Radiant heat in a single room with a small water heater?
Been reading a learning here for some time, thank you, but now I have a question of my own. I bought a 2,500 ft split level home in southern Wisconsin. The house had hot water radiant heat and no A/C. The boiler was from the 70’s and there was a flat roof. We have a sun room off the back of the house that is 10X16. It is on a slab and the owner added radiant in floor heating under the granite floor tiles.
We ended up removing the boiler and all the pipes and radiators and putting in a 2 stage high efficiency furnace with 2 zones. We had a gable roof put on so the upstairs could have ducts and ceiling vents. It was quite a project and has been working pretty well except for the sunroom where I keep my office. There is a 4 foot wide opening from the main house with no door. In the summer with A/C on, it is only a few degrees warmer that the main area, but in the winter it is a problem. It runs about 7-8 colder and the granite floor currently reads 58f with my infrared thermometer.
The copper lines are still in the crawlspace that lead to the sunroom. My HVAC quoted $3K to connect them to a water/water heat exchanger to my 50 gallon gas water heater. Seems like a lot and I was wondering if I could get a small 5-10 gallon water heater and make it a stand alone unit? I would need a heater, circulation pump, pressure tank and thermostat? Doesn’t sound that expensive or difficult. Anybody done this type of job or have any insight?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Michael,
The first step is to do a heat load calculation. A typical 6-gallon water heater is rated at 3,000 watts, which is 10,239 BTU/h. If your heat loss calculation exceeds that number, you'll need a bigger water heater.
The go-to site for do-it-yourself hydronic systems using a water heater is a company called Radiantec. The type of system you are considering is called a closed system (rather than an open system or an indirect system). Radiantec will sell you a kit with pumps and controls if you want to go through them.
.
In many (or even most) areas using a water heater solely for space heating would be a code violation.
There are some tiny electric boilers out there costing ~$800 that would be more appropriate (as they are purpose made for the application) even models that are single-phase 120VAC, eg: Electro Industries EMB S-2 (2.5 kW, ~8500 BTUh/r) or in the unlikely event that that's not big enough for the load, the EMB S-4 (5kW, 17,000 BTU/hr). There are more choices at 240VAC, but those two models may be the easiest for a DIY installation. The manual lives here (and elsewhere- the manufacturer's website was being balky today):
http://www.houseneeds.com/upload/pdf/electro-embsminiboilermanual.pdf
Dana, I though the interdiction against using water heaters for space heating was only if they were also used to supply domestic hot water?
Malcolm: It's not universal, but in my state (and several others) using a water heater solely for space heating is expressly disallowed, but IS allowed for combined heating & domestic hot water. The rationale will vary from state to state, but many states have minimum AFUE requirements for space heating equipment, and water heaters are not normally tested under an AFUE protocol. Other states require ASME certification for hydronic boilers, which water heaters don't have.
For the record, the low temp heating system in my house uses a tankless water heater in lieu of a hydronic boiler, but because the domestic hot water is also heated with it (although not directly), it's still a legal installation. The potable hot water never passes through the heat exchanger of the tankless, unlike many/most tankless water heater combi setups.
Thanks Dana. A lot of the hippie houses around here on Vancouver Island used hot water tanks with Heath-Robinson delivery systems. I have no idea what our code says about it.
Dana, I'm a little puzzled by your description of your hydronic setup. Either the potable water is directly heated by the tankless and the hydronic is run through a heat exchanger or the potable is run through the heat exchanger and the hydronic is directly heated. I can't see any difference between these two setups with regards to potable water safety. Or are you alluding to people running potable water through the floor (and does anyone still do that)?
Thanks for all the responses! I am going to call Radiantec about this tomorow. Thank you.
Rob: In my system the tankless is on the heating system side, and does NOT have potable water running through it. There are no pumps on the potable side of my system- the potable heat exchanger is immersed in a 48 gallon tank of heating system water that serves as the heating system's buffer tank, set to 130F. With all zones calling for heat the bottom of the tank can drop to the 115F range, but it's still over 120F at the top. The only time it drops below 115F is during long draws of domestic hot water. This is the approximate topology:
http://www.ergomax.com/Radiant.jpg
I have diverse radiation, from radiant floors to cast iron rads to a hydro-air handler, some of it micro-zoned which is why it needs a buffer tank.
From a potable safety point of view it doesn't matter which side of the HX the tankless is operating, but if the heat exchanger is a low-mass plate type, there needs to be pumps on both sides, of HX, and the pump on the potable side needs to be compatible with potable water. (In Martin's diagram in response #1 there is no heat exchanger. ) From a number of burn cycles point of view having the buffer tank avoids short-cycling the tankless on short hot water draws, and with the minimum burn times the total number of cycles is about the same as in a hot-water only setup, but with longer, more efficient burns.
In MA it's legal to use potable water in the heating system with a few caveats built into the code: There are minimum water exchange rates that must be met for all zones and all parts of the system even when there is no space heating load to prevent stagnation issues. This is pretty easy to meet with the minimal amount of water in a hydro-air coil. To do it with a radiant floor requires more design work, but that can also be done. With these so-called "open" systems the pumps also have to be more expensive bronze or stainless potable-compatible units, as do any zone valves, etc.
Thanks for the explanation Dana, I learn a lot from your posts. I am more familiar with the flat plate exchanger with two pumps - but your system makes a lot more sense. I don't want to get too far off topic, but what size pump are you using to recirculate the buffer tank?
The primary loop pump driving the tankless to buffer tank is currently a 1/20 HP 3 speed Taco 0015, which is capable of ~3 gpm+ though the tankless HX when set to high speed, with a freshly cleaned screen-filter. (The tiny screen filter in the tankless needs annual cleaning to keep the gpm in a suitable range.) If the pump ever craps out I'll replace it with an ECM drive pump of similar power.
I normally run it at low speed and ~2-2.5 gpm when the screen is clean, but will bump it up if the reported flow on the tankless controller drops too low before I clean the filter again.
All,
I am a complete novice with little technical expertise, but a love for research. I have learned lots on the site and appreciate everything as we upgrade our home for comfort and energy efficiency.
The home is a 30 year old ranch of about 2,000 square feet in Maine, fairly well insulated, with a boiler for three zone baseboard and radiant heat and hot water. one of the zones is a 12 foot square sun room on a tiled concrete slab, with fairly good quality windows on two sides with very high quality insulated shades, the unheated garage on the third side, and the house itself on the fourth side. My plan is to replace the boiler with a two zone model for backup heat; adding a high efficiency heat pump for the primary heating and cooling and a new heat pump hot water heater.
My question - the sun room has radiant heat which we enjoy your round. Since I don’t want the boiler for primary heat, it won’t run for the radiant heat in that room. I’d be most appreciative of a step by step description of how to replace the boiler-driven zone with a single purpose system for this radiant heat. At this point, the plastic piping that carries water from the boiler goes through a mixing valve to bring the water to 110 degrees. There is a circulating pump that moves the water through the system. I’ve spoken with one plumber and he said there was no good solution, but this thread convinces me he’s wrong. I'm looking for the best solution including the equipment I should use and the design of the system.
Any advice or suggestions would be great.
Ray,
You wrote, "My plan is to replace the boiler with a two zone model for backup heat." You mentioned a "model" -- a model of what? Another boiler?
If this becomes your backup heat source -- this "model" -- what will you use for the primary heating source?
Q. "I need a step-by-step description of how to replace the boiler-driven zone with a single purpose system for this radiant heat."
A. You're asking for a primer on hydronic heating. There are lots of ways you can make mistakes. If you aren't a plumber or a heating contractor, I urge you to call an expert and pay for the installation work. If you want to live dangerously -- hydronic heating systems can explode, after all -- can Radiantec and try to get D.I.Y. advice over the phone. (I don't recommend this approach.)
Martin, Thanks very much for your reply.
On your first question, my thinking is to replace the existing boiler with a two zone boiler for backup and a heat pump system for primary heating and cooling. I don't want to run the boiler at all in shoulder seasons, but I do want the single room radiant heat.
On your second point, I absolutely agree; I will NOT do this project myself. Unfortunately, the plumber I mentioned said there wasn't a solution. What I'm hoping someone can provide is a simple explanation of how to do this most efficiently so I can work with a skilled expert to get the job done.
Hope this makes more sense.
>"...my thinking is to replace the existing boiler with a two zone boiler..."
>"Hope this makes more sense."
It makes NO sense, mainly because there is no such thing as "...a two zone boiler...".
If the existing boiler is still operable what probably makes most sense is to keep the existing boiler, and decommission any zones that are being completely replaced by the heat pump system or use them as backup, and decommission the water heater zone (or embedded coil, if it's an embedded tankless hot water). This may be as simple as turning some valves and disonnecting/combining thermostats.
What is the boiler make/model ?
Zone controller make/model?
How many feet of baseboard (broken down zone by zone )?
Are any zones going away completely, or are you simply looking to consolidate multiple zones?
Ray,
You can use one of the smaller electric boilers suggest above.
If your under-slab insulation is decent and the room is not very lossy, your heap pump water heater should be able to heat this as well. To connect to the HPWH, you can use a sidearm in reverse thermosyphon (installed near the top of the tank without a heat trap) to feed the existing mixing valve/pump loop. Keep in mind that you would be limited to around 4000btu of heat and you might want to set the floor heat thermostat to turn off in the mornings when there are large hot water draws.
Solar hot water panel or panels......
Tom,
In Maine, the times when space heating is required aren't correlated with the hours of sunshine. Remember, Ray is talking about a sunroom. A sunroom doesn't need any heat when it's sunny. It only needs space heat on cloudy days and at night.