My Hot Tub Hookup Experience - Glycol Loop

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I thought I would share my hot tub hookup just for into for those who may be googling this and need some info like I did.

I have an Eko 40 Boiler with 1000 gallons of storage - coupled with a Vesta System from nofossil. We acquired a free hot tub that was only a few years old and had either a broken heater or controller; according to the original owner. This summer I build a deck off our main level to support the tub. I took the side panels off the tub and insulated the best I could around the outside using 2.5" rigid foam and a few cans of spray foam.

I am running a loop off my system, in the floor joists in the basement, to the corner of the house closest to the hot tub. Here I run the loop through a 10 plate heat exchanger which exchanges heat with a glycol loop to that runs outside of the basement sill plate and alongside the deck floor joists then up through the deck into the bottom of the hot tub. From there it runs through a 55kbtu/hr titanium tube exchanger before running back into the house. This glycol loop has it's own pressure tank, air valve, and grundfos alpha pump running on the lowest power setting.

Since the tub is not right up against the house, I opted to run this glycol loop. My EKO 40 puts out a max of ~73 F (163F) water and the storage top doesn't get much hotter than this at the moment due to the limitations of the boