance burns will increase system efficiency, but will lower the effective maximum output of the tankless- there's a balance to be struck as a matter of practicality. But unless you have multiple showers going on simultaneously, the buffering of the tank means you generally don't NEED max output- the mass of the tank masks the shorter draws that occur during the longer draws from the tankless.

With a 10 gallon tank, 100KBTU/h recovery burn and a 10F hysteresis on the aquastat you're looking at about a 30 second minimum burn- perhaps longer than a hand-washing burn on a normal tankless setup but still less than ideal. A minute or more would mean the tankless always ran near it's raw combustion efficiency. There would still be far fewer burns though- the modulation during the minimum burn would alway be much higher than a low-flow hand-washing draw, and many short draws would occur between burns. This is really no different than a boiler & indirect-tank except that the "boiler" in this instance is a tankless, and instead of a heat exchanger and a boiler-water loop, the tankless is heating potable water directly.

[Scott D. Plumber]

My house is set up with Rinnai and Geo. I used the Mixing valve and there are two ways to run it. !. Mixing valve turned up and Rinnai’s off during the summer. Free hot water. 2. Mixing valve turned down and feed it 90* water (My MV goes that low) This helps the Rinnai’s achieve full capacity.

You don’t want to run a close delta T because the Tankless (any of them) WILL condense inside the unit and ruin it! Especially on a long draw. Also, it may require a higher flow rate to fire if feeding it too hot incoming water. These things are designed to take cold water and make it hot. They do not care for high temp incoming water. This is also why an aquastat is advised on recirc loops and why the Metlund De-mand system is a preferred recirc for residential and some commercial applications.

[Dana]

My house is set up with Rinnai and Geo. I used the Mixing valve and there are two ways to run it. !. Mixing valve turned up and Rinnai’s off during the summer. Free hot water. 2. Mixing valve turned down and feed it 90* water (My MV goes that low) This helps the Rinnai’s achieve full capacity.

You don’t want to run a close delta T because the Tankless (any of them) WILL condense inside the unit and ruin it! Especially on a long draw. Also, it may require a higher flow rate to fire if feeding it too hot incoming water. These things are designed to take cold water and make it hot. They do not care for high temp incoming water. This is also why an aquastat is advised on recirc loops and why the Metlund De-mand system is a preferred recirc for residential and some commercial applications.

I can't imagine how it can condense even at 1F delta T as long as all water temps are above 122F (the dew point of natural gas exhaust with 10% excess combustion air at sea level) with some margin, and how a higher delta-T fixes it. (Under 120F I can understand... )

Can you 'splain me how this happens?

I've also never quite understood what design features keep condensation from occuring on the heat exchangers with 40-50F incoming water temps too, which may be related. Do tell!

Clearly the mixing-valve approach is the least complex, least expensive and still reasonably efficient approach. (I can't imagine what hydroflow doesn't like about it.)

[Scott D. Plumber]

The first summer in my house, I turned off the tankless and ran free DHW for about 5 months durring a/c season. As soon as I ran of of hot water once, it flipped it back and never touched it again. In the first 14 months of being in the house I used 334 gallons of LP. We have a gas range used almost daily, the gas grill is hooked to it, used about 3 - 5 times a week minimum, a gas Kingsman vent free fireplace that my wife runs a lot durring the winter. (She loves it!) and the two tankless. 334 gallons in 14 months with all that is pretty good I think.

As far condensing in low delta T uses, I'll have to look up the "why" part. All I can tell you now is it does...and will. Seen it too many times. For instance I went to 5 jobs with one contractor to look at failed HX's on tankless heaters. All 5 were hooked up to First Company Air handler units but without the higher flow pumps. All 5 would only get aobut 1.5GPM flow through the tankless. All 5 set at 140* and getting about 120 -115* back. (That's less than 20,000 btu deliverable heat to the house!) All 5 would never satisfy the t-stat durring the cold days (imagine that) and ran a lot. Long run times at low delta t, with the unit basicly stuck on low fire caused condensate on both sides of the HX which ate them up in aobut 13-15 months. And high gas bills.

The way I see it you still have pretty low temp flue gas because you are not firing very much on low flow conditions for long periods of time and a large section of the HX/burner is unfired for long periods of time.

[Dana]

The first summer in my house, I turned off the tankless and ran free DHW for about 5 months durring a/c season. As soon as I ran of of hot water once, it flipped it back and never touched it again. In the first 14 months of being in the house I used 334 gallons of LP. We have a gas range used almost daily, the gas grill is hooked to it, used about 3 - 5 times a week minimum, a gas Kingsman vent free fireplace that my wife runs a lot durring the winter. (She loves it!) and the two tankless. 334 gallons in 14 months with all that is pretty good I think.

