Combi Boiler-Newbie needs answers

[Bob1958]

I live at 8300 feet SW of Denver in the foothills. Presently I have electric baseboard heat, and I want to switch to hot water baseboard heat. I presently also have a Powervent gas water heater with PVC stack exhaust through floor above and through the roof.

Plumbing store guy in Denver thinks I need the Combi -NTI Vmax153P (16 gallon reserve tank) based on my estimated 500 feet of 3/4 inch PEX and 67 feet total of baseboard heater, and I plan 4 zones. He is quick to sell but short on answers to questions I have:

Do I need this much boiler?
Do I need an external pump for the heating of baseboards? How much pump and If so what triggers the pump to turn on?
Is there just water in the system or do I add a lubricant?
Could I just go tankless?

Any help is greatly appreciated, as are any suggestions for other combi boilers
Thanks,Bob

 

[djdavenport]

I can burn a little harmless bandwidth until some real experts weigh in. I'm at 7,500 feet in a 1,800 sq ft house. We have a tankless combi that services both in-slab radiant and DHW. It was put in about a year and a half ago (I didn't research it. Plumbing crisis hit and I said "yes." Fortunately, it's been a good choice. Gas bills down about 40% year over year, long cycles in condensing mode, and the fulfilled promise of endless hot water.)

I think knowing a lot more about your space and having a thorough heat calculation done will be invaluable. And the trade-offs between a tankless, a combi, a stand-alone modulating boiler with a indirect or some other flavor in terms of usage and size will make more sense once you know what you're looking at in terms of heat loss. Also, having an Outdoor Temperature Reset with properly dialed in curves can make a big difference in terms of efficiency.

In terms of controlling the beast...typically, each zone will have a thermostat which is wired to a multi-zone controller. When the thermostat calls for heat, it tells the controller two things--fire up the boiler and turn on the circulating pump (and/or zone valve, depending on how your system is designed.) Used to be that a typical circulator like a Taco 007 burned a fair amount of power, and so having a single circulator and controlling the zones with individual zone valves rather than having, in your case, four circulator pumps, was much more efficient in terms of power usage. These days, with ECM pumps that use a ridiculously small amount of power, the difference is less pronounced, and using zone pumps rather than a single pump and zone valves is probably closer to being a wash. The boiler, itself, will be controlled by its own system, typically an aquastat with hi and low temperature limits dialed in.

I think there is some debate about whether, in a closed system, it is necessary to use a corrosion inhibitor. Ideally, if your system doesn't leak and there is no need to replenish it with "new" water (which has dissolved oxygen in it and is, therefore, corrosive) you're probably fine with just water. If there is a freeze hazard, you can run a water/glycol mix. But, the typical pump doesn't need any additional lubrication. Just make sure it is mounted in the proper orientation.

Again, I'm no pro. Just a interested homeowner, but I've learned a helluva lot in the past 18 months since on a 18 degree February day we simultaneously lost the water heater and the boiler. It's really good that you can research your options during the non-heating season and make a informed choice. Good luck.

 

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[Bob1958]

Thanks DJD, that is very helpful, I live in a raised ranch that is 2200 sq feet, half of it is finished basement. Every year we get a week in December or January where it can get to 15-20 below at night, but most of the winter get between 5 above and 5 below. Some of the piping will be on outside walls downstairs. I was planning a 1 pump 4 valve zoned system. I am guessing that the thermostat in a zone would turn on the external pump and boiler like you said, and at the same time it opens the zone valve?

I haven't done a heat calculation in the past, I have done a lot of my heating by wood stove and haven't worried about it as on really cold nights I'll get up and put in more wood. Even so my electric bills can be as high as $200 more than in the summer.

Some of what you wrote about the condensing option and the out door temp reset will require my researching as at this point in time is above my pay grade. Any other thoughts are going to be helpful.

From what i have read on this forum getting a boiler that is too big in BTUs for my needs can actually shorten the life of the boiler. Is 600 BTUs per foot of baseboard an accurate formula? I also have been told I lose 25% efficiency of my boiler at my 8300 feet altitude.

Like I said any thoughts are appreciated and helpful.

