New to GBT forums here but I have a project coming up that I could use advice on. We're looking to install hydronic radiant heat in a slab-on-grade accessory dwelling unit (small cottage 700sq ft) in northern california. The heating demands are relatively light (never drops below freezing) but I am wondering if forum members would recommend a gas boiler or an electric boiler. The downsides are the $/kw in CA which are ~15c/kwh (high), but we may supplement with solar. Upsides to sticking with an electric boiler are that we won't have to run a gas line back there (saving some minor cost) and it could simplify the installation overall. Any thoughts? Thanks

Also consider a Chiltrix heat pump. The % of time the building is occupied will effect the economics significantly.

Gas would be more operationally economical than electric. However, if your heating load is relatively small, your electric bill might be quite reasonable. You might want to consider a NextGen electric boiler:

NextGen Boiler

Going the NextGen path will save you the significant cost of having to also purchase a circulator pump, expansion tank, air vent, and differential pressure relief valve separately. A NexGen boiler installation is extremely DIY friendly, especially when compared to a modulating condensing gas boiler. The NextGen boiler also has a Summer mode that cycles the circulator pump, keeping it from seizing up...a common problem when the hydronic system has a significant period of non-use.

You should first do a heat loss analysis and then do a hydronic radiant design before actually selecting your heat source. We have free software on our website to accomplish this or hire someone competent and dependable to do this for you.

Cool, thank you for the recommendations and quick replies. I was wondering how the operational cost of this system (hydronic with an electric boiler) vs. just electric mats that use resistance-heating, would be for a project. Does anyone have a reference to a forum posting that does this analysis? Tried searching and couldn't locate a direct "hydronic with electric boiler" vs. "electric resistance mats" comparison. I understand this depends entirely on the system and weather, but curious for any feedback about mild-climate conditions. Thanks

It really depends on your building makeup and how much heat the building needs. 1KW/HR is equal to approx 3400 BTU/HR, wether it is making hot water with resistance elements or directly heating wires in the concrete. The electric in the floor might be a little easier to damage and perhaps less resistant to slab cracking. The advantage of using hot water, is that I think the tubes are more resilient/resistant to damage in the slab. There are also a whole lot more ways to make water hot. Gas as you mentioned, Wood/pellet boilers, electric resistance, electric heat pump at 3X+ efficiency in a mild climate, solar direct, Solar electric resistance and solar electric powering the heat pump... Some of these will work the best/most efficient with some hot water mass storage. But the point is that the water gives you options for your energy source, the slab resistance element do not...

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In mild climates where the diurnal swing is modest, high-mass radiant floors make less sense. I use radiant ceilings in many of these applications. If you know that electricity is going to be the power source for decades to come, an electric radiant ceiling system is more responsive and can be equally as comfortable as a slab. Generally speaking electric heating systems require less maintenance than hydronic systems but it is certainly rare when the extra cost of electricity makes it the smarter financial choice.

Will the new cottage have a bathroom?

In terms of heating system acquisition cost and overall comfort, one can’t beat slab-on-grade hydronic radiant floor heating.

Hydronic radiant floors make great sense in mild to cold climates or in highly energy efficient buildings using low R-value floor finishes (e.g., polished concrete and tile) where a floor surface temperature between 70F and 85F will generate the required BTUs per square foot of floor surface area to satisfy the design maximum heat load requirements. Floor surface temperatures between 75F and 85F are the most comfortable (i.e., the warm toes factor). However, floor surface temperatures between 70F and 75F are not uncomfortable either...you still have a totally quiet and draft-free heating system (i.e., unlike a hot air furnace or mini split system). If you use slab temperature sensors and boiler outdoor reset, you will not have any controllability problems with slab-on-grade hydronic radiant floor heating. This is especially true in highly energy efficient buildings which are largely insensitive to large outdoor temperature swings.

Hydronic radiant ceilings make great sense in very cold climates, code minimum buildings or buildings using high R-value floor finishes (e.g., carpets and hardwood floors) where you simply can’t generate the required BTUs per square foot of floor surface area without exceeding a 85F floor surface temperature which can cause the occupants to complain of uncomfortably hot and sweaty feet. However, electric/hydronic radiant ceilings (and hydronic radiant floor tube & board/plate systems) have a much higher acquisition cost than slab-on-grade hydronic radiant floors. Therefore, we always recommend our clients to consider mini splits in lieu of electric radiant ceilings (and hydronic radiant floor tube & board/plate systems), especially if they also require air conditioning too. Much lower electrical operational cost to go the mini split path in lieu of electric radiant ceilings. Hydronic radiant ceilings might make sense if alternative fuels to electric are available to heat the water.

In the end, you simply need to determine the acquisition cost and operational cost of each heating option and select the one that makes the most sense for your specific situation.

... buildings using high R-value floor finishes (e.g., carpets and hardwood floors) where you simply can’t generate the required BTUs per square foot of floor surface area without exceeding a 85F floor surface temperature which can cause the occupants to complain...

I suggest that high R value flooring requires a higher water/slab temperature but does not increase the required floor surface temperature or cause uncomfortable feet.

Less dense flooring (like carpet) remains comfortable to the bare feet down to lower temperatures (eg 70F) than something like concrete or tile (limited comfort range). Ie, concrete/tile often requires higher surface temperatures (like 79F+) to be comfortable. Achieving this is likely to overheat the room and waste energy.

Very true, using high R-value floor finishes (or using low R-value insulation below the slab or using tube & board/plate systems) will increase the required water supply temperature needed to generate the required surface temperature and required BTUs per square foot of surface area to satisfy the design maximum heat load requirements. It is indeed the floor or ceiling surface temperature that generates the required BTUs per square foot of surface area. Needing an increased water supply temperature to generate the given BTUs per square foot of surface area results in increased hydronic radiant emitter inefficiently (i.e., lower performance) and increased operational cost. Here’s an excerpt from the directions of our free hydronic radiant floor heating design software:

“Once your hydronic radiant floor heating system is installed and operating, it may be used to assess the performance of BOTH your building design and your hydronic radiant emitter design. With regard to building design performance for a design indoor temperature of 70 degrees F and design outdoor temperature of 20F, we consider a 74 degree F or less hydronic radiant floor surface temperature as excellent building design performance (i.e., a low-load, energy efficient building), between 75-80 degree F as average performance, and greater than 80 degree F as poor performance (i.e., a high-load, energy inefficient building). A hydronic radiant floor surface temperature of 85 degrees F or less is recommended for barefoot friendly residential floors. With regard to hydronic radiant emitter design performance, we consider a 15 degree F or less temperature difference between the hydronic radiant floor heat source supply temperature and the hydronic radiant floor surface temperature as excellent hydronic radiant emitter design performance, between 16-25 degree F as average performance, and greater than a 25 degree F as poor performance. One should always keep in mind that it can be very difficult and expensive to subsequently improve the performance of your building design and your hydronic radiant emitter design after they have been constructed.”

Note that when you talk about required surface temperatures and water supply temperatures (i.e., building and hydronic radiant emitter performance), you also have to specify the associated design indoor and outdoor temperatures (or more accurately, specify the actual building heat loss under these conditions). Many HVAC companies and product manufactures often just state something nebulous like their tube & board/plate systems only require 100F supply water.