Unusual set up: indirect as preheat for small combi?

[Greenmountain]

I'm new at this, and am not sure if my idea makes sense so looking for some feedback.
Boiler would be a Rinnai E50 and indirect a Heat-Flo 60g, three small zones plus the indirect as a 4th zone.

I'm thinking of routing all my zone returns to flow through the indirect (manifold to indirect input) before returning to boiler. There would also be a tee in this line so that the indirect is a zone as well. Check valves would prevent backflow. Temp setting for the indirect storage would be roughly 65-70F.

The basic philosophy is to one, preheat incoming cold water (water comes in at anywhere from around 34-45F) to increase the DHW output of the combi (set to Max temp output from combi then through mixing valve), and two, to scrub heat from the return flow to maintain high condensing efficiency. I think it should also help to minimize short cycling.

(The stated min flow rate for the indirect HX is 14gpm. I'm not sure yet what the combined flow rate of all zones will be, but if necessary I could install a bypass controlled by a PRV, though 14gpm seems like plenty of capacity.)

This also essentially makes any standby heat loss from the storage negligible since heat loss in winter is to a heated space, and very little heat loss will occur in summer since storage temp will be very close to average ambient.

Btw, the inspiration here is cost opportunity... I've found very good deals on the equipment, so not looking to compare to a system at retail pricing, just want to ask if anyone sees problems or inefficiencies.

Thanks!

 

[Greenmountain]

additional question: in reading through the E50C instructions, so far I'm not coming across any options to adjust DHW output temp.
It just says:

"The 3/4" NPT adapter fitting with flow reducing valve must be fitted in the cold water
connection.
A flow regulator valve is supplied with the boiler in a 3/4"NPT adapter fitting. The
flow regulator valve ensures that a quantity of water is supplied which has a outlet
temperature of 120°F (assuming a cold water temperature of 45°F). The quantity of
water is virtually unaffected by the water pressure."


Anyone know if the output temp can be set higher on this model??



It also suggests:

"A thermostatic mixing valve should be installed on all E combi boilers on the domestic
hot water side to prevent scalding."

If max temp output is 120F, I'm assuming a TMV would be in case of some sort of serious malfunction?

 

[Sponsor]

 

[Fitter30]

Need to see how your going to the system with a diagram. If your going to pipe boiler water thru the water heater side to capture the heat from return water i would want a heat exchanger to separate them. If the tank temp is 120* and put less than that temp through the tank coil it will removing heat from the tank. Both boiler and dhw don't run at the same time. If 50,000 btus are needed for heating at peak load you can't have 40* water feeding heating side.

 

[Greenmountain]

ok, apologies for my chicken scratch drawing... but here's a very rough schematic.

I've posted on "heatinghelp.com-thewall" also and the feedback I got there was pretty negative.
seems the risk of pathogenic growth in the "warm(65F+/-?)" indirect storage is too high, though I wouldn't mind a second opinion on that...

Wondering if decreasing the storage volume and adding flow through it by using it for laundry/utility sink might add enough flow/water change that risk of pathogens could be diminished enough to where the system could work?? (the 60gal Heat-Flo I was originally considering is no longer available, so could shop for a smaller tank)

I know that there are circumstances where water sits in pipes at 65-75+F... but how much water for how long seems to be a part of risk assessment for pathogenic growth, true?

So, just to restate and summarize my reasoning for this idea:

1. assuming sufficient radiation in all zones, the additional heat scrubbing from the returns should greatly help keep return water at condensing temps

2. given how cold my incoming water is (34-45F), and the relatively low capacity of the combi, a pre-heat should help get my DHW up to 140F

3. in terms of efficiency, very little energy is wasted maintaining high volumes of storage at high heat

4. could maybe also help reduce short cycling

 

[Fitter30]

Don't recommend preheating dhw with boiler water in case you need the capacity. Heres a controller for zone and dhw priority. Your dhw looks ok might need expansion tank if on city water. Use taco e series pumps for energy efficiency and built in check in pump 1.
https://www.supplyhouse.com/Taco-ZV...MIiuGA29HU8wIV8ntvBB3E8APxEAQYAyABEgIKavD_BwE

 

[Dana]

Pre heating tanks of potable water into the Legionella high-growth temperature zone (85F-115F) is a generally bad idea. In many locations the normal summertime incoming water temp is 65-75F, and yes it is a Legionella hazard, but not a high hazard. Increasing flow won't fix that (any tank size would be a problem), but as long as the temp stayed below 75F the risk is somewhat bounded.

Heating the tank to 160F for several hours will usually kill off active colonies. Heating it to 140F will kill it too, but it will take days or weeks. If that can be done periodically as a mitigation strategy the Legionella hazard can be managed. The extreme red zone would be 100-105F, storage temps commonly found in hot tubs, which are (unsurprisingly) one of the most commonly isolated sources of clinical cases.

Also note, the minimum firing rate of of the Rinnai E50 is 13,000 BTU/hr, which is low enough for stable output temperatures at 105-115F at 1 gpm flow with an incoming water temp <70F, but becomes increasingly impossible as the incoming temps inch north of 90F. That can cause burner flame out on some models, overtemp error shut down in others. I don't know for sure how the E50 behaves under low-flow, high incoming water temp conditions, but it's not how the thing was designed to be operated.

A much less risky pre-heat strategy would be to install a drainwater heat recovery heat exchanger downstream of the main shower drain, feeding cold water side of the water heater & shower mixer with the heat exchanger output. A 50% or higher efficiency heat exchanger would pre-heat the water into the 70s or higher, but only while the shower is running- it wouldn't stagnate for hours or days at a high risk temperature. The benefit would only be for longer simultaneous hot water/drain flows such as showers, but that's usually a large chunk of the hot water use in a typical residential application.

 

[Greenmountain]

Thanks Dana...

In your response to my other post, you mentioned you keep your Ergomax at 130F, but occasional it dips down to 110F when your larger zones are pulling heat. I've been reading about legionella and 130F does kill it, but as you mentioned, it takes time.

There does seem to be a significant lack of consensus out there on time/temp for safety, but here is one chart I found:

source: https://www.pmengineer.com/articles/90962-legionella-and-water-temperatures-go-hand-in-hand


In your case, do you feel that your DHW sits at 130F long enough to kill stuff off before hitting the shower?
Or is it that there's less storage volume, more continuous flow, or are you on treated city water, or?

Here's a quote from the above article:

"If semi-instantaneous heat exchangers are used for water heating, setting them at 60° C (140° F) will be of little or no benefit for Legionella control if water will not be held at that temperature long enough to kill Legionella. Installing a mixing valve downstream of a semi-instantaneous heat exchanger could actually promote rather than hinder Legionella growth by adding surface area (for biofilm development) to the system."

In this regard, how is ANY on-demand water heater safe? In my other building, incoming well water is heated to 120F by an on-demand tankless...
so at no point in the system are controlling temps reached, much less for the time needed to kill bacteria.

According to the CDC, legionella infections are rising pretty steadily. I wonder if increasing use of on-demand DHW heaters might be a piece of this?
Also, I've read that copper itself is hostile to bacterial growth, so maybe increasing use of pex is another risk factor?

I DO have a UV "sterilizer", but according to the feedback I got on "heatinghelp.com", UV treatment is, at best, not dependable. I'm not sure I agree with this, but either way, in my case, either the UV is working, or I don't have pathogens to begin with, or everyone who's ever lived in the house has a great immune system! (I am in touch with previous owners, so 50+yrs of occupancy history with no waterborne illness)

I'm not intending to implement the system I've been thinking about, but still, I guess the whole time/temp/volume/flow paradigm for pathogenic risk management still feels pretty fuzzy to me.

 

[jad3675]

In your response to my other post, you mentioned you keep your Ergomax at 130F, but occasional it dips down to 110F when your larger zones are pulling heat. I've been reading about legionella and 130F does kill it, but as you mentioned, it takes time.

There does seem to be a significant lack of consensus out there on time/temp for safety, but here is one chart I found:

In your case, do you feel that your DHW sits at 130F long enough to kill stuff off before hitting the shower?
Or is it that there's less storage volume, more continuous flow, or are you on treated city water, or?

The stored water in a ergomax/turbomax is non-potable, so the storage temp doesn't mater.

 

[Greenmountain]

The stored water in a ergomax/turbomax is non-potable, so the storage temp doesn't mater.

ergomax/turbomax are "reverse" indirects... There is DHW that sits inside the DHW Hx inside the boiler water

 

[jad3675]

ergomax/turbomax are "reverse" indirects... There is DHW that sits inside the DHW Hx inside the boiler water

I know, I have a turbomax. There's 20ft of 3/8 tubing in a 26 gallon turbomax. That's about 1/10 of a gallon of water. Since the system is essentially flushed each time you open a hot water tap, the risk of legionnaires is almost non-existent - there's no 'sitting' water in the coil like you get ina traditional tank with a dip tube.
To put it another way; are you worried about the hw supply lines harboring legionnaries? There's probably more warm-ish water in the lines running to your kitchen sink than there inside a reverse indirect.

 

[Dana]

Thanks Dana...

In your response to my other post, you mentioned you keep your Ergomax at 130F, but occasional it dips down to 110F when your larger zones are pulling heat. I've been reading about legionella and 130F does kill it, but as you mentioned, it takes time.

The thing about an Ergomax is that the potable water doesn't stagnate. There is less than a gallon of potable water in the heat exchanger coils, and gets fully purged with almost every hot water draw. And unlike a potable tank, the coils are designed for high turbulence, which pretty much scrubs the potable side of the heat exchanger coils.

This is true of most tankless water heaters too.

In your case, do you feel that your DHW sits at 130F long enough to kill stuff off before hitting the shower?
Or is it that there's less storage volume, more continuous flow, or are you on treated city water, or?

Here's a quote from the above article:

"If semi-instantaneous heat exchangers are used for water heating, setting them at 60° C (140° F) will be of little or no benefit for Legionella control if water will not be held at that temperature long enough to kill Legionella. Installing a mixing valve downstream of a semi-instantaneous heat exchanger could actually promote rather than hinder Legionella growth by adding surface area (for biofilm development) to the system."

In this regard, how is ANY on-demand water heater safe? In my other building, incoming well water is heated to 120F by an on-demand tankless...
so at no point in the system are controlling temps reached, much less for the time needed to kill bacteria.

It's true that if a Legionella colony gets established in a tankless or reverse-indirect it takes a lot to kill it. But due to the turbulence it's a bit harder for biofilms to get established.

I'm personally on treated city water and since the stagnation temp is well above the high-growth temperature range don't really worry about it much, but I might take the tank up to 160F or higher once every year or so if I were on a well. A tankless will spend MUCH more time in a higher-growth temperature range, but if you're on an annual or bi-annual de-liming schedule that would probably take care of it. (Haven't researched that, but the biofilms would presumably form on the lime layer.)