High LP use with Bosch 151?

Gcracker · December 2017

I posted here once previously (below), as my new modular and tightly sealed home (1100 sq ft) with a new Bosch 151 single zone FHW into +/- 30' of copper baseboard that seems to either misconfigured, misplumbed, or otherwise wrong. My two 120g LP tanks were filled on 11/22/17 and today (12/28/17), they are below 20%. The LP is used exclusively for the combi.

I am concerned that this seems to be fairly high usage for such an energy efficient system, in a well-insulated and air-tight home. It directs my attention again to my combi, and whether it was put in properly.

I am wondering what the experts think is normal LP usage for this system in an average modular very tight and insulated home with average temperature of 22.4-degrees F (Avg low of 13.3). Does ~150gal of propane for 30 days seem right?

My builder and the executor of any warranty service says it's normal, and that I am more likely to bother the HVAC installer than find a problem, but, as the below post details, something doesn't seem quite right. I still can't seem to alter room temperature unless the kitchen kick space heater is on.

Do some HVAC companies do quick visual inspections to help a homeowner determine if there is a plumbing or sizing problem?

Thank you for any insight you can lend.

https://forum.heatinghelp.com/discussion/163363/pri-sec-pump-mismatch-or-gravity-back-flow-homeowner-needs-help-long?

Jamie Hall · December 2017

Did anyone do a heat loss calculation for your house? If they did, with that number and the design temperature used, you can determine quite easily what the heat loss per hour per degree is (just keep dividing!). Then with that number, you can get the expected heat loss for your average outdoor temperature by multiplying that number by the delta T.

Once you have that, you can compare that to the amount of gas used -- which is somewhere, in your case, around 20,000 btu per hour. Comparing that to the expected heat loss will give you a pretty decent estimate of overall system efficiency.

(Considering today -- it's -4 where I am -- I wish I could come even close to that -- I'm running through close to 150,000 BTU per hour!)(of course, I have about 7 times the floor area, too).

EBEBRATT-Ed · December 2017

Download the Slant Fin app and do your own heat loss for the first step. Guessing doesn't work

30 feet of baseboard will only give you less than 20,000BTU/hour.
To me this could be kind of light for 1000 sq feet but like I said, no guessing do the heat loss.

You may be short of baseboard

How many BTUs does a Bosh 151 do??

hot_rod · December 2017

150 gallons for 30 days? That is 5 gallons per day.
If that is a 150,000 input, at full fire it consumes over 1-1/2 gallon per hour. A gallon of lp at around 90,000BTU.

Ironman · December 2017

You could have a leak on the propane tank or line.

I agree about the lack of BB. 30' is gonna give you 15k btus output with 170* AVERAGE water temp. But why in the world would you want to run a mod/con at high temp like that? You're loosing all sorts of efficiency and defeating the purpose of the mod/con.

You're also experiencing the flaw of most combi boilers: in order to produce enough domestic hot water, they have to be grossly over-sized for space heating. The boiler is 151k btus and you only have enough BB to emitt 15K!

Solid_Fuel_Man · December 2017

@Ironman I'm glad someone said it. 30 feet? Get more baseboard and lower the temps and I bet your fuel usage will go down. Because 20,000 BTU's per hour at 22.4 degrees is really high for 1100 feet.

For perspective my heatloss for 3,200 feet at -40f is 40,000 btu.

Jack · December 2017

The third rail in this discussion is the use of a combi in low heat load systems. Your minimum fire of 36kbtu probably substantially exceeds your actual heat loss...at design condition. The Combi is always selected for DHW loads, and well, its has plenty of capacity on the heat side. In this case probably around 7X. How efficient is a your boiler at low fire, because it is rarely, if ever getting off minimum fire. I agree that more baseboard will help, but you are still putting a bandaid on a grossly oversized heating system.

Gordy · December 2017

Gentlemen, this is a classic case of slapping a high efficiency boiler in a high efficiency envelope with a low efficiency emitter. The installer, or builder didn't have a clue as to make a high temp emitter work with lower supply temps to get the boilers rated efficiency.

The clues are in that outdoor reset, or supply temp sensor were not even installed.

Put a HE boiler in get the rebate done. Such a waste of equipments possible potential.

However with a combi you will suffer from over sizing on the CH side to get the output for DHW, but there are some combis out there with higher tdrs to combat this scenario.

Your builder, and installer are praying on your lack of knowledge. Your getting a 90 plus boiler that may never see mid 80's efficiency until some changes are made. More emitter, implement odr. Get the return temps to the boiler below 130 to condense, and pickup efficiency points.

lchmb · December 2017

I was at a home recently with the same issue. An NTI boiler hooked to two air handlers. We have seen some colder than normal weather but these folks have used 150 gallons of LP to keep the house at 63 degree's in 3 weeks... The installer left the heat curve for a radiant zone which made it run non stop using up plenty of gas..

rbeck · December 2017

With all the calls I take everyday, every week all year long I may be off, but I bet that 80% or more of all boilers are 100% oversized.
I would also bet that 30% or more of all outdoor reset systems have either been not installed, disconnected or adjusted to always shoot for a much high temp than needed.
Oversizing LP units affects the fuel bill more than oil or natural gas.
You are right, most combi's are way oversized for heating and should be limited to the proper applications. The only way in most cases the combi has any chance of working properly in both applications is if it is a 10:1 turndown.
If you size a combi for heating the house there is usually a higher hot water demand than combi's can supply.

ced48 · December 2017

In a slightly smaller home, with a 55K boiler, I use 50 gallons of propane per 1000 degree days of heating demand, with domestic hot water included.
I think you need about 3 times the baseboard that you have, and your boiler needs to be able to cycle down to less than 15,000 btu's to even begin to do a decent job of heating your new house,

GW · December 2017

Not too many variables-

you're leaking fuel

Your LP company is ripping you off

Another appliance or appliances is/are gobbling fuel

your combustion is poor and that needs to be addressed pronto

Your burning cleanly but the boiler is puttering away cycling like a dog, wasting energy

Your circulation is poor, can't move the energy.

Your heat emitter system is needing some luv, poor heat transfer.

your home is leaking heat at an abundant rate, any icicles?? Attic access open? Your snow looking good on the roof? better or worse than the neighbors?

or, last but not least, a combination of the above

Normally speaking, fuel burned equals heat generated.

Edited....I have heard that LP company theft exists, probably does and probably the nasty 1% of our society

EBEBRATT-Ed · December 2017

@Gcracker said

" I still can't seem to alter room temperature unless the kitchen kick space heater is on"

That's my clue that he is short on baseboard for his heat load.

Yes, their are other issues maybe the wrong boiler and maybe on set up properly etc.

Gordy · December 2017

^ it's a new 1100 sf home with 15 k of baseboard. minimum input modulation on that boiler with lp is 46.4 k. Cycle haven. No condensing. If it ran at min.

1 gallon of propane has 91333 btus
150 gallons in 30 days. However I don't trust % gauges on propane tanks as accurate measurement.

That's 13,699,950 btus for 30 days
456,655 btus per day avg.
19,027 btus per hour avg.

.21 gallons an hour avg. consumption. 4.99 gallons a day.

Hot water is not accounted for.

Sorry one must consider the whole system not just the boiler. No a btu made is not necessarily going to what it's made for. Out the vent.

EBEBRATT-Ed · December 2017

@gordy, I agree the builder didn't do the right thing here. Multiple problems

ced48 · December 2017

I just read more of the provided information on this install. I am not a professional installer, just a retired home builder that has installed a few heating systems, including my own Lochinvar WH55. Yet with my limited knowledge of heating systems, I can say without doubt, this install is a total disaster. Don't waste any more time and money, get you builder to replace the entire system. The work that you paid for should be grounds for this so called pro to be stopped from working until he is taught how to do the job

Gcracker · December 2017

Thank you very very much! You all have given me very much to think about. I will download the slant fin app and do a heat loss calc. Beyond that, I'm afraid the builder/contractor are going to give me the run-around rather than do any of the necessary changes.

I am hoping to be able to come to them with some hard facts, and this thread gave me very much info to help me with that. I think the roughly 15k max of 30' of baseboard, and the min load on the combi of 46.4k means it is always cycling.

I also mentioned the lack of the ODR or the factory-recommended and included room thermostat to the HVAC contractor, who said "your system is too small to need that ODR, and the Bosch Comfort Sensor is for a different type of system." He gave no explanation for these opinions, however.

