Help with hot water heating system

Jer74

Good Morning

I have a few questions about a heating system that is not working very well, but first a bit about the house.

The heating system in question heats my 3 car garage that has an apartment above. The garage floor has heating loops in the concrete, and the apartment has baseboard style hot water radiators. The garage is approximately 1200 sq/ft and the apartment is slightly smaller at 1000 sq/ft, both with 8' ceilings.

Currently the heat is supplied using a Rinnai tankless water heater, and the research I have done is indicating that this system is sub par at the best of times. I have not been heating the garage to allow full priority to the apartment and am only able to get it to 64F with an outside temp of 32F, and we are supplementing with electric heat.

My questions are:

1- does this seem reasonable, or should i be expecting higher heat temps in the apartment? The water heater maxes out at 120F but when checking with a temp gun the output water measured on copper pipe is only around 90F, and the water temp at the baseboard radiators is only 85F.

2- Should I be looking at a Combi if I want to replace this system, or should I stick with the hot water heater for the DHW, and look at adding a small boiler to get this system working properly?

3- What should the output temp be on a boiler or combi unit to heat the upstairs correctly? I know there is probably more to the equation like water speed, and radiator surface area, but what is typical?


Thanks in advance for any advice you can offer.

Jeremy
B.C. Canada

Hot_water_fan

3. "Typical" is 180 for baseboards, but if you can keep the house at 64 with 85 degree water, you can use a much lower temperature. Lower temperatures are more efficient and should be used as much as possible. An outdoor reset will adjust the temperature higher as the outdoor temperatures decrease to keep the house at setpoint.

Does the current tankless heater heat both DHW and the apartment?
What's the total linear feet of baseboard?

Jer74

Thanks for the reply

Yes the system does both the heat and DHW. The DHW goes direct upstairs, and the heat goes through a plate style heat exchanger to the heat loop. I can measure the actual upstairs baseboards, but off the top of my head:

2 bedrooms 4' each = 8'
Living room 8' = 8'
Kitchen 8' = 8'
Bathroom 2' = 2'

total = 26' approx

Jon_blaney

Is the floor the floor between the garage and apartment insulated? If not, the cold garage is sucking heat from the apartment. Make sure the unit is designed to be used as a boiler. There may be adjustments that can be made. This type of equipment is kind of fussy and requires a skilled tech to get it right. Be careful, do not get som hack to look at it. If I was going to replace it, I would get a a old school cast iron boiler. The tech support is much deeper.

Hot_water_fan

The heat exchanger (which you need in this situation) contributes to why your water doesn't reach 120 - you lose some heat in the process. Try turning on the garage loops for a few days, I doubt you're constrained by heater size.

Jer74

I feel like this system was not designed right in the first place, and no this is currently not a combi unit. I have made the mistake of hiring two different guys to do some repairs over the years and feel like they had no idea. I have since learned the system and done all maintenance and repairs myself (not an expert but am a handy guy, and a heavy duty mechanic by trade). My brother in law is a plumber (he is a total hack, so will not be working on my place...lol) and can get good discounts on new equipment. Am thinking about a Combi, but not sure if i sure leave the unit for DHW, and add a boiler for the heat. Little tight for space but could be doable once i could determine the size needed.

Jer74

Thanks Hot Water Fan

I have turned the garage on yesterday so we will see if that helps, maybe the radiant heat will head upstairs a bit.

Hot_water_fan

Little tight for space but could be doable once i could determine the size needed.

Ha usually the answer is the smallest one you can find, boilers are notoriously oversized.

Jer74

Is there a calculation that exists to determine what the inlet temp on the radiator needs to be to heat a determined space, or is there to many variables?

Jer74

Is a small electric boiler even an option? or are they more for a small space like heated floor in a bathroom? Currently this unit I have runs on Natural Gas, but depending on power draw I could easily add an electric unit that is dedicated and maybe eliminate the heat exchanger? I may be able to also use gas, i will just need to be sure my service is enough for both units, and the figure out a second venting route.

