Does Buffer Tank with Modcon Make a Precise TDR Match Less Critical?

D107

(HO here) It seems to me that with a mod con and buffer tank, firing cycles would be longer and the system could sip from the buffer to meet the heating load rather than from a highly modulated burner, though I'm guessing it's not that simple. (Let's say for HTP's Versa Hydro.)

Brewbeer

HO here with mod-con (Lochinvar WHN055) and buffer tank (Boiler Buddy 30). See photo linked in my signature.

At low fire (20%), the boiler is putting out about 10k BTU/hr. For my system, with all three zones calling, any outdoor temp above about 38F results in the boiler producing more heat than the emitters can radiate into the house, which results in boiler cycling.

My house has 3 heated levels and a somewhat open floor plan, so heat rises up through the structure via convection currents, which historically resulted in a cold lower level when the house was heated by a forced air system. The buffer allows the baseboards on the lowest level to continuously provide heat, which has eliminated the cold lower level that always present with the forced air system.

Comfort has been increased immeasurably over the forced air system.

Rich_49

TDR is always important . The lower the TDR the longer a period each season the boiler has the ability to short cycle . The lower the TDR also allows for smaller buffer to be required if at all .

All this is dependent on what the heat loss of the structure is throughout the entire heating season , say from 50* to 5* ODT . room by room heat loss and current survey of existing emitters will give one the information required to make the proper choices and design the best possible system .

HomerJSmith

It seems to me that you are talking apples and oranges. They serve different purposes. A buffer tank is just a load like the heat emitters and Turn Down Ratio is an energy saving function. The TDR functions while the boiler is firing and the buffer tank does its function when the boiler isn't firing (stored heat energy) although it does store the heat energy when the boiler is firing, extending the firing time and extending the off time.
But what da I know.

D107

Question arose because in considering a Versa Hydro for a 40Kbtu home, the TDR minimum is above the likely minimum shoulder season load. So, as I understand it, in that scenario the buffer tank aspect of the Hydro is supposed to mitigate the short-cycling, therefore making less critical whether the TDR itself is a perfect match for the house. So perhaps title of thread should be 'does a buffer tank make a precise TDR match less critical?' Certainly not an 'either or' issue.

NY_Rob

^ is the home one large loop or is it multi-zoned with zone valves?

D107

3 zones now on 3 circs, plan is one ecm circ with three zone valves. two zones cast iron, one zone baseboard. FYI had paid heat loss with great contractor, awaiting proposal. He has been very generous with his time and answered many of my questions and will continue to do so as I also get additional clarifications from various sources so I can speed up my comprehension on this.

HomerJSmith

Talk to HTP tech. What you are saying does makes sense. Go ahead and install a buffer tank, won't hurt. But isn't it the water in the DHW tank that supplies the low temp heating supply water thru a flat plate heat exchanger? Why wouldn't the tank water be the same as a buffer tank?

D107

I'm not installing a separate buffer tank, the Versa Hydro IS a combo boiler-hwh-buffer tank. My contractor knows the system very well and can easily explain this to me when we next confer; we discussed it briefly but I missed a few details.

NY_Rob

David107 said:

3 zones now on 3 circs, plan is one ecm circ with three zone valves.

What you can do during the shoulder seasons is simply "combine" zones with a standard low voltage test lead w/alligator clips connected to the hot terminal of two or more zone valves. This way when one zone valve activates it opens the other zone valve and your BTU capacity just went way up preventing short-cycling. It also prevents regular cycling because it will heat the zone that's not calling for heat, so it too will go longer between heat calls.

It sounds simplistic and at first it seems like it wouldn't work and it would heat a zones that don't need it. But the nice thing about heating with 120F water is that it takes a long time to change the temp in a room, so running an extra zone for 20min or so in a shoulder season isn't going to overheat it... but it will hold off a call for heat a bit longer to that zone and it will stop short zones from short cycling.

You can also raise the ODR SWT temp to give you hotter SWT during the shoulder seasons so it will heat the areas quicker and then probably not call for heat for a while.

HomerJSmith

I'm missing something. I don't see how you would get short cycling with that unit. Raise the tank temperature and adjust the mixing valve and set the differential to 30 deg., would that work.

D107

I saw the 10 to 1 TDR on the 100K space heating module, which would mean the minimum firing rate there would be 10Kbtu, whereas probably ideal lowest would be 5K on a 40kbtu loss on a shoulder day. Their brochure says: "Modulation, along with thermal mass, eliminates the short-cycling effect of small zones or minimal DHW draws." So it seems the buffer aspect (thermal mass) would mitigate whatever the TDR couldn't cover. Seems that without the buffer aspect on a 50 degree day it might short cycle because it might need a low firing rate of 5K. So therein seems to lie the genius of this unit. (If I was going straight mod con, I might be advised to go for 60k or 70k unit with 10 TDR.)

Rich_49

The space heating module has no firing rate . It simply moves the hottest water out of the top of the tank through the FPHX and back down to the bottom of the tank . When the lower sensor senses the tank temp get to a pre determined temp it will then fire the burner . It will stay on at it's lowest firing rate only when the low end modulation is being used for space heating and using the minimum TDR amount of heat .