A complicating factor is that these modular homes are usually built for a buyer to spec, and this one was built to sell. Many of the features and decisions that are usually made by the home-buyer were either made by a random uninvolved contractor, or not at all. I don't know if the HVAC contractor considers me the customer, or the builder. I certainly don't feel served, but I won't give up.

One last question raised by a comment here: Should I be trying to run lower FHW temps for the condensing boiler? I saw a reply that said to shoot for return temps around 130F. Is a rule of thumb that I should run the lowest FHW temp possible to heat the space?

Thank you again, and I will check back in when I have my calc figures. I am hoping to find that adding a certain number of BB feet will help.

Gordy · December 2017

Sadly the hvac contractor’s loyalty ended at the builder’s wallet, until the builder puts the pressure on. As far as he is concerned it’s between you, and the builder.

From what hvac contractor has told you solidifies my assumptions as he is incompetent. I would ask for his card so you can “not”recommend their services to others......

EBEBRATT-Ed · January 2018

@Gcracker to answer your question with condensing boilers the lower the water temperature they run at the higher the efficiency. Return water at 130 or below lets the boiler condense and efficiency go up.

Because your probably short on baseboard you can't lower the water temperature...you won't get enough heat. Basboard output changes with water temp the higher the temp the more heat you get but boiler efficiency goes down

Brewbeer · January 2018

The more baseboard you have, the lower the temp you can run the boiler and meet the heating load need.
The boiler is another issue, if the lowest your boiler can produce is ~45k, that is always going to be a problem with a house that has a design heat loss of ~20k, or less.
If re-doing the baseboard is possible, this is the option I'd recommend you pursue. Install as much as you can. Use a high output baseboard.. Offer to pay the cost of materials. Do the room by room heat loss calc and post it, we can help you size the radiation.

Gordy · January 2018

You can only do so much with that size boiler. Adding emitter helps, but the loads will still be satisfied quickly even at a lower water temp. Buffer tank could help.

NY_Rob · January 2018

From what I have read, new modern homes built to recent code standards can have very low heatloss... like 10BTU's/Sq-ft or even less.
Your "tightly sealed 1100 sq ft home" could realistically have only 11K BTU's heatloss on the coldest days of the year, and considerably less on less than frigid days.
30 ft of baseboard (assuming 170F Average Water Temp) is probably sufficient.

A mod-con with a minimum output of 36K BTU's is outrageously oversized for this application and will be a short cycling disaster with a short life.

With such low heatloss, there's literally no Mod-Con boiler (running at efficient return temps in condensing range) that won't short cycle on all but the coldest days.

To "do it right" you probably need to almost double your radiation, and replace the boiler with a 10:1 high turndown small mod-con like the HTP UFT-80W which has a minimum output of 8K BTU's.

Just to keep things in perspective.... a modern gas stove top burner puts out 8K BTU's.....

Gordy · January 2018

The lochinvar cadet cdn 40 is a nice little unit with an optional dhw module, or just go indirect. Bare bones odr, 5:1 tdr low end 8k output. There are options that don't involve 10:1 tdr out there. In the right situation. Add an indirect of sufficient size. If I don't use more than 40 gallons of dhw in any of my hot water usage patterns, why do I care how fast it makes it.

Solid_Fuel_Man · January 2018

I honestly don't know why combis are such the rage. I small boiler with a decent size indirect will work much more efficiently, and last longer in all but extreme cases. I just don't get it.

I ordered two 70,000 BTU boilers and it took over a week to get them. "Oh, would never sell a 70,000 BTU boiler, we have the 110,000 and the 155,000 here though, are you sure 70,000 is big enough?"

Yeah.......

hot_rod · January 2018

Solid_Fuel_Man said:
I honestly don't know why combis are such the rage. I small boiler with a decent size indirect will work much more efficiently, and last longer in all but extreme cases. I just don't get it.

I ordered two 70,000 BTU boilers and it took over a week to get them. "Oh, would never sell a 70,000 BTU boiler, we have the 110,000 and the 155,000 here though, are you sure 70,000 is big enough?"

Yeah.......

I was thinking the same about combos, WHY?

