Stupid homeowner questions.......

Generally, Deirdre,

Low-Mass high efficiency boilers are an excellent way to produce abundant hot water in summer. Older style high-mass cast iron boilers by comparison take a while to ramp up to temperature so you can see the inherent inefficiency in that. My own boiler (Monitor MZ) has low water content and low mass so it can heat my tank from a cold start in about twenty minutes (tank at 50, water to 130). Once the tank is warm the "touch up" takes five minutes if that.

I will let others opine about the specific boilers you are considering. Each has their following and their detractors (cars, neighborhoods, airlines - everything has an opinion attached to it!). The others who may comment tend to be on the installation and service side, so see the full array from installation to warranty issues, service and support.

Hope this helps-

Brad

Trinity

The Trinity is a low mass boiler with a plate type heat exchanger built in for the domestic hot water. You are correct in saying that the boiler will need to heat up in order to generate hot water, but that boiler only holds about a gallon of water (give or take), so the heat-up is meaningless.

It takes a LOT of energy to "instantly" produce domestic hot water.

It takes an ENORMOUS amount to "instantly" produce domestic hot water in quantities greater than say one tub, one clothes washer or one dishwasher combined with anything more than a simple lavatory.

The flat-plate heat exchangers attached to some boilers definitely work--just make absolutely certain that you're aware of the flow limitations and NEVER oversize such a boiler just to meet the DHW load! In other words the boiler is FIRST sized PROPERLY to meet the space heating load and such sizing will define how much "instant" hot water can be produced.

Most modern high-efficiency boilers (and by "high efficiency boiler" I'm NOT referring to a conventional design with a flue damper and pilotless ignition) use what is called "Domestic Hot Water Priority". This means that the FULL output of the boiler is devoted to producing DHW when such is needed. The logic behind this is that DHW demands are usually short in duration and the house won't have a chance to drop too much in temperature while DHW is being produced. DHW priority can however be a serious problem if you've set your thermostat(s) down at night, turn them up when you wake, and expect to "get ready using DHW" at the same time the house is RECOVERING from setback!!!!

The solution to the problem of DHW demand higher than can be produced by a boiler SIZED TO THE HEAT LOSS OF THE HOME is storage. This is called "indirect" domestic hot water. Remember though that your high efficiency boiler wants to give priority to DHW. This means that the heat exchanger in the indirect tank must be capable of transferring heat from the boiler just as fast as it can be produced at maximum boiler ouput. This ability does not come without significant expense--think $$$$ because heat exchanger surface does not come cheaply! For that reason, I recommend only the finest quality indirects that will likely outlast you AND your children. The heat exchanger in my big horizontal Viessmann tank looks like the bottom and sides of a coffin made from 1 1/2"? stainless steel tube--if my body would fit through the 4" or so inspection hole, I'd say bury me there...

The typical alternative is a stand-alone DHW heater. They're cheaply made, short lasting, poorly insulated and terribly inefficient with natural gas, but they're cheap and easily replaceable by even a basic handyhomeowner.

Dear D

I admire your determination to understand what you are buying. Keep it up! You'll get it, especially if you hang around here and keep asking "stupid homeowner questions"

I would be a little concerned with the DHW output of the Trinity for a house the size of yours. You probably have at least a couple bathrooms and if by chance both showers were being used at the same time you would likely run out of hot water. If you can control your hot water usage to a point where you never use more than 3-4 gallon per minute it may work OK for you. I have seen however, far more people disappointed with this type of hot water production than not.

I feel that a good quality indirect water heater fired with a high efficiency condensing boiler is about as good as you can get. This would be considering recovery time, initial dump load, (filling a tub while running the washing machine etc.) and system efficiency.

IIRC you were working on a two temp system using radiant floors and a higher temp for rads or hydro air? If so, copy and paste this address and go to page 29. It will show you a piping schematic of how an indirect ties into a boiler and give you an idea of how it works. This particular drawing is from the Vitodens installation manual but the concept would be the same for other boilers also. You can also see the various components that are required to achieve the multiple temps needed for your system. It's not as complicated as it looks. Just follow the water flow through the system as it leaves the boiler on the left. Please feel free to e-mail me directly. I'm a little far away to do an install for you but if I can help you with some advice I'd be happy to give it. I'm picking up a pretty strong vibe that you are trying hard to sort out the wheat from the chaff. Keep asking questions.


http://www.viessmann.ca/web/canada/ca_publish.nsf/AttachmentsByTitle/docVitodens200big-ii.pdf/$file/Vitodens200-44-60-ii.pdf

Being that this is the whole manual, the file is fairly large (2+Mb) so it may take a while to download if you don't have hi speed.

The Trinity boilers are fine, not the best, but fine. I have installed many of them with no more problems than I have with any other boiler. A Buderus is a good step up. I would use a storage tank to store your hot water. I do not think much of the combo unit. Use it only if you do not have room for a tank.

Dave in Denver

Residentially when driven by a high-efficiency boiler, yes.

Deirdre- Storage of DHW

I am glad the others responded as they know the equipment far more intimitely...

I did not know that the Trinity as described had an integral plate exchanger for so-called "instantaneous" HW production. Personally I do not prefer that method.

Rather, I was thinking (forgive me, assuming) an indirect; a large coil within a storage tank.

If I had a boiler with an integral plate exchanger, I most certainly would install a Super-Stor or other storage device. The flow rate of the plate exchangers is rather fixed and geared to one major appliance (laundry OR shower) at any one time, but not both.

What I love about The Wall: Like several coats of paint, everyone chips in until things are covered.

Brad

The last word

Vitodens.

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

I get the impression that we are supposed to size the boiler to solely the heat load-- but if that is the case, how is the hot water being supported?

I believe your house is large and old, correct?

If so, ANY properly sized boiler with have quite a lot of output--just not enough to instantly meet a domestic hot water demand of much more than a single hot shower.

Indirect domestic water heaters just look like tanks. But instead of having a burner and a flue (like in yours), they have an internal heat exchanger the connects to the boiler.

As mentioned, most high-efficiency boilers produce domestic hot water on priority--DHW gets the FULL attention of the boiler when called for. Just like your current water heaters, the boiler cannot meet a large demand INSTANTLY, but water hot water is stored in the tank, drawn from the top with replacement water going to the bottom. Because hot water rises--just like hot air--you get significantly more hot water than can be instantly produced. The capacity of the indirect domestic water heater should be based on a realistic estimate of peak demand. And again, since the indirect is usually supplied via priority you want an indirect that can instantly absorb the FULL output of the boiler.

Be forewarned that if this system uses hydro-air and if you set back the thermostat at night and expect the house to warm while everyone is doing their morning business, you will probably be very disappointed.

Steve

Your "blending" sounds like a good application by a great and practical engineer. I was thinking of entire zones heated solely via hydro-air where most or all circuits are frequently running at the same time.

Size the tank not the boiler

If you know going in that you have a big dump load of hot water such as a two person tub or that the scenario you brought up occurs regularly, you account for that with the tank volume. Sizing the boiler to provide 5, 6, 7 GPM will nearlly always result in severe oversizing. Best to allow for that with the tank and let the boiler do its thing at a normal pace.

The Vitodens has a sweet feature that will keep it in condensing mode nearly 100% of the time while making domestic HW. It can be programmed to maintain a temperature a certain level above the domestic tank while in DHW mode. I think 36* is the default. So if your tank is at say 90*, the Vito will fire to 126*. After hitting that 36* differential target, it will modulate down and keep that same delta T as the tank temp comes up. So, if you have a tank setpoint of 120*, the Vito will finish off at 156*. With a normal 20* drop, that's still right at condensing temp range. After hitting the tank setpoint the Vito will purge the excess heat from the boiler into the DHW tank and reurn to heating mode. Nice.

First thing to figure out is how many BTUs you're going to consume in that hour.

Let's say the top of the indirect where you draw the water will average 130°F for that hour.

Then let's say that the cold water is 55°F.

I consider 115°F a hot shower--some like it hotter--but I'll still use 115°F.

Assuming you're using a single, standard showerhead and haven't removed the flow restrictor, you get about 2½ gpm for a shower.

(130° * 2 gpm) + (55° * 0.5 gpm) = 287.5

287.5 / 2.5 gpm = 115°

So, you need 2 gpm of hot water @ 130° for your 115° shower.

2 * 15 minute showers = 30 minutes.

30 minutes * 2 gpm = 60 gallons.

60 * 8.33 pounds per gallon = 500# of water.

Each pound had to be raised from 55°F to 130°, a difference of 75°.

75° * 500# = 37,500 btus in 30 minutes for the showers. To meet this load instantly the boiler would have to transfer heat at a rate of 75,000 btus per hour.

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Now, let's add in the washing machine. I'll assume two full loads--one with "hot" wash, one with "warm", both with "cold" rinse.

Wash cycle will take about 15 gallons of water. I'll assume the solenoid mixing valve will flow 3 gpm straight "hot" and 5 gpm (2.5 each) "warm".

Your hot load will fill in 5 minutes. 15 gallons * 8.33# per gallon = 125#. 130° - 55° = 75 btus/#. 125# * 75 btu = 9,375 btus in 5 minutes. "Instant" satisfaction would require 112,500 btus/hour.

Your warm load will fill in 3 minutes. 7½ gallons of this are hot. 7.5 * 8.33# = 62.5#. 130° - 55° = 75 btus/#. 62.5# * 75 btu = 4,688 btus in 3 minutes. "Instant" satisfaction would require 93,760 btus/hour.

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So, the peak flow will be 2 gpm for the shower + 3 gpm for the clothes washer (hot load) = 5 gpm. Using the btu/hour rates required for each this would mean that for "instant" satisfaction, the boiler would have to transfer energy at the rate of 112,500 + 75,000 = 187,500 btu/hour. Unless this is an extremely large and/or extremely "leaky" home, a properly sized condensing/modulating boiler will likely have less than half of this output ability.

The solution to the problem is storage. How much?

Let's say the boiler is capable of transferring heat at the rate of 75,000 btu/hr to the indirect while operating at nearly full output.

In this "peak load" hour, we consumed 37,500 + 9,375 + 4,688 = 51,563 btus. WELL within the capacity of the boiler to fully "recharge" the indirect. At 75,000 btu/hr it would take the boiler about 41 minutes to replenish.

You might be thinking, "I don't need storage." WRONG! During one 15 minute period (shower & hot load of clothes) you consumed 18,750 (shower) + 9,375 (clothes) = 28,125 btus but the boiler could only transfer 1,250 btu/minute * 15 minutes = 18,750 btus. There is a deficit of 28,125 - 18,750 = 9,375 btus.

Where do those 9,375 btus have to come from? Storage!

Each pound of heated storage water has 130 - 55 = 75 btus available for use. 9,375 / 75 = 125# of water. 125# / 8.33 = 15 gallons.

Does that mean you only need a 15-gallon indirect tank? NO! Why? Because as you use the hot water in the tank, it's being replaced with cold and that water can't be heated to temperature as fast as it's being added! Fortunately, hot water naturally rises in a tank. The fact that you draw from the top and replace at the bottom enhances the effect as the water doesn't get too much opportunity to "mix" in the tank--it stratifies.

Let's say that the top 1/2 of the tank stays well within temperature. That means you need 15 * 2 = 30 gallons of storage. (Would those with more experience please comment on that "top half" estimate?)

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Let's say now that those two 15-minute showers occurred simultaneously and see how things change.

37,500 btu required for the showers. 18,750 btus available from the boiler for a difference of 18,750 btus from storage. 18,750 / 75 = 250#. 250# / 8.33 = 30 gallons. 30 * 2 = 60 gallons of storage.

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VERY important to remember that these calculations assumed that heat at the rate of 75,000 btu/hr can be transferred from the boiler to the indirect! This is well within the ability (even at a quite low and efficient supply temperature) of the 42-gallon (smallest) Vitocell-H 100. But not all indirects are Viessmann... Check ratings! With all else equal, a lower heat transfer ability will require more storage. This is what I was talking about when saying that the indirect should be fully capable of instantly absorbing the full boiler output!!! If it can't you need more storage and much more importantly this means that if domestic hot water priority is used the boiler will have to devote itself to DHW for a longer period of time. While this is happening you do not get space heating!

Your great response should be in hot tech topics

Thanks so much. Hope Deirder's still with us. I'm inspired to go right now and buy another $10 brick to support the site. Also for Mr. Ebels post on same subject and Brad's post on an AC thread that explicitly outlines how to AC a basement with a portable unit. Hope homeowners appreciate all this free expert advice.

Thanks, I'll be digesting your post later this afternoon.

David

The large size of your 80 gallon indirect I guess allows you

to keep the water at 120deg (less fuel usage), rather than a 40 gallon tank at 140+ degrees yes? Then you don't need an anti-scald valve. Some on the wall recommend keeping water at 140 to avoid Legionaire's etc. Is that a widespread recommendation? Does the anti-scald cut down on water flow as I've heard? (though the higher temp increases output I know since you can mix more cold water with it.)

Thanks,

David

Thanks All!

I was out of town for a few days and just returned to all the info posted above. Thanks everyone for all the discussion, even if a significant portion is above my head....

I have gathered that you all think I should consider something more efficient than the Trinity and add DHW storage tank.

Can you guys suggest the best combination of equipment that would be a)reasonably effecient b)reasonably priced and c)relatively easily installed by a plumber who also holds an HVAC liscence but who probably isn't up on the latest technology?

I guess the most complicated thing is my bizarre mix of radiators: 600 square feet of pex in concrete in kitchen and downstairs baths (hi use areas that we would like to keep cozily warm), 700 square feet of suspended pex (no plates) under hardwood in formal living/dining (rarely used space), traditional cast iron radiators in guest bedroom, office and sunroom (with bypass valves). In addition, we are considering adding baseboard heat and a hydonic towel warmer on the second floor (currently served by a gas furnace).

Your mix of radiation will certainly complicate matters. I easily see a three-temperature system. Lowest will be the tube-in-slab. Middle will be the iron radiators. Highest will be either the suspended tube or the baseboard but they are unlikely to be both well-served by the same curve even if they can use the same maximum temperature. In that regard it's almost a four-temp system...

Search and search for a knowledgeable pro--it will take such to even begin to meet your goals of reasonable efficency, price and installation. Do be aware that you're talking about a rather complex system. Complexity adds to price (materials, design and labor) so meeting your other goals means that "reasonable" with regards to price is relative to that complexity.

maybe that's the way--hire a wallie to design

and have your local guy do it. from the wall and elsewhere you can probably get photos to help you out. And your taking photos of certain areas would help in the design and to check at various stages of completion. Perhaps it might be worth it to pay that 100 mile away wallie to come out once to survey. Or someone he knows near you. I think you figured out your own solution.

Best,

David

Before you blame the lack skill of the locals, don't forget that the system you propose as well as your stated goals require a very high degree of hydronic artistry. There will be many ways to "balance" efficiency, comfort, cost and complexity in this system with concessions made to at least one of those criterion.

Please understand that I offer the following as a sincere assessment--I'm not chiding you. I'm just trying to help you see how you must consider everything as a working, complete system.

It appears that you're creating difficulty for yourself. Isn't the slab portion new or a planned addition in the short term? If so, your choice of tube-in-slab complicates matters greatly. You're adding yet another wholly different form of emitter to a system that already seems to be pieced together.

Did you inheret the heating system in its current form? If not, what have you added/changed? The bare tube radiant floors? If so, again, this was not the best choice considering your cast iron radiators. Placing the tube in nice heavy extruded conduction plates would have been a much better choice even if the initial cost was higher.

Why the furnace on the 2nd floor? Were radiators removed or was this previously an unheated area renovated into living space? Since you already have ductwork there, have you considered a traditional A/C system for the 2nd floor? Depending on layout such may even be able to serve much of the ground floor with relatively minor modifications and only requiring a small (perhaps even a simple mini-split) for your kitchen addition.

Also, did you remove radiators in the areas with the bare-tube staple-up? If so, do you still have them? Since those rooms will be little-used, it may make things simpler (and even more comfortable) if you replace the radiators and abandon the radiant. It might also be possible to use the radiant for a "base" heat level with radiators handling high loads and/or relatively quick heat if you plan on keeping these little-used spaces cooler than the rest of the house.

I do understand your desire for "cutting-edge" technology, but such technology is MUCH more involved than just using cutting-edge equipment! "Cutting-edge" technology is system based! EVERY component must work together to produce order from chaos. With the system you propose it's a tall order just to reduce the chaos caused by your wild mix of emitters--let alone the weather and your comfort desires.

Is this the solid brick three-story Victorian?

Am I correct that the immediate problem is an underperforming system that's extremely expensive to operate?

Even your new boiler is likely GREATLY oversized for just the ground floor when there is occupied and heated space above.

If the current system is set up anything like I imagine (poorly I'm sorry to say), I can see some SERIOUS problems.

Your bare-tube radiant floors want HIGH temperature. If this is the house I'm thinking of and the rooms served are the front ones with the big, curving windows it's likely that the floors have insufficient output potential even at the highest practical supply temperature of around 190F.

What this means is that the radiant floors can't put out too much heat--WAY, WAY, WAY, less than the boiler produces. BUT, the thermostat serving the radiant rooms is likely calling almost constantly as it's rarely satisfied.

When this happens the boiler short-cycles LIKE MAD. This spells DISASTER for efficiency.

In the meantime the portion heated by radiators also needs heat. Considering this is a gravity conversion, as soon as it calls, it will INSTANTLY absorb nearly every available BTU from the boiler. Your return and supply temperature will drop LIKE A ROCK. The radiant portion that needs high supply temperature will now have almost nothing to work with.

One possible way to deal with your radiant floors is a buffer. A buffer is nothing but a tank of water that STORES heat. This is VERY similar to domestic hot water and the exact same principles apply. You can heat that buffer tank to the highest practical temperature and then draw energy for your bare-tube floor with its return going back into the tank. You'll want to store at least 30 minutes worth of energy in that tank at design load before the TANK calls the boiler for a "recharge".

See how complicated this is becoming?

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You seem bright and interested. Go to www.hvaccalc.com and buy the "homeowner" version of the heat loss program. This is a VERY easy to use program! Spend some time with CAREFUL measurements and reasonable assumptions regarding construction. A well-labeled sketch of each floor plan with windows and normal ceiling height included will GREATLY assist you and such shouldn't take more than a few hours.