Do I Have Enough Energy?

rhlj342

New here, and I have been wrestling with the question about a prospective hydronic system I want to install..."Do I have enough energy?". A bit long, so bear with me. This is a second home, mountain cabin at 8000'. We have heated the place for 60 years with space heaters and a wood stove...so we have been getting by. I don't like to leave unattended space heaters going overnight, and the problem I want to solve is leaving the toasty bedroom in the morning to a very cold rest of the space. The hydronic system would need to keep those spaces (and the bedrooms) at 60 degrees overnight, and the bedroom we are sleeping in at 65-68 degrees. So, here are the conditions:

1) I have recently installed a new propane hot water heater heater (40 gallons and 33,000 BTU's). I wish I had thought of this before I bought the new one but I didn't, and I don't want to buy a new one...so this is what I have.
2) I have determined that the heat requirement for all spaces (4 zones) is 28,000 BTU's. Zone 1 4,000 BTU's, Zones 2&3 7000 BTU's each, and Zone 4 10,000 BTU's
3) I am using a system delta of 20 degrees and an inside/outside delta of 50 degree's (we don't stay if the temps drop below 15-20 degrees as we begin having water problems, so 15 degree's is my worst case number).
4) The building is pretty tight, but poorly insulated (built in 1921). Unheated areas of the home will drop from 70 degree's at bedtime to 40 degrees by morning if it's 25 degrees outside.
5) Planning on a closed loop with plate heat exchanger and 140 degree water out.
6) The 4th zone would run very seldom, so the most common load would be 18,000 BTU's, although I still need to plan on the 4th functioning.
7) I will be zoning domestic HW as a priority on the system, but we are not taking showers at night...the time I am most interested in heat.

My assumptions as follows:

1) If the load is either 18,000 or 28,000 BTU's, the system will not be calling for heat continuously. (Maybe 30 minutes every hour)
2) If that's true (because many nights will only have a delta of 35-40 degree's) then the heat demand will only be 9,000 to 14,000 BTU's/hr.
3) I'm figuring the hot water heater I have is maybe 75% thermally efficient which results in +/- 25,000 BTU's transferred into heat. (Not enough for full-time operation of all 4 zones by itself, and certainly not enough for domestic hot water).

Questions (here is where I get confused)

1) The system delta of 20 doesn't seem to work out mathematically. If I ran 8 gpm through a system whose output is 28,000 BTU's, it would seem I could expect a system delta of 7 degrees ( 8*8.3*60 = 3984#'s 28,000/3984 = 7.03 degrees).
2) If I slowed the flow rate to 4 gpm, I should have a system delta of 14 degrees. This would represent a BTU requirement of 4*8.3*14*60 or 27,888 BTU's.
3) Is that math correct, and does it indicate that with all 4 zones running full-time my bucket of BTU's runneth empty?

So, I fully understand this is less than Ideal, but can someone answer the question "Do I have enough energy available at night to knock the chill off to 60 degrees or so?". I feel like I do, but I know it's marginal. We can make up the difference with space heaters in occupied rooms as we have always done. I just don't want to spend the money and effort if a) the hot water heater will run all night to keep up (noise, as it's close to the bedroom we sleep in) or b) it simply won't keep up.

Have any thoughts?

Thanks in advance!

jumper

Have you considered timers & thermostats for space heaters?

hot_rod

The derate for efficiency and you altitude, I’d expect more like 20,000 BTU/ hr out of that water heater. Certainly not much to heat and provide DHW

Jon_blaney

What is the recovery time for the heater? What is your planned heating temperature schedule?

Jamie Hall

OK. To start, it's power (BTUh) you are looking at -- and for -- not energy( (BTU). Often confused.

Second, the question is what is the effective power of the water heater? As @hot_rod said, that 33,000 BTUh input water heater may have an available output, derated for altitude and efficiency, of around 20,000 BTUh to 25,000 BTUh.