As far condensing in low delta T uses, I'll have to look up the "why" part. All I can tell you now is it does...and will. Seen it too many times. For instance I went to 5 jobs with one contractor to look at failed HX's on tankless heaters. All 5 were hooked up to First Company Air handler units but without the higher flow pumps. All 5 would only get aobut 1.5GPM flow through the tankless. All 5 set at 140* and getting about 120 -115* back. (That's less than 20,000 btu deliverable heat to the house!) All 5 would never satisfy the t-stat durring the cold days (imagine that) and ran a lot. Long run times at low delta t, with the unit basicly stuck on low fire caused condensate on both sides of the HX which ate them up in aobut 13-15 months. And high gas bills.

The way I see it you still have pretty low temp flue gas because you are not firing very much on low flow conditions for long periods of time and a large section of the HX/burner is unfired for long periods of time.

The key here probably isn't the so much the delta-T as it is the sub-122F return water temps and the low firing rate(=low fire-side turbulence). If you were pumping 10gpm through them instead of 1.5gpm at the same delta-T (for a ~100K burn) it's conceivable that condensation droplets on the fins of the copper HX get scrubbed clear by the sheer draft of the flue. (Is THAT how they manage 50F street water without sustaining damage?) Or is it possible that end-of-burn flue purges are also designed to blow condensate off the fins, and that continuous burns don't see enough purge cycles? I don't know, but I'd sure LIKE to know.

At any firing rate or delta-T, if the return water is above the dew point of the exhaust it simply can't condense on the HX, (but surely will in the flue.) Even with the same delta-T out of the air handler coil, at same pump flow, if you bumped up the output of the tankless to where the minimum return water temp was 125F they'd probably last for a decade or more. Of course they'd still be freezin' their butts off if the heat delivered didn't exceed the heat loss of the house- sounds under-designed on a couple of fronts.

Lowest modulated fire probably isn't the efficiency sweet-spot either. At lowest fire the turbulence on the fire side of the HX is lower resulting in laminar flow of the exhaust gases over the fins, and the stack temp rises (!). As with water-tube heating boilers, highest combustion-efficiency may in fact hit 86-88% when they're going full-blast, but drop to around 82-83% at quarter-output. (Haven't measured it, but I probably will at some point if I can't find that data from other sources.) Below some firing level the low turbulence on the fire side just kills it on efficieincy though. But for DHW apps it's useful to not have the thing flame-out when the water is at a trickle, so some run down under 10KBTU outputs at lowest fire these days. It could be that at 15-20K the combustion efficiency of those units is only 70%, but if you bumped it to 30K it would be a respectable 80 (?).

These folks measured the raw combustion efficiency of a Rinnai in an air-handler/DHW combi system at between 83-86%, but they don't give details on the modulation levels or water temps at which those numbers were measured. The design heat load was 12kW or ~41K, so presumably it was at least 40-60K when the air handler was running, probably a bit higher during heavy DHW loads:

http://dsp-psd.pwgsc.gc.ca/collectio...2-106-108E.pdf

Their cousins, the copper-tube hydronic boilers (eg RayPak) seem to tolerate ~110F return water on long burns without damage, (that's with ~10% excess combustion air and low-80s combustion efficiencies) but even the multi-stage versions don't drop below ~25% of full-fire. There must be something to fin-tube HX technology that's at least somewhat tolerant of low temp as long as the fire-side flow/turbulence is high enough.

Tankless units serve radiant slabs at even lower return temps all the time, seemingly without rapid deterioration of the heat exchangers. But I'm wondering if there's some minimum modulation setting at which they need to operate to avoid self-destruction? It would be nice to get to the bottom of this, since tankless heaters are being designed into combi heating systems at an increasing rate. Takagi seems to be marketing their stuff as generic wall-hung boilers (without any low temp or low fire caveats in their literature) but Rinnai has its air-handler combi, and Navien has it's heating-box combi heat exchanger setup too.

From your experience it sounds like the mixer-valve solution for heat pump + tankless in series has very reasonable efficiency, and it's clearly a low-cost low-maintenance no-headache solution. If the cold-water sandwich issue isn't annoying (which it won't be in warm-water areas the way it is with sub-40F mid-winter water in other areas) there's no reason to do anything different.