Hot_water_fan

Is there a calculation that exists to determine what the inlet temp on the radiator needs to be to heat a determined space, or is there to many variables?

Not super important in this situation in my opinion. If you can get to 64 degrees using 85 degree water at what is probably half of your design load (32 degrees out), piping it with a new boiler and without the heat exchanger using a temperature < 120 probably works. Anything under 120 is great for efficiency, especially if that occurs at design temp.

mattmia2

Many variables.

Start with a heat loss calculation for the building.
That will tell you how much heat you have to provide to each room. If you want to leave the garage unheated then you will have to figure in that floor as being to the outside for the purpose of figuring out how much heat you need in each room (but not for the overall of the building it you also include the heating of the garage in the total).
Once you know how much heat you need per room you can figure out what water temp will get that much heat out of your emitters. You can also figure out the size of the boiler from the total of all the heat loss to the outside.(also need to consider the DHW load if you are using it for DHW).

Since I don't think you have a chimney or similar vent, you probably want to use a mod con boiler or combi boiler that can vent through plastic pipe out the sidewall. You will need to get someone with the right equipment to set up the combustion on this, they need to be adjusted to the site conditions after they are installed.

What is the DHW load? Is there a shower in the apartment? Washing machine? Or does it only supply a couple sinks? Or multiple full baths? If the load is small a combi might make sense.

Jamie Hall

There are a number of variables -- but not too many.

The first variable which you need to determine, though, is what the expected heat loss for the space is. That's not hard to do -- Slant/Fin has a very nice calculator for that, here: https://www.slantfin.com/slantfin-heat-loss-calculator/

Then with that you need the length of the baseboards available -- in your case, you have that : 26 feet.

The heat output of baseboard varies -- as you are aware! -- with temperature, but is about 600 BTUh per foot with the water temperature entering at around 180. It's slightly variable with the type of baseboard and flow, but that's a good start. It drops very nearly linearly from there down to an input temperature o 100, where it's close enough to nothing to not count.

So, for example as a very VERY rough guess, let's suppose that your apartment works out to a design heating load of 20,000 BTUh (that would be a pretty well insulated space). You have 26 feet of baseboard up there. a quick calculation will show that you need... almost 800 BTUh per foot of baseboard, which is more than it can manage even at 180, so... in this case, you kind of punt and say OK, I'll run the water at as high a temperature as my boiler can produce.

Which brings us to the boiler. Note: boiler. A tankless water heater simply cannot produce water at anything like the required temperature. Never mind that it's the wrong appliance for the job in the first place. Unhappily, you don't that many much power -- but you need it at a high temperature, so a small cast iron boiler is going to be your best bet.

Jer74

Thanks, just to be clear, if i have 120 flowing into the radiators you think it will be fine, so a small dedicated boiler with no heat exchanger would be best case scenario?

Hot_water_fan

Thanks, just to be clear, if i have 120 flowing into the radiators you think it will be fine, so a small dedicated boiler with no heat exchanger would be best case scenario?

Yes, that's the best scenario. The temperature can be increased immensely, so if 120 is too low (only relevant a handful of days per year), not a big deal. The smallest condensing boiler is about 50k Btu, so you'll have plenty of boiler.

Jamie Hall

Jer74 said:

Thanks, just to be clear, if i have 120 flowing into the radiators you think it will be fine, so a small dedicated boiler with no heat exchanger would be best case scenario?

A small dedicated boiler would be best. If you want to heat domestic hot water with it, you could use an indirect.

But

I get the impression that you think that having 120 F water flowing into the radiators will magically bring you enough heat. Maybe. I have to make some assumptions here, but you mention being able to maintain 64 in the house now, probably with about 32 outside. OK. If you want to raise that to 75 -- which should be your goal, even if you don't do it while you live there yourself -- you'll need 30% more heat transferring to 10 degree warmer air just to do that when it's 32 outside. That gets you up to somewhere in the vicinity of 110 degree water right there. Now I don't know where you are, so it's possible it never gets colder than that. But if it does...

Hot_water_fan

Unless there's some undisclosed constraint here, whether it's 120 or 140 at design day isn't all that significant since the high temperature can be adjusted (often automatically depending on the boiler).

Jer74

OK, so there is a couple details that I left out:

1- The apartment above the garage is rented out to a young couple with a baby recently. the previous tenant was a young engineer who traveled for work alot and never complained about heat.

2- The DHW supplies 3 sinks (1- kitchen 1- bathroom 1- garage washroom) a shower, a washing machine, and a dishwasher.

3- I am in the pacific northwest about 3 hours north of Seattle in Canada, just north of the 49th parallel. It does get below 32 from time to time in the winter, usually a few weeks down to 15 ish but not sustained for long.

I have downloaded the Heat loss factor app and will try and get some numbers this evening to be sure of the size of boiler I need. I will just need to try and find out what the factors for the glass and doors are they ask for in the app.

Thanks for all your help, I really want to get this right the first time.

PS.... any thoughts on the electric boiler option?

Jon_blaney

You have to compare fuel cost by fuel time. Then decide what you can afford. An electric boiler uses a lot of juice.

Hot_water_fan

@Jer74 don't bother with the heat loss calculation in this one situation. It won't provide any actionable info: every gas boiler will be wildly oversized for the application. You'll be in the 10-20kBtu range, no gas boiler will be anywhere close to that small. MAYBE you can find a cast iron of 30kBtu, but then you run into the venting problems.

Electric boiler would work just fine. Depends on your utility rates whether it'll be cheap to operate. If you keep the water baseboard you could one day switch it out for an air-to-water heat pump. Otherwise, you could just do electric baseboard and skip the boiler hassles.

Jer74

Yes I did consider the electric baseboard option, but not crazy about tearing up the finished space to run wire etc.... I was leaning towards a Combi unit, but after reading multiple disappointments in different forums I am shying away, I would hate to spend the money on one and have it not work, but.......

From what I am hearing i need to increase water temp going to the heating system, and my current unit is just not capable of getting there. On a Combi unit, could I expect the boiler side (heat side) to output water in the 120 to 140F range that is needed? and have the DHW down to a non scalding temp? Is a Combi a bad idea, or have i just been reading bad experiences that may be due to poor designed systems?

Sorry for all the questions, but I am learning here... I think lol

Jer74

Also....

Currently I have a circ pump flowing through my heat exchanger on the W/H side, and a circ pump through the heat exchanger and out to the baseboards. I have read that a Combi has an internal pump, would it have a dedicated heat loop then? And would I be able to eliminate the two external circ pumps and heat exchanger?

again...... many thanks

Hot_water_fan

A boiler (combi or otherwise) can reach about 200 degrees, so you're fine. You don't have to worry about water temperatures. You won't need a heat exchanger, you will need a system circulator (or several based on your zoning). There's not really any advantage to a combi except for the space savings.

Jamie Hall

I would carefully check your hydro rates vs. gas -- I seem to recall that BC Hydro (when it works...) has quite low rates, and so an electric boiler might be just the right choice.

Jer74

Thanks Hot Water Fan!

Jamie - Yes we are very lucky here with both Hydro and Gas, the rates fluctuate a bit but I think they are very close for now. I just did a little digging and a 55K BTU electric unit has a 63amp input power....ouch. I will look a bit more but I may not have that much left over, and i really don't want to lose my welder, plasma, compressor, etc.... in the shop. The electric would really simplify the install not needed to find another space to vent that wont interfere with the existing H/W heater.

mattmia2

A good combi is a little bit less reliable than a dedicated mod con boiler just because it has more components, it has some sort of diverter valve to select between the heating loop and the HX for DHW and an internal circualtor, but if you pick a quality product and apply it correctly it can be reliable. An indirect heated by a mod con is another probably a little bit more reliable option that would only require one vent. being able to make enough DHW will be the deciding factor on how big a boiler you need if you do it with one appliance. The smallest mod con you can get and a large indirect would probably match your load best.

Jamie Hall

That need for a lot of amps has tripped up more than one electric boiler idea! But... power is power, and you can't get away from it. Can you upgrade your mains service?

Hot_water_fan

I can’t imagine you’d need more than a 30kbtu electric boiler, which is still a lot of amps of course

JakeCK

Why isn't anyone talking air to water hp? If electric is cheap, he can run as low as 140 swt, and is already thinking about an electric boiler... 

Jer74

Thanks all for your feedback and suggestions, I really appreciate it!

Taking all of your feedback into consideration, I am leaning towards the Combi system. My brother in law priced one out for me at $2700 (Canadian dollars so about $2200 US) for a Navian unit. I know this may be the least reliable of the options, but if I can get 7 to 10 years out of the system I may be in a better spot to upgrade to something more robust then. My current Rinnai unit has been in service for 16 years and during the winter never shuts off.

I just need to figure out a typical install drawing that would work for my system, and from what I have researched it would be something like this:

1- DHW has its own circuit, so very straightforward

2- hot feed for heat out to air eliminator with expansion tank mounted below -> to circ pump controlled by thermostat -> to manifold/TEE fitting branching to my two zones -> through circuits and back to zone control valves for each circuit -> Back to boiler/combi

Couple questions assuming this is correct:

1- my current system has a make up water valve, on a combi unit would this be introduced on the heating loop, or the cold water feed for the DHW? or is it even needed?

2- Am I correct in putting the zone control valves on the return in this application?

3- My current circ pump has a three speed adjustment on it, should I replace with the same or is there a specified speed that this pump should run at GPM?

4- Does a Combi use the same dirct vent system that my current Rinnai uses? I saw a couple pictures with two exhaust pipes coming of the top of the unit? Is one exhaust and one intake air? Or are thay available in the pipe in pipe design?


Thanks again for all the help and suggestions, and feel free to beat me up a bit on this design/idea.... I have thick skin

Jon_blaney

Watch the Mikey Pipes Youtube videos on these Navien units. Based on what I have seen, I would go elsewhere. All these new school boilers seem expensive to buy and maintain. I would go old school.

rick in Alaska

Just two things here. That water heater can have the heat output increased up to 140 by adjusting a dip switch inside it, and if that isn't enough, you can put on a different controller that will allow it to go to 180 I believe. You would need a mixing valve for the radiant and the domestic if you go this way though.
Second, with an infrared thermometer, you would need to paint the pipes with a flat black paint, or some other way to take the shine off it to get an accurate reading.
Rick

Jer74

Thanks for the comments

Jon, thanks for the advice, I am torn on this decision but am leaning combi because of the space and ease of installation as my system would only need minor mods to make it work. I will look at the Navien video, Is there another brand/model that is more robust? The Noble unit seems good (lochnivar?) but I dont have any real world experience with either.

Rick, I did flip the dip switch #6 on this unit and it allowed adjustment to 49 Celsius, but now its stuck there? wont go up or down from 49. Great tip on the black paint i had no idea! I will give that a try

mattmia2

@Ironman puts in a lot of HTP stuff, he can give you an idea of how durable those are.

You probably will want to pipe it primary-secondary with a boiler pump and one for each zone. Some scheme to temper it for your radiant too, either a mixing vale or injection mixing, might want to look at a control that can do outdoor reset.

Make sure the output of the boiler can handle your DHW load if you are using a combi, the DHW will probably set the boiler size. You will need to look at your max draw and your incoming water temp.

rick in Alaska

Make sure when you are trying to change the temperature setting that there is not any flow happening as the control cannot be changed when it is running.
Rick