The modulation that the heat pack has is also tied to the flat plate , supply and return sensors will modulate the circ speed based on programming and actual S&R temps being seen by the sensors. At the beginning of a space heating call while the burner is off , the top tank temp starts to drop and as it degrades the circ will change speed appropriately against the fluid temp on the space heating side based on the supply and return temps .

With a newer , very tight house with very low loads ( 3-4000 BTUh ) the 008VV in the heat pack can get into a bit of trouble below 4 volts requirement and do funny things . Best thing to do then is adjust the default 20* delta in the programming to , first 15* then if necessary 10* to straighten that issue out . We found that out in a remote design job in Maine which I designed on warmer days when one or 2 baseboard zones called needing only .6 total combined flow or less . The job was designed for 1 circ / 2 zone valves but the local plumber thought 2 VT2218s would be better . Because of the very low resistance that these circuits offered we had to send him 2 Caleffi 132 quicksetters in 1/2 to create the resistance and insure max gpm . Yes , I did get the phone call about the wrong size ( 1/2) for the 3/4 circuits that I sent . A couple reducing couplings later and upon restart the VTs 4* delta at very low load was moving water at a respectable 16* .

You can't make this stuff up . The Versa is a really nice , well performing machine . It'll be better when and if they make it in a model for the low load house with a 003 or 006 vv in the heat pack . That would have been perfect for the house just described

NY_Rob

David107 said:

...If I was going straight mod con, I might be advised to go for 60k or 70k unit with 10 TDR.

It would be nice if something like that existed....

Lowest mod-con output is still 8K BTU's (from the HTP UFT-80W).
The Lochinvar WHN056 is 56K BTU output, but not a 10:1 TDR, sadly it only modulates down to 8.3K BTU's.

A 50K BTU or 60K BTU mod-con with a 10:1 TDR would solve lots of problems and would (imo) be a big seller.

DC123

Seems to exist in Europe. Viessmann vitodens 300-w has a low of 1.9 kw, which translates to 6500 btu/hr. 1:10 TDR.

SuperJ

I’ve got a older Polaris tank in my house I run a 16-25degf differential on a couple micro zones TRV Panel rads. It’s a 100k non modulating input BTU. With the 34 gallon buffer I run less than one 5min cycle per hour in the current shoulder season conditions. A buffer tank helps a ton especially when you don’t have a good TDR.

JeremyG

Maybe a daft question... but can some of the short-cycling concerns be addressed with a good thermostat? I have much better temp regulation with an Ecobee than with a dumb thermostat on my old and massively oversized CI boiler (though starting to think about a mod-con upgrade in conjunction with finishing the basement). The Ecobee has tunable parameters such as minimum runtime and temp "swing". I'm guessing Nest and others offer the same.

The house usually stays within 0.5ºF of the setpoint, whereas my old 'stat would blast past it. (This has a nice secondary effect of keeping the radiator temps much cooler — around 110º — unless it's really cold. So I have confidence that the house will still be comfortable with cooler boiler temps.)

Gordy

^In short to any great degree of mattering, no.

Actually a crappy thermostat with loose room setpoint will cycle the boiler less. In other words a longer heat cycle with a looser setpoint.

When you tighten the room setpoint the boiler is going to kick on, and off more, and probably run a shorter cycle to keep that narrow setpoint.

Mass in any system is your friend, if controlled correctly. The whole premise of buffering is to smooth out those short cycles to longer burns. All the buffer tank accomplishes is adding mass.

Even older gravity systems with large piping are great for added mass. Built in buffer tank.

That’s why the HTP versa hydro is a great unit in certain applications.

JeremyG

Yeah, I get that the smaller hysteresis might contribute to short-cycling. My point was that and the minimum runtime can be configured on a "smart" thermostat. So you might overheat a little bit in the shoulder season, but you could ensure that there's a 10-min (or whatever) boiler firing cycle.

Not meaning to take this thread off-track. I've gained a lot of useful info from this one and others to help inform my future upgrade project. (Maybe I'll keep as much of the gravity-fed iron pipe as possible!)

NY_Rob

JeremyG said:

... My point was that and the minimum runtime can be configured on a "smart" thermostat. So you might overheat a little bit in the shoulder season, but you could ensure that there's a 10-min (or whatever) boiler firing cycle.

You can set the "min runtime" to whatever you like on the t-stat, but if the SWT hits setpoint the boiler will cease firing regardless. "Min runtime" on the t-stat only guarantees the CH pump will run for the min runtime not the boiler.

You're better off enabling "ramp delay" and "limit combustion %" if your boiler has these options- and most mod-cons will. This way it doesn't throw the full output of the boiler at the CH water the second it fires up.

IMO- the electronic t-stats are really a detriment to mod-con based systems. They react too fast and don't seem to anticipate correctly for very slow rising temps found in low-temp systems. I went back to the old style Honeywell mechanical t-stat in one area of my house, and it seems to control the heat better than any of the electronic t-stats I've tried. When the room temp gets below setpoint it snaps on, stays on (even with people walking past it and doors opening and closing) and shuts down a little above setpoint. No bouncing up and down and cycling the boiler needlessly like the electronic t-stats did. And... it was only about $25 IIRC!