I am currently gathering up bids for my in slab radiant on my new home build.  I am putting radiant in about 4500 square feet split between potentially 3 or 5 zones.  I would like a boiler to heat all of these floors and to heat my water in a hot water tank.  I have bids using a Navien boiler, a bosch, and an HTP.  Any favorites here or any one I should stay away from?  The HTP bidder probably is at the bottom of my list right now due to other reasons.  Looks like they are all calling for 100-110000 btu boilers.  Thank you

Well, you first need to do a proper heat loss analysis (e.g., ASHRAE, ACCA Manual J, DIY, etc.) to best size the boiler. Then you need to consider fuel rates for your area to best determine which fuel and associated boiler type will provide the best operational economics. Often this is a gas condensing/modulating boiler. And finally, you need to research the boiler brands for best reliability and customer service before selecting the winner. You might find our software useful for your quest:

Borst Heat Loss Analysis Software

Borst Integrated Heating System Performance Software

acat, I am in the same boat. Getting ready to buy one for my house I am doing. I asked someone on this forum that I respect and that does this type work everyday and they recommended Navien. From the research I have done it doesn't look like a good idea to buy the all-in-one units that do both the hot water and radient heat. The reviews indicate lots of problems (see reviews on Amazon). I am going with a broiler for my heated floors and one for hot water.

The combo units seem like a compromise of both and not end up doing either really good. You need a lot of BTU's (~200k) for domestic hot water but for heating the house it sounds like you might only need 50K or less most of the time. I know those units claim to be able to modulate from 15k to 200k but at what end are they actually efficient at? Where I live in Michigan I spend 95% of my NG bill on heating the house and very little on heating hot water so I want a unit that is best at heating for my radiant floors. The como units seem like they might be aimed more for the domestic hot water side.

I have not been impressed with the Navien myself. Especially not for combination duty, thought I don't think less of it than I do of most on demands, I don't think much of most on demands either.

HTP has a newer series of boilers with 10 to 1 turndown ratio (in heating conditions) that I like a lot, the "UFT" series. They include a 199-19k combi unit that I think much more highly of than I do of most on demands both because of the modulation rates and the design of the on demand heat exchanger which I think will be far less problematic then most on demand heat exchangers.

Beware that most on demands report 10 to 1 turndowns, but that is at a domestic water temperature rise. In space heat, to actually achieve that kind of turndown with a typical on demand means you'd need prohibitively high flow rates and ridiculously sized pumps which will eat up a lot of your fuel savings and probably reduce the lifespan of the unit. The UFT series does not suffer from any of those problems either.

I generally prefer a purpose built boiler for space heat and a tank of some kind for domestic, either indirect with appropriate purge controls, or a standalone unit. In rare cases (at least, in a heating dominated climate) a heat pump water heater may be a good choice too.

Combi boilers are great in the right application, it does take a lot of BTU on the boiler side to make the domestic water,
About 100,000 BTU for a 2.5 gpm flow depending on the unit/temp rise.
These units will have a delay for hot water unless the boiler is equipped with a small stand by tank. Without that you have 45 seconds just for hot water, then it has to get to the fixture.
Whole house recirculation, cant do it with a combi, these are typically designed to prioritize the domestic water based on flow, the whole house pump will lock out the heating side while running.
That said when we design boiler systems and pump panels there are a few things to consider,
Size of house is the start, I assume larger homes 2,500+ typically will need a side arm tank (not an absolute but a start point), now whole house recirculation can occur with pumps and or convection from the side arm tank, boiler size can be reduced, tank capacity can be sized for a quick fill in the case of a soaking tub, multi headed showers , if there are multiple bathrooms the demand of several can be factored in.
I do not think there is a big hit on efficiency due to scale.
That said we design and ship a lot of combi boilers with on demand isolated DHW.
I just feel there is a point in the scale of a home where these combi units do not fit the bill. A typical larger home budget usually can afford the additional cost of a tank and pump.
Another reason to consider a side arm tank would be solar domestic water application, a tank is needed to store and collect. Combi boilers will be tripped up when presented with preheated water.
Dan

Great info. Dan, Any recommendations on brands?

We would be happy to look at your project and make recommendations for appliances and systems we provide,
Give you some ideas when looking at options out there,
Web site is BlueRidgeCompany.com
Dan

We are in the process of installing 6100 sq ft of in-slab pex. My designer/engineer specified PT boiler, boiler buddy tank and PT indirect water heater. The boiler water circulates to the boiler buddy tank and then circulates as needed to the floors or the indirect water heater for DHW. We designed for low floor water temps at ~95F-105F and the boiler water tank will be around 120F or what is needed to have DHW at a comfortable level. The controls for the zones also monitor outside temps to adjust the system water temps as needed.

Sounds like a good design,
We will be glad to help but is best to go through our web site,
blueridgecompany.com
Dan

Posted By acat on 18 Jun 2016 11:14 AM
I am currently gathering up bids for my in slab radiant on my new home build.  I am putting radiant in about 4500 square feet split between potentially 3 or 5 zones.  I would like a boiler to heat all of these floors and to heat my water in a hot water tank.  I have bids using a Navien boiler, a bosch, and an HTP.  Any favorites here or any one I should stay away from?  The HTP bidder probably is at the bottom of my list right now due to other reasons.  Looks like they are all calling for 100-110000 btu boilers.  Thank you

Without the heat load numbers there's no way to assess whether 100-110K is the right size or (as is all to common) ridiculously oversized.  A typical code-min 4500' McMansion in my neighborhood would come in with a design heat load of about half that at 99% outside design temp of 0F-5F.  A tight better than code house that size with a reasonable shape can come under 40,000 BTU/hr.

Modulation ranges are not infinite.  If the minimum modulated output of the boiler is more than half your design condition load it's not going to modulate much.  With a reasonable amount of thermal mass in the water & slabs to help it probably won't short-cycle, but it's downright silly to install a modulating boiler so oversized that it can't actually modulate, or worse, so oversized that it short-cycles on zone calls.

So, first things first, a room by room, zone by zone load calculation, using aggressive rather than conservative assumptions on air tightness.  While it's human nature to up-size things "just in case",  if you oversize it by 3x you'll have sufficient boiler to heat the place at temperatures not seen since before the last ice age, and end up operating at lower efficiency, with reduced equipment life.

Rather than trying to figure out which is the right boiler, try figuring out which is the right contractor.  If none of them offered to do a Manual-J type calculation or didn't ask for the Manual-J performed by a third party, they're all suspect.   But rather than asking an HVAC contactor to do that work, it's better to have a disinterested party such as an engineering firm who is only selling the accuracy of their numbers, not equipment, hydronic design, or installation services. 

They still need to be steered.  Air infiltration defaults in the software packages are often above currently code-legal leakage.  In addition, unless directed otherwise the number crunchers are likely to assume ASHRAE 62.2 ventilation rates without heat recovery, either of which can send the final numbers skyward.  Legal maximum leakage is 3 air exchanges per hour at 50 pascals (3ACH/50), which is dead easy to hit if any attention at all is paid to air sealing during construction.  A better assumption on a home you care about and are having built to your specifications would be air tightness on would be no more than 2ACH/50, which is also pretty easy to hit (but not too easy as an after-the-fact retrofit air sealing.)  Direct whomever is running the numbers to also assume a Heat Recovery Ventilation system, at rates no higher than ASRHAE  62.2.

When you know the design heat load you'll be in much better shape for picking a boiler. I'm guessing you won't need anything more than 80K for a boiler, and if it's a decently performing house design you may do just fine with a ~50K boiler.  Only when you get to that point is comparing different boiler models a reasonable activity.

Once you get your load, it's worth reviewing something like this (for the boiler you are considering). Best to spend most of the time in the upper left.

Everyone of the bidders has performed a manual J. They all have my prints and spent some time putting numbers and bids together. I will be using the same company for both my HVAC and Radiant. They all have designed it to utilize a boiler to heat my radiant floors in 3 zones and to heat my domestic hot water. They all spec'd a 60 or slightly above gallon hot water storage tank. I guess I don't know if that makes it a combo unit boiler or not. Beings they all have spec'd a 100-120 btu boiler I am going to assume they know what they are talking about. Some have included a boiler buffer tank and some have said it is not required. thanks

I concur with Dana...sounds likes way too much boiler to me too for what should be an energy efficient low load building. However, if the manual J was done correctly, that's what it is.

It is best not to need a buffer tank, but if you do need one, you do need one... Here’s the proper way to make that determination:

Borst Buffer Tank Design Software

Excerpt from the software instructions:

“This calculator enables you to properly design/size a buffer tank for use in a hydronic radiant floor heating system. A properly designed/sized buffer tank will prevent the hydronic radiant heat source (e.g., boiler or heat pump) from short cycling which will significantly decrease the life expectancy of the heat source.

When low thermal mass heat sources are combined with zoned hydronic radiant distribution systems, it is possible for the heat source to short cycle when only a couple of these zones call for heat. This occurs because the rate of heat produced by the heat source is much greater than the rate of heat used/released by these zones. Low volume thermal mass heat sources in combination with low volume hydronic circuit and distribution systems cannot absorb this excess heat rate without experiencing a rapid rise in temperature. This causes the heat source to reach its high temperature limit very quickly resulting in very short on times and short cycling. While electric heating elements may tolerate short cycling, gas valves, oil burners, ignition systems, and compressors will have a significantly reduced life expectancy.”

"Rather than trying to figure out which is the right boiler, try figuring out which is the right contractor. If none of them offered to do a Manual-J type calculation or didn't ask for the Manual-J performed by a third party, they're all suspect. But rather than asking an HVAC contactor to do that work, it's better to have a disinterested party such as an engineering firm who is only selling the accuracy of their numbers, not equipment, hydronic design, or installation services."

I think that is a great statement Dana1!

As he alluded to, there are a bunch of ways that without some careful study of the plans and codes, that the calculated numbers could seem way high. The HVAC provider doing the calculations to size the system that they will be selling you always seems a little like the fox guarding the henhouse to me. You have the quotes in your hand, you can see how much money is at stake here to install such a large system. Is it perhaps worth it to spend a little more for an independent survey? That of course is for you to decide. It might in this case save you at the least, the cost of the independent study, in reduced initial equipment cost, and improved efficiency and increased service life(and less hassles) of a properly sized plant. At the very least if the numbers agree with what the bidders submitted, you will have the peace of mind knowing that yes indeed it does need a 110KBTU/HR boiler system...

Posted By acat on 20 Jun 2016 11:24 PM
Everyone of the bidders has performed a manual J. They all have my prints and spent some time putting numbers and bids together. I will be using the same company for both my HVAC and Radiant. They all have designed it to utilize a boiler to heat my radiant floors in 3 zones and to heat my domestic hot water. They all spec'd a 60 or slightly above gallon hot water storage tank. I guess I don't know if that makes it a combo unit boiler or not. Beings they all have spec'd a 100-120 btu boiler I am going to assume they know what they are talking about. Some have included a boiler buffer tank and some have said it is not required. thanks

Assuming that they actually know what they are talking about is fraught with risk.   I can't tell you the number of HVAC contractor performed Manual-Js  I've seen that came out more than 50% higher than a competent aggressive third party's Manual-J. Like I've already stated, the numbers seem really quite high for even a code-min house in most areas. In contractor Manual-Js it's common to see outside design temps used that are 5F- 10F colder than the actual 99% temperature bin for the location, combined with higher than code-min interior design temps, incorrect window & wall U-factors (sometimes lower performance than current code min.)  If you didn't pay for the Manual-J, the odds that it was really done right are low, but not impossible. If the bidders performed the Manual-Js, did they (or will they) give you printed out copies?  If yes, how much variance was there on the room-by-room numbers between the different contractors? Do the construction details actually match what you're building?

A few months ago I saw an HVAC contractor's Manual-J for a high-performance house that used all code-minimum U-factors and higher-than code air leakage.  Not surprisingly (and all too commonly) it came in more than 2x higher than a HERS rater engineering Manual-J for the same house that used the exact construction details, that also used the lower ventilation load appropriate for the heat recovery ventilation & very tight house specification.

As for the domestic hot water, in 99 cases out of 100 you would not need to up-size the boiler for domestic hot water performance.  The size of the potable hot water tank varies with hot water load, but not the size of the boiler.  For a 60 gallon indirect simply running the indirect as a priority zone will always work.  A 50,000 BTU/hr boiler delivers substantially more heat per minute than a 50 gallon standalone hot water heater's burner, and recovery times will be quick.  With high-mass radiation like concrete slabs you would never even notice that the heat was off during the tank recovery- it might even be hard to detect with room and slab temp instrumentation.
 

Combi-boilers rarely fit American home applications. A properly sized ModCon with SS indirect is the most common and appropriate answer for low-temperature <140­°F AWT applications such as hydronic radiant slabs.
When designing radiant slab systems the heat load and floor coverings in each zone indicate the design water temperature. An average can be used to serve all zones in most well designed systems.

If I going to compare Electric Boiler and Gas Boiler I would go for electric boiler for sure. And I recommend Bosch Electric Mini-Tank Water Heater Tronic 3000