But now I live with 3 of them

The one at my mother in laws is a 16 year old Laars Mascot, 160,000. I have opened it twice for cleaning, never replaced a part or had it fail. I believe it is a rebadged Baxi.

I also have a Lochinvar Cadet in the shop and a Nobel in my home. Properly applied they work great. Only the Cadet in my home is close to the correct size for heating at design, still I included a small buffer.

In the shop I have a portion of the 500 gallon wood buffer connect to the Cadet, so it never short cycles with a 30K design load.

The nicer combi models have ramp delay and electronic limiting to tame the heating side down to 20% of max fire. The max fire of course for DHW.

So they are appropriate in many applications where small space dictates a single unit, or the loads match, or you buffer and limit their firing.

The combis are opening some interesting hydronic markets for small, tight space applications.

ced48 · January 2018

Seems to be enough of a demand for a 30k boiler. I wonder why the manufactures are not stepping up to fill this void?

Solid_Fuel_Man · January 2018

I've said that too, a 50,000 BTU 10:1 would be a great boiler size IMHO.

I bet that a manufacturer would not sell many though, I hope I'm wrong about that though.

Gordy · January 2018

As older homes get replaced, and or upgraded to low load homes that market will shine. Supply needs demand.

Gordy · January 2018

With low load homes it becomes questionable if HW heat of any kind is worth the cost. The advantage however is alternative green energy to heat the water if such a method.

Canucker · January 2018

I've thought of going down that road @Gordy I would love to add my detached garage as a separate zone from the house. It gets a lot of Sun all year and I would think solar heat would be nice for shoulder seasons, fed from the garage. @Mark Eatherton has me thinking of using the driveway as a collection unit

Gordy · January 2018

And that is the advantage of HW heating, and DHW. You have options to heat the water. Solar, wood, electric, gas,etc. solar is the biggy, many ways to harvest, and store that energy when available for later use. Same with wood.

Gordy · January 2018

Radiant is the lowest required temps to utilize those options, and extend usage by mixing down when panels are done correctly to utilize the lowest water temps possible.

The ceavate is in low load homes you may only need just above set point water temps. So the love of warm floors goes away. Then it becomes when does radiant make sense.Neutral should be the goal no matter what anyway.

I think radiant ceilings in a low load structure reaps the rewards of cheaper panel assemblies, and not having to grief about not having the floors warm, floor covering hassles,cabinetry, and furniture. Ceilings gives the abilities of higher tube densities than floors to keep those water temps low.

Gcracker · January 2018

I thank you for all your comments, and even the off-topic market descriptions. This all has helped me immensely. After work, I will be able to do my heat-loss calculations, and will post back here.

I have two questions:

High-output emitters; can you point me in a direction to research those? Of course I will google it in the meantime.
Do you think the possible design flaws in the plumbing of the Pri/Sec loop (>4" separation) can be affecting this as well?

I am new to the Pri/Sec concept, and at first, I just couldn't wrap my head around the idea of not having the flow always directly through the exchanger. Now I think I understand that concept, but am not sure it has been implemented in a way that serves the primary purpose of sending hot water out to my baseboards and getting cooler water back to the boiler to heat. I will take new measurements since my lowered pump settings (described in my original post, linked in this post), but it still seems that radiators alone should be able to heat the space, without a kick-space heater running at full bore, and that my return temperature shouldn't be so drastically different. Also of concern is that, if the p/s loop is poorly plumbed, could it be pulling it's own hot ware back into the return side instead of sending it out to the baseboards?

The following is not important, and just my rant/vent/opinion/wishes/dreams:

I really wish I could just say, "Something's wrong here," and the builder and HVAC folks would do their magic and help.

I'll stay optimistic and wait for my heat-loss figures. Perhaps my home really does only need 10 to 15k BTU. Yesterday was -25 in the AM, and climbed up to 9 by 3pm. This morning, it was -10. The house did not drop temperature overnight, so it can maintain temp, but the need to have the kick-space heater fan on high makes watching TV difficult, not to mention it seems like it is supposed to be an accessory, not a required component.

I have seldom-used rooms with doors closed and baseboard vents closed completely to help get more heat into the main spaces. I think the last time I looked at the Combi special functions, I saw that it is always running at 30% on the FHW side. I would guess that is its lowest possible modulation. I don't know if my flow rates are right, if I have enough baseboard, or if the plumbing was done properly. I only know that I have filled our tanks three times now, and we expected to be using roughly half the energy we have used so far, for the entire year, based on our research and the claims of the builder. January 2 and we've already spent $1,350 on propane.

Oh, one last question: When we moved in, we were horrified to discover that this combi can't modulate low enough for DHW when we shower, so we get hot/cold cycles after the first 2 to 5 minutes of showering. The builder/plumber/HVAC folks came and removed the water saving shower heads, and told us to take two showers at a time, if possible. Does that seem like another flaw in these combi units? the inability to heat a DHW flow less than 2.7 gpm seem like a flaw with shower head rated for <2gpm, and mixers set at some hot and some cold would mean you are rarely flowing even 1.5gpm through the DHW system when using a water-saving shower.

Thanks for all your help! I wish the folks taking my money were half as helpful as you lot of strangers!

ced48 · January 2018

I don't know how to say it more clearly, you have been screwed-Your heating system can not work properly with the components that were installed, all you can do is make it work better, but not right. If your builder will not help, hire an attorney.

KC_Jones · January 2018

Gcracker said:

Oh, one last question: When we moved in, we were horrified to discover that this combi can't modulate low enough for DHW when we shower, so we get hot/cold cycles after the first 2 to 5 minutes of showering. The builder/plumber/HVAC folks came and removed the water saving shower heads, and told us to take two showers at a time, if possible. Does that seem like another flaw in these combi units? the inability to heat a DHW flow less than 2.7 gpm seem like a flaw with shower head rated for <2gpm, and mixers set at some hot and some cold would mean you are rarely flowing even 1.5gpm through the DHW system when using a water-saving shower.

Thanks for all your help! I wish the folks taking my money were half as helpful as you lot of strangers!</p>

It is a flaw of the contractor who clearly has no clue what they are doing. People come on here a lot blaming equipment, the reality is most of the time it's a contractor problem.

The contractor chose the wrong equipment, and applied it to a poor design and doesn't understand what they are installing. You have a contractor problem plain and simple.

Personally I would hire a knowledgeable contractor to do a consult on what the proper design should have been, then hire an attorney to go against the installing contractor. The results are either he pays to have a proper system installed by someone who knows what they are doing or they rip out what you have to install the proper system.

And people wonder why contractors get a bad reputation. This is a perfect example.

NY_Rob · January 2018

^ Like my bro in-law who lives in upstate NY says "around here, to be a contractor all you need is a puckup truck and a magnetic sign".

His last wood burning stove "contractor" burnt his house down because he never secured the flue pipe and the sections disconnected from each other inside the exterior wall!

Permits, training, certifications... lol... got a truck? You're a contractor

KC_Jones · January 2018

NY_Rob said:
^ Like my bro in-law who lives in upstate NY says "around here, to be a contractor all you need is a puckup truck and a magnetic sign".

His last wood burning stove "contractor" burnt his house down because he never secured the flue pipe and the sections disconnected from each other inside the exterior wall!

Permits, training, certifications... lol... got a truck? You're a contractor

I live in PA, we didn't have state wide building code until 2003. Before that......

Ironman · January 2018

The boiler should be able to fire and modulate with as little as .5 gpm domestic flow.

Again, you have an incompetent, dishonest contractor.

Gordy · January 2018

The whole purpose of the lesser gallon per minute shower head is to be able to do more DHW applications at the same time. They told you to remove them, and take two showers at the same time..........SMDH

Gordy · January 2018

As far as high output emitters. There is various ways to go about this. Panel rads are very elegant in my opinion, and can be sized for a specific water temp. Towel warmers are another elegant emitter.

Then there is high output base board. Or add more of the same to what you have. The more emitter the lower the water temps.

Gordy · January 2018

Primary secondary piping with closely spaced ts, or a hydraulic separator simply put allows two systems to operate at different flow rates with out interfering with the flow rate of each other.

With a mod/con boiler the goal is to always have the system side flow (secondary) greater than primary side (boiler). This assures that the boiler will always get the lowest return water temp possible for better boiler efficiency.

High LP use with Bosch 151?

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