 

[djdavenport]

Now, you're asking stuff above my pay grade. With combi boilers, you need to be aware of what the turn-down rate is. They may be rated for 120,000 BTUs, for example, which might be fine for your coldest design days, but way oversized for when the weather gets warmer. But, if the boiler can modulate down to, say, 15,000 units and that balances your heat requirements for the rest of the heating season, you're likely okay. Otherwise, you could end up short-cycling and that is bad from an equipment and efficiency point of view. Also, with a combi, you need to make sure that, while not overpowering the heating side, you still have enough BTUs to handle your DHW needs. I think the conventional wisdom is that combi boilers can work pretty well if there is sufficient emission on the radiation side so that they don't short cycle but also that you aren't trying to feed three carwash sized showers on the DHW side. If you have large hot water requirements, they might not be the best choice. We're a family of two, so that's never been an issue.

But, that said, there are lots of other choices, and guys will be along who can actually give some decent advice.

 

[NY_Rob]

.. it can get to 15-20 below at night, but most of the winter get between 5 above and 5 below. Some of the piping will be on outside walls downstairs.

I'm not an expert either... but I believe you will have to run your system on a glycol (special for heating systems- not the stuff used in automobiles) mix... and the boiler you choose should have a "Freeze Protection" mode that will run the pump based on the temp of the water in the system piping vs. heat calls to keep the pipes from freezing.

67' of emitter split between four zones will create short cycling problems for your boiler... is there any way to combine into two zones or even one zone?

 

[Bob1958]

I could, I have four 9 footers, four 6 footers, a 4 and a 3. How much temp drop would happen between inlet and outlet of a 9 ft. baseboard heater?

Shoot I'd love not to need manifolds and zone valves!!!

 

[NY_Rob]

I could, I have four 9 footers, four 6 footers, a 4 and a 3. How much temp drop would happen between inlet and outlet of a 9 ft. baseboard heater?

Virtually none after just 9ft.

A 67ft zone would give you a nice DT (Delta Temp) between input and output... probably between 12-15 deg.

 

[Bob1958]

Thank NY_Rob. So if the DT is less (4 zone) then it would create more chance of short cycling? Would there be any advantage to a 2 zone system -upstairs -downstairs?

 

[NY_Rob]

A lot depends on the minimum fire rate of the boiler you choose and the temperature of the water you heat with.
If you want to go with a mod-con boiler you need return water temps below 130F to condense (see chart).

If you use a 20f deg temp drop (DT) between supply and return (industry standard figure for calculation purposes) assume you started with 140F supply water (SWT) and 120F water returning (RWT) to the boiler. Your AWT (Average Water Temp) is 130F... so according to the slat-fin chart for fin tube radiators- 130F water gives off 260BTU's per ft, you have 67' ft of baseboard with fins (do not measure the enclosures... just the finned lengths)... this will radiate 17,400BTU. A mod-con like the HTP UFT-80W can modulate down as low as 8K BTU, so no short cycling with 67' radiation at 130F SWT.
Now, cut that up into two (assume even amounts of fin tube) 33.5' zones.... and you can only put out 8,700 BTU's at 130F AWT... still okay for the UFT-80W.
But, if you look at the Boiler Efficiency Curve chart, you really want much lower supply water temps than 140F... you would like to have maybe 120F SWT with 110F AWT to get 91% efficiency out of your boiler.

At 110F AWT (160BTU's/Ft) that 67' of fin tube puts out 10,700 BTU's which still won't short cycle the UFT-80W, but a 33.5' zone would only put out 5K BTU's... which would definitely short cycle the boiler with it's minimum 8K BTU fire rate. To stop short cycling you'll have to use higher SWT and one look at the Boiler Efficiency Curve chart will show you what that does to efficiency.

 

[Bob1958]

That Vmax 153 modulated down 10:1, which suggests only down to 15 k so I could easily short cycle at the higher SWT. So do you think I could get away with an 80ish K BTU boiler? Or 100k due to my altitude?
Thanks for your very helpful post!

 

[NY_Rob]

^ Now you're starting to see the light... and probably know more than the guy trying to sell you the 140K BTU boiler with a minimum fire rate of 15K BTU's.

At the highest SWT available... 180F, your 67' of fin tube can only put out 39K BTU's... so yeah and 80K BTU boiler will be sufficient for your radiation.
I have been told the ballpark heatloss for a modern 2x4 built home is in the 15-20 BTU/SqFt range... so 40K BTU's sounds pretty close for your 2,000 SqFt home.

The HTP UFT-80W is not a combi unit, so you would need an indirect storage tank (like the SuperStore Ultra) for domestic hot water.

The other thing to keep in mind is that mod-con boilers adjust their SWT based on the outdoor temperature and an outdoor temp sensor that gets installed outside your house. The theory is that you don't need 150 F water to heat your house when it's 45F outdoors- you can heat it with 115F water and save $$.

I'll use the "Outdoor Reset Curve" for my boiler for my house as an example.
Examine the curve and you'll quickly see why it's important to have a boiler that is closely matched to your radiation output. In my example you'll see I only use 115F SWT when it's 45F or higher outdoors- so my radiation has to be of sufficient length to radiate 8K BTU's from 105F ATW. In reality I could heat the house on warmer days with less than 115F supply water, but my shortest zone would short cycle due to it's output being less than the minimum 8K BTU's the boiler can deliver.

 

[Bob1958]

This has been very helpful, I have learned alot this weekend. Thank you!

So I have read that at my altitude boiler efficiency derates 20%, if I want 80KBTU, does that mean I should be looking at a 100KBTU burner?

 

[NY_Rob]

If your heatloss is say 40K BTU's... no need to get a boiler that puts out more than your heatloss or more than your radiators can emit.

If you have 67' of fin tube that can put out 580BTU's/ft, your max radiation is 39K BTU's.
Plopping in a 100K BTU boiler isn't going to change the fact that your radiators can only put out 39K BTU's of heat. You can only use 39K BTU's out of the 100K BTU's available.

Now, if you're planning to add radiation in the future- get boiler that will handle the additional load. But even of you doubled your present radiation the 80K BTU output of the HTP UFT-80W would cover that.

Download the Slant-fin Heatloss Calculator app and do a heatloss estimate before going any further, that will give you a rough estimate of your heatloss.

 

[Dana]

Mind you, the Slantfin load tool is going to overestimate reality, sometimes by quite a bit, but it's way better than a WAG.

If you have a heating history on the place it's possible to run a pretty good whole-house heat load calc based on wintertime meter readings (with a deduction for power use based on shoulder-season periods.) With the addition of wood heat it gets more complicated, but it's not impossible. That won't tell you how much heat load there is per zone or room, but you can work proportionally with Slantfin's numbers, but without a good handle on it just going with Slantfin's number would be close enough. It would oversize the radiation, but that's a GOOD thing for condensing efficiency.

Combi boilers are usually a LOUSY fit for most houses. They're better suited to large lossy houses with big heat loads but only moderate domestic hot water needs. The high minimum firing rate becomes really problematic if you're breaking it up into zones, since each zone has to be able to emit the lion's share of the min-fire output to avoid short cycling on zone calls. The napkin-math on that can be found here.

The min-fire input of the 153P is 15,400 BTU/hr. At 95% combustion efficiency that's about 14,600BTU/hr out. With typical fin tube at an average water temp (AWT) of 120F necessary to hit those efficiency numbers it would take about 70' of baseboard PER ZONE to balance the boiler input to zone-radiation output.

Don't get to enamored of any particular solution until you have narrowed down on the heat loads per zone, and how much radiation it's going to take to deliver that much heat with an AWT of 14oF (assuming you're looking for condensing efficiency). There is no point to a modulating condensing boiler that's so oversized that can't condense or modulate. Sometimes you're better off using a condensing water heater with a heat exchanger separating the potable from the heating system water when breaking it up into zones. The thermal mass of the stored water keeps it from short cycling on zone calls even on micro-zoned systems- even zones with only 5' of baseboard.

Even with derating for altitude the house has to either large, leaky, or uninsulated (or all of the above) to need more than 80,000 BTU/hr of boiler. The design heat loads of a tight, insulated 2x4 framed 2500' house would usually come in under 60,000 BTU/hr @ -14F (the 99% outside design temp for Leadville.) For a 2200' raised ranch it'll be under 50,000 BTU/hr, maybe closing in on 40K @ -14F.

For a second-opinion on the Slantfin tool's numbers try loadcalc. Like Slantfin, it too is prone to oversizing well into double-digit percentages compared to what a properly aggressive Manual-J on a professional tool would deliver. Make the most aggressive assumptions that are even remotely reasonable for air tightness and R-values and it'll still oversize by a bit.

Whatever load tool you use, set the air tightness to something very tight.The default settings for "typical" in most load tools assumes a much leakier house than would be tolerable for comfort (or even keeping the pipes from freezing) at 8300' in Colorado. Air leakage default settings are the single largest oversizing error for most freebie tools.

 

[Bob1958]

Thanks guys! My attic is insulated 6 in or so. But I think I am pretty leaky. House was built in late 70s. I will play with the heat loss tool.
I have been thinking about keeping my present water heater until it dies, and put in just a boiler, and scrap the combi idea. I just don't want to cut another holle in my fairly new roof.

 

[NY_Rob]

FWIW- mod-cons can't share a common vent with any other appliance if that's the case here...

 

[Dana]

Thanks guys! My attic is insulated 6 in or so. But I think I am pretty leaky. House was built in late 70s. I will play with the heat loss tool.
I have been thinking about keeping my present water heater until it dies, and put in just a boiler, and scrap the combi idea. I just don't want to cut another holle in my fairly new roof.

It's common to side-vent mod-con boilers through a wall rather than running the venting through the roof.

It's common to heat hot water with a mod-con using an indirect fired tank that has a heat exchanger coil rather than it's own burner & flue. When the indirect is controlled as the priority zone, it gets the full output of the boiler, for very fast recovery. The output of a 60-80KBTU/hr mod-con is about twice that of typical 50 gallon water heater. Even though the zone controller suppresses space heating whenever the indirect is calling for heat, the recovery time is fast enough that it doesn't really matter (that is, unless you have a dozen people showering in rapid succession on the coldest hours of the coldest night of the year.)

 

[Bob1958]

Ok based on the whole house heat loss Calc, My heat loss estimate is 42672 BTUs per hour. That sounds like alot when I think I have somewhere between tight and medium seal. I used Alamosa's outside temp of -16 as most of our temps fall above that. Most of our night temps Dec-Feb are below Zero. So now that i have the 42.6 KBTU per hour number how does that fit? My slant fin like you said would max out at 39K

I can side vent, I'd need to run it about 25-30 feet horizontal.

That NTI 153 P I was talking about has a 16 gallon reserve tank (Plus)for hot water as well, it is an indirect water heater-the picture looks like a coiled or looped array. ARe you suggesting a seperate unit that is not a conventional water heater? As you'all suggest tho' I don't need that much (153) juice. My wife points out I need at least 2 zones-one for upstairs and one downstairs, esp. if I still burn wood some.

Way too much research!!!
Thanks again!

 

[Dana]

The 99th percentile temperature bin at the Alamosa airport weather station is -11F. The -16F must be the 99.6th percentile bin (which is sometimes used, but not really necessary for residential.)

For an 2200' house a load of ~42.6K @ -16F is credible, but might still be higher than reality. The energy use load calculation would tell you how tight the house really is, if it were possible to track the wood and wood stove efficiency anywhere near as accurately as the electricty use (which is hard, to be sure.)

With a design heat load of ~40K and a min-fire output of about 14K for the 153P it's do-able, but not ideal, and a potential short cycling disaster if broken up into multiple zones, since each individual zone would need to be able to emit the 14K at condensing temperatures. If you're really planning on only 67' of baseboard and cutting it up into co-equal zones of 17' each, the individual zones would only emit 3400 BTU/hr at condensing temperatures which is only 25% of the min-fire output, of the boiler. That's a short-cycling disaster. Any zones are shorter than 50' (10,000 BTU/hr out at condensing temperatures) are a real short cycling problem for a boiler that can only dial back to 14,000 BTU/hr out.

The 16 gallon tank is only storing domestic hot water as a small indirect-fired tank, which is fine, but it's not a heating system buffer that would help suppress short cycling. The range of temperatures used for heating with a mod con are both higher and lower than what would be appropriate for domestic hot water, and even if it had the internal controls to use it as a buffer tank (which I don't believe it does) it wouldn't be a good idea to use it that way. For control of legionella it's normal to store potable water in tanks at 140F, which is above the condensing temperature of natural gas exhaust.

 

[Dana]

BTW: It's good to have at least enough radiation to emit the load at the 99th percentile temperature bin at an AWT of 140F. With that much radiation the boiler operates in condensing mode at least 90% of the time, and pretty much hits it's advertised AFUE efficiency numbers. With less radiation than that it can't, since it doesn't condense enough of the time. For baseboard an AWT of 140F delivers about 300 BTU/foot of baseboard.

So for a 40,000 BTU/hr load you're looking at 40,000 /300= 133' of baseboard, which is literally twice the 67' you were planning on.

With 133' of baseboard divided into 4 equal zones you would have 33' of baseboard per zone. That still pretty stubby, with substantial short-cycling potential for a boiler with a min-fire output of 14,000 BTU/hr like the Vmax 153P, but would be fine for boilers with a min-fire output in the 7000-8000 BTU/hr range. (HTP UFT-080w, Lochinvar WH-056 or WH-086, etc.)