Now. Is that sufficient power to heat the structure? In your comment 2, you list the power requirements (how did you determine those, by the way?). You compare those requirements to the power available from the water heater, and while you don't have enough power for all four zones at once, never mind domestic hot water, you do have enough to heat any two zones at once, or zones 1,2 and 3 at the same time, or heat domestic water. You will not have enough power to recover after domestic hot water use if any of the four zones is calling for heat.

Now don't confuse power available or used with the flow rate needed to deliver it from where it is put into the hot water to where it is going to be used. The power is given by that standard hydronic formula, and as you have noted if the flow is greater the delta T will be less, for any given power. You are quite correct in stating that with all four zones running at once you run out of power -- but that isn't the result of the hydronic relationship, that's simply because you can't get any more power out of the system than you put in.

rhlj342

Thanks for all the comments.  I don't like the idea of freestanding electric space heaters unattended whether or not they're on timers or not. I just don't trust them. It's the main reason I'm looking at some hydronic supplemental heat.
As to the second comment, I'm really looking for heat at night. We can manage the heat during the day with both gas and electric as well as the wood stove. Heat during the day is really not a problem. It's just at night when I don't want to run any of those heat sources in other parts of the building unattended. We could dial back all the zones or turn them off during the day, and devote the hot water heater to DHW.  
The derating for altitude is a good point. The heater says it should recover in about an hour, but neither of us are very green when it comes to showers. We both take 20 minute showers, and we never run out of water even if those showers are concurrent. With the fuel being propane, and the flame being adjustable I feel like we may get more than the rated BTUs, but that's just supposition.
I calculated the BTU requirements needed using the formulas for room volume, windows and doors and infiltration loss. It came out pretty close to the 45 BTUs per square foot recommended for that area. I understand what you mean about not conflating energy input and flow rate. I can get the energy I need, I think, with a lower flow rate and a higher delta. I was initially looking at 8 gallons an hour just to pull out additional BTUs but reverted to 4 gallons an hour because I don't think I have the additional BTUs to pull, so the 8 gallons an hour becomes superfluous. Zones one, two and three are my primary concern, because that's the main floor of the building. If I could just keep the temperature up to 55 or 60 at night in two of those three zones, I would be a happy camper. I would want the third zone to be at 65 to 68° where we are sleeping.  If zone 4 is occupied, then supplemental electric heat could be used because it would be attended.
In the mornings and during the day, it would be nice to have one of the four zones functioning, but I don't need all of them during that time. Thanks again for the help! A newbie with newbie questions lol

rhlj342

By the way, another question... Do any of you have experience with hot water heater boosters, and do they work? I also have access to 99,000 BTU tankless hot water heater that could potentially be used as a booster. Unfortunately neither the hot water heater nor the tankless supply enough energy and or flow to use one for DHW and the other for heat. The tankless only supplies three gallons per minute. I have heard that tank boosters actually do work, and my provide the extra thermal energy I need. Thoughts on that?

Jamie Hall

OK, but... you can't pull any more power out of the water heater than you put in with the burner, and you can't dump any more power into the spaces than the radiation will put out at whatever temperature water the radiator is provided with. The produce of delta T and flow rate is a constant set by the power of the water heater, and fiddling with flow rate will only change the delta T -- not give you more power or less.

SuperTech

I honestly think that you are using the wrong tool for the job. Get a properly sized and installed boiler, let your water heater take care of what it was designed for.  

rhlj342

Jamie, yes I understand that. When I embarked upon this, I saw that the radiators I wanted to use put out more BTU's with a higher flow rate, so I started there. If I had the BTU's to give, doubling the flow rate would deliver +/- 10% more BTU's (meaning that it was capable of radiating more at a higher flow), BUT as you point out, I have no more BTU's to give.
Super Tech - I began this acknowledging it was the wrong tool, and stating that I don't want to buy a new tool. I was just trying to determine if my math was right in order that I might successfully hammer in a nail with the heel of my shoe.

fentonc

Have you considered installing a permanent electric baseboard (or other permanent, thermostat-controlled resistive heat source) instead of plug-in space heaters?

Jon_blaney

To be honest, I don't think this scheme is going to work well. There are too many other options. Install electric base board, get a better wood stove, get some insulation, a boiler maybe. If you want to use a water heater, get one with about three times the output. Sometimes you just have to spend the money to get a decent result.

rhlj342

@fentonc I have, and it remains a potential solution. I think I have just become enamored with the hydronic concept. It has given me a chance to learn about something new, and this kind of stuff is not how my brain works (not at all an engineering mind). I really enjoy things that challenge that deficiency, so I think that has fueled my interest in this solution. Electric heat may be the easier solution. I think fire in a 100 year old wood building has always had me squeamish about running high powered electric devices through old wiring. If I went that direction, I would have it all re-wired. This is a mountain vacation home where we spend +/- a month a year, so I was also looking to minimize the investment.

rhlj342

@Jon_blaney You may be right, Jon. That's why I came here. I knew there were much more informed heads than mine. We are spending a lot of money in other areas of the home, and the truth is, I don't want to spend the extra money if I can't make what I have work. Having become enamored with the concept, I was hoping I might pull it off. I don't need a perfect system...just a supplemental one, but the consensus seems to be I don't have the hardware to make it go.

Jamie Hall

Let me try this one more time. The heat output of a radiator is a function only of the average water temperature. It is NOT affected by flow, except that a higher flow will, with a given input temperature, have a higher average temperature for the radiator -- a direct consequence of the lower delta T.

EBEBRATT-Ed

@rhlj342

I really think electric heat is the way to go with your limited use it would be cheap to install and it can't freeze up. New 20 amp 240 volt circuits new from your panel will avoid using any old wiring.

28,000 btus = 8200 watts=34 amps at 240 volt. You can put 16 amps on a 20 amp circuit

rhlj342

@Jamie Hall We are on the same page

rhlj342

@EBEBRATT-Ed Yes, and I have plenty of room in the panel to do that. I'll just need to do all new wire runs for those. I don't have room for baseboard heat, so maybe I can look at some in-wall models. I was originally going to go with ventless propane, but they don't work at that altitude...another thing that drove me to hydronics.

rhlj342

@Jamie Hall This is what I was going by below. Given that the radiator is capable of radiating additional heat at a given temperature, a higher flow rate will lead to the emission of additional BTU's. The curve diminishes rather rapidly after about 2 gpm, but provided that the BTU's are available within the fluid AND the radiator has the capacity at a given temperature to re-radiate those BTU's, increased flow will impart more BTU's to the emitter and thus to the room.

https://images.thdstatic.com/catalog/pdfImages/d9/d98964e1-ebe5-45b3-b70c-9d9681cf6ec1.pdf

A KS-2004 for example, will emit 2700 BTU's at 1 gpm and 3140 BTU's at 5 GPM at a water temp of 150 degrees. It is rated at 4000 BTU's, and while it appears at high water temps it can exceed it rating, it will never achieve the 7000 BTU's of the 7000 BTU unit at any temperature. That's what I was trying to say about flow, temperature and capacity. I just may not have said it very elegantly.

Jamie Hall

What you are seeing is the effect of flow rate on average temperature. For a given inlet temperature, the higher the flow rate the smaller the delta T, and therefore the higher the average temperature.

rhlj342

Agreed...same page

hot_rod

You could get the tankless for DHW and use the water heater for about 20, 000 btu/ hr

What size cabin, 45 btu/ sq ft seems pretty high?

EdTheHeaterMan

Hydronics is a great way to heat a building comfortably, but are you looking to to the job properly or affordably? I think you will find that doing the job properly is the only affordable way to get what you want. After you try to get something done affordably, and discover it is not good enough, then you spend a little more, and find out it is still insufficient, You are already into the project too far to change course to Electric Baseboard or Propane wall units. Now you undo the heat exchanger and get a space heating boiler, and say to yourself in 2024, I should have listened to @Jon_blaney and @SuperTech

bburd

@op You have said this is a vacation home, usually unattended, and that the outside temperature regularly goes below freezing. How will you protect a hydronic system from frozen and burst pipes? Glycol?

retiredguy

Have you considered a vented, propane wall heater. They come in a variety of BTU inputs, can be gravity heat with no electricity required or electrified with a fan for better air circulation and a relite pilot. Or, you could install a smaller forced air propane furnace. A 40,000 input/ 32,000 BTU output furnace with minimal ducting would be a much simpler solution. There is no water to freeze, no radiation to buy, and no guessing if you have enough BTU's available.

For the hot water radiation, have you considered a pick-up factor. That is the extra heat required to raise the room temperature from very cold to comfortable. If you would want to consider this as a potential solution just"GOOGLE" or search for Williams vented propane wall heaters. There are numerous options. Also, I think that installing a hot water heat system is way too complicated for a part time cabin.

rhlj342

@hot_rod I have thought about going the tankless route. I have access to a gently used one that would provide about 3 gpm...not enough for either side, but I thought about tying it in as a booster. The cabin is in New Mexico at about 8,000 msl. The bottom floor is about 1100 sq. ft., with a room upstairs that is about 225. I agree that the 45 BTU/ft is high only because we would not be there during a worst case scenario in sub-zero temps. As I mentioned, we bail out at 15-20 degrees because of water issues. Given that, 45/ft is more than enough. It is board and batten with very little insulation.

rhlj342

@EdTheHeaterMan Yes, that's why I came here to get some input. Again, I'm not looking at this system to replace another or to depend on for total heat. We have heated the place with space heaters and a wood stove for 60 years. I just want something to keep the place from getting into the 40's at night in unoccupied areas.

rhlj342

@bburd Glycol, yes. This is why I chose to go the closed loop route. I don't want to mess with draining it every fall only to find out there is a low spot that froze and broke. I understand also that I will lose some efficiency with glycol as well.

rhlj342

@retiredguy I began this whole journey looking at ventless, but they don't perform well at altitude. Williams is the only manufacturer that makes a high altitude kit for ventless so it led me to them. They very pricey, so I began looking at vented options. Propane heaters are my #1 choice, but I would need three, meaning three roof penetrations and going with Williams is not even a budget consideration due to their high cost. My solution may be to do what we have done for 60 years; Live with cold morning spaces. It's pretty quick to heat up with the gas range, wood stove and electric heat. I'll be spending about $25,000 on the place this year with upgrades, so based on the input I'm getting here, I will either go electric (which I don't trust) or just maintain the status quo. Part of the fun has always been the fact that it's NOT like home, although the older I get, splitting wood in 30 degree temps is less fun

hot_rod

The oil filled electric heaters seem a bit safer, freestander or baseboard, also electric panel rads.

If you want a fast heat up hydronically, you really need a fan convector of some sorts.

If you had a higher output water heater 50,000 or more, it might be worth a Hydronic solution, but...

The Bradford White Combi Cor we’re perfect for this type of job, 65 or 75 K, no power required to keep the running.

rhlj342

@hot_rod thanks for those suggestions. I'm not really looking for a rapid heat up, just something to run during the night that would keep the space at 58 to 60°. Then in the morning entering the day we can achieve the heat level we want with space heaters and other supplemental heat.
Bob, what do you think about using the tankless hot water heater I have access to as a booster for the propane tank heater? It's a smaller one that they say will give me about 3 gallons a minute with an input temperature of 55° and an output of 140. The output temperature drops with additional flow above and beyond the 3GPM, but it still may be an effective solution as a booster. You ever seen a system plumbed like that?