Adding dhw priority to tekmar 261

SweatyInToronto

I guess at this point the question is whether to go with a standalone boiler for the DHW, or retrofit in an HX? If a HX was used, then it looks like a Tekmar 263 or 274 is needed to be sure to have post-purge , timeout, just in case.

Would you think the boilers can handle the dwh demand and the space heat on design days?

A standalone just has to last 10 years until the hydronics are replaced and a full "combination" high efficiency solution can be put in.

hot_rod

SweatyInToronto said:

I guess at this point the question is whether to go with a standalone boiler for the DHW, or retrofit in an HX? If a HX was used, then it looks like a Tekmar 263 or 274 is needed to be sure to have post-purge , timeout, just in case.

Would you think the boilers can handle the dwh demand and the space heat on design days?

A standalone just has to last 10 years until the hydronics are replaced and a full "combination" high efficiency solution can be put in.

To answer you question you would need a heat load calc for the building, only then you would know how much extra capacity the boiler have for DHW.
A DHW load would be good to know or calculate also. a basic fixture count would get you in the ball park.

If the DHW load is not large, as you suggested you could heat and create DHW at the same time, without a priority function, especially with over sized boilers, which based on a WAG they are oversized. If both boilers are never running non stop on design day, then they are oversized

There there would be no need to abandon the 261, just zone the DHW HX with a pump or 1-1/4" zone valve.
A basic aquastat on the DHW tank sends a call to the HX pump, the relay or end switch on the zone valve calls on the boilers via the 261 or parallel wired to boiler contact.

Really no need to throw new or extra controls at the system once you settle in on the DHW method, at the very most an isolation relay or single relay control, shown here.

SweatyInToronto

hot_rod said:

SweatyInToronto said:

If the DHW load is not large, as you suggested you could heat and create DHW at the same time, without a priority function, especially with over sized boilers, which based on a WAG they are oversized. If both boilers are never running non stop on design day, then they are oversized

So this approach would do simultaneous space heat and dwh? Either that or using the setpoint priority could potentially provide enough btus all around, but does it deal with thermal shock avoidance?

Case 1) If the boiler exchanger was hot from the hydronics but the dwh loop had cooled down overnight and then started up and shot a load of 80 degree water into a very hot HX.

Case 2) If the boiler HX was hot from dwh and the space heat was just on after WWSD for example.

I do want to protect the boilers especially if they will be providing both domestic hot water and space heat. For that I gather I'd need post purge and the upgraded control - am I incorrect?

hot_rod

Post purge basically pulls residual heat from the boiler when it shuts off. It’s not really a protection function, more of an energy savings attempt.
A 3 way thermostatic boiler protection valve would protect from any cold return. It’s a non electric piping device.

with oversized boilers and low mass heat emitters, I highly doubt you have return temperatures issues.  Again, until we have a reasonably accurate heat load number for the building, it’s all speculation.

SweatyInToronto

Ok I'll see what I can come up with.

Still post purge would have the effect of redicing the boilers temperature significantly, wouldn't it? 

So the 3 way valve would be able to mix in water from the hot loop if there is demand to heat the cold loop so the differential is not greater than x degrees say? 

Would installing on two boilers create a lot complexity? In software there is the KISS principle. I don't want to over engineer a hot shower and maybe there is a reason it's standalone now as was mentioned!

Can u recommend a good site for the heat load calc though?

Thanks! 

hot_rod

The post purge isn't all that sophisticated, usually just a time function. In most really boxes it is a non adjustable fixed time period. Obviously you do not purge below the temperature of the DHW tank or heat emitters. So if the tank is at 140 and the boiler shuts down at 180, you just try to purge down the useable heat into the tank, or building.

Did you mention the boilers are 20 years old? If they have run clean, without corrosion in the HX or flue piping, without boiler return protection, you probably do not need to add it? Many boilers run from a cold start their entire life. You want to get up above condensing conditions within 10 minutes of run time is a general rule of thumb for a non condensing boiler.

If you need, or feel you want to add return protection, I'd use a 3 way thermostatic valve that actually detects return and adjusts accordingly. Shown below.

A bypass, or bypass with pumps are not 100% assurance of return temperature protection.

You might just determine the outside dimension of the building, do a window count and do a heat load on the entire building to get a close estimate. It would be quicker than a load calc on every single room and get close enough for the number you need to compare boiler size to load size.
Engineers, contractors, designers, or wholesalers that do heat loads on a regular basis in your area could probably guesstimate within a few BTU/ sq ft of a load, it something you get a feel for after doing 100's of them


www.slantfin.com has a free heat load calc. It is easy to use and they have a online turtorial.

SweatyInToronto

Thats awesome I shall give it a spin. Thanks again. 

Boilers are like 15 years. No protection.

It's thermal shock scenarios after adding a dwh loop that I'm concerned about more than condensation.  On cold start the boiler and water are approximately the same temp so yes there would be condensation for a time but hopefully burn off quickly like you say.  

However when adding a second loop for dwh, the chance of cold water heating hot boiler exists in a few scenarios above.
At least it seems that way to me.  Have been reading. 

I might be over emphasizing the risk but until I know better I have to err on the side of caution. 

hot_rod

It's rare to see DHW tanks pulled down to room or incoming temperature? Usually they call for heat when they drop 5-10° degrees below set point. So set them at 140, when they drop to 130 or so the boiler will see whatever temperature is in the tank. i doubt folks run the hot waster until it is incoming temperature of 50° or lower.

Possibly after a long power outage you could see a long cold start if that is a common occurrence then by all means add a return protection mechanism.
Or standby generator

Zman

On occasion, I have seen indirect hw heaters that have been sized large enough to drag a boiler into condensing temp range.
Since you are doing the heat exchanger sizing, you should easily be able to avoid that issue with the manufactures sizing software.

SweatyInToronto

Update: decided to go with a replacement stand alone water heater as we felt it was better to install the indirect or HX with a new system rather than try to retrofit it into an existing very old system, for sizing and other reasons. Thanks for all your feedback - it's was educational and hopefully will help others as well.