How Do I Control This?!

Kestrel

Hello esteemed comfort-makers!

I'm moving ahead with my conversion from scorched air to hydronic, and I have (of course) had an idea that, ahem, improves the system, but increases its complexity, and I'm not totally clear on how to best control the system.



Old plan (first pic below) - TT Solo 110 boiler, TT Smart-60 indirect, and Boiler Buddy 30gal buffer to avoid short cycling.  Heating side is manifold supplying 7 panel rads (second floor and basement) each with own TRV, and a second manifold supplying joist space radiant tubes on first floor.  Both manifolds downstream of a Grundfos Alpha.  TRVs open and close individual rads, and the main floor is controlled by a thermostat and floor sensor that control a 2-way zone valve.  Boiler controlled by ODR and sensor in buffer tank.  DHW totally separate, with own aquastat.



New plan (second schematic below) driven by desire to add solar hot water at later date, also to minimize footprint of boiler room.

Same TT Solo 110 boiler, but using TT SME-80 multi-fuel tank with 70gal DHW and 30gal primary side, and with coil for solar hook up.



My dilemma is - how to control this when both the buffer tank and DHW tank at same temperature?  The buffer temp needs to vary seasonally, but the DHW needs to be constant.  Plan is for design temp of the secondary system to be 150'F

At design temp here in Seattle (20'), at the coldest part of the year the buffer is higher temp than DHW, so thermostatic valve at outlet of DHW brings it down, and all is OK.  However, at warmer parts of the heating season, buffer may need to be below DHW temp.

Do I need a mixing valve upstream of the Grundfos Alpha to bring down the heating water temp during those parts of the year?

If I need to keep the buffer tank hot enough for the DHW, can the outdoor reset control the mixing valve?

Gordan

I was tossing around similar ideas...

I don't think that your latter design will be an improvement in terms of efficiency. The right way to do what you're describing would be with a multi-valent (stratified) tank, but those are terribly expensive. Otherwise, most of the heating season you'll be overheating water just to mix it down.



 In my case, I was struggling to find the way to use my reverse indirect as a buffer for space heating also, but I decided instead to get the PS 60, which modulates nice and low, and not buffer on the space heating side. The large water volume of the heat exchanger does have some buffering ability, as does the 15 gals or so of water in the distribution. I didn't pipe it pri/sec; I'm pumping through with an Alpha. I haven't had any issues with DHW recovery yet, but my hot water usage is moderate to small - two small children and two adults. Your mileage may vary. You could also look into one of the new fire tube Knights, as they offer a wider modulation range without losing the advantages of the fire tube design.

Kestrel

Thoughts on ODR

I guess one major goal was to have a system that would be straightforward to add solar input at a later date.

The other goal was to minimize the footprint of the mechanical room - and so consolidating the Boiler Buddy and the DHW tank was appealing



What I realized was that putting the home heat buffer volume (there to minimize short cycling) in direct contact with the DHW volume means that the buffer needs to be kept above a minimum temp that is needed for DHW, and therefore cannot be modulated down with a DHW curve controlling boiler firing, at least at the lower end of the ODR range.



So, the design temp of 150 is higher than DHW needs, so it seems that buffer/DHW temperature could range from 150 down to 120, but then no lower.  (Is that do-able?  How would one configure controls to do that?)



Alternately, I could keep the DHW/Buffer volumes at 150, temper the DHW output with a thermostatic mixing valve, and temper the heating water temperature  upstream of the pump with a 3-way mixing valve under the control of an ODR, like a Taco i-series.

I was talking to the supply house hydronic designer who suggested just using a fixed 150 for heating at all time - the consequence being that the radiators and floor emitters deliver heat faster during off-peak parts of the season, but an expensive (and breakable) extra feature (the i-series) is eliminated.



That got me thinking about ODR - and lowering the water temp in parallel with rising outdoor temps means that the heating time the the various emitters are active is kept more constant by delivering fewer BTUs/time when fewer BTUs are needed.  Keeping the water temp constant means that the emitters deliver the required heat in shorter amounts of time during the shoulder seasons - which might be fine, or might mean that there are bigger and more frequent temperature swings.



If I over-size the radiators, that would allow lower water temperature in the first place, and might mitigate swings in temperature, yes?

Thoughts?

Jason Quinn

Just a thought....

Was "googling" around after reading your thread and saw an HTP Hydro-Versa. It appears to combine DHW, space heating and I believe solar into one package. Sorry, I forgot to copy the link.

Gordan

IMO, you're chasing your own tail a bit :-)

Here are my recommendations, which are my opinion only:



1) You're investing in a mod-con, so make sure that your space heating - which is most of your load - operates at temps that are as low as possible. Adequately sizing your radiators will, yes, lower the required supply temperature - it will also provide some mass to act as a buffer. You don't need a separate buffer; there's nothing magical about buffers, as they're just a way to add thermal mass to a low-mass system, which is not what you'll have with those radiators. As long as flow through the system is adequate, you'll keep that boiler firing for nice long cycles.

2) You're paying for a boiler that's got a built-in ODR controller. Use it! Don't tack on external controls that duplicate the function of internal ones, unless you're getting really fancy - and I doubt that there's often a need to get really fancy.

3) Solar can always be added pretty easily on the DHW side. Solar space heating... well... be realistic about your insolation, first cost for an adequate collector system. Your heating load suggests that you're not in the sun belt. You may not really get much out of solar by way of space heating, so it may not make sense to compromise your design just to accommodate a heat source that will not realistically be available for most of the heating season.

So, all of these combined suggest:

4) Separate your DHW and space heating functions and tailor supply temps to each. A dual-coil indirect will accommodate a future solar DHW option; you can just pipe both coils to the boiler now to get a larger exchange surface (and better efficiency) and repipe the bottom coil to solar later.

Kestrel

good advice!

Gordon

Thanks so much for your insights.  I know I was getting dangerous with adding more bells and whistles.

I like the idea of distinct DHW and heating circuits.  On that note - any recommendations for an indirect with a second coil?  Expense is an issue, though we could go a bit more than the TT Smart 60 we were originally planning.

As for buffer/thermal mass - I was concerned about both achieving hydraulic separation and avoiding short cycling.  If I have 7 or 8 indempendent radiators with TRVs, and a large joist-space tube system, is short cycling a realistic worry?  Can I dispense with the originally planned boiler buddy?

Thanks for your thoughts and advice.

Gordan

You'll want to tune in your ODR curve...

...so that those TRVs normally stay open. You do that by "dropping" the curve until it can barely keep up with load, then bump it up just a tad. That way you'll have good flow through those TRVs, and there will be no fear of short cycling. You're right in insinuating that all that thermal mass on the emitter side is of no use as a buffer if it's isolated by valves from the boiler, but doing the above should address that. You may have to tinker with it some if you find that on a particularly windy day it's really not keeping up, but like most things in life, it's a compromise, so find one that you and your boiler can live with. If you don't want to compromise, you spend more money and more space and get a dedicated buffer tank, decoupling boiler flow from system flow and adding thermal mass to the boiler circuit, and now you can be all sorts of out-of-whack with your system and the boiler won't feel it. But why would you want to be all sorts of out-of-whack?



 As I mentioned, you could further mitigate any short-cycling concerns by getting a boiler that modulates lower.



If you're willing to tune things to this degree, you may find that piping things primary-secondary is completely unnecessary and you can just pump through the boiler. These fire tube exchangers have very low head loss, certainly at the flows you'd be putting through them at max load.



I don't have any experience with dual coil indirects but you could do a search on the site and see what people like. The solar forum might be a good place to ask about them, too. Lots of manufacturers make them, including big ticket ones like Buderus and Viessmann, Laars, HeatFlo, Stiebel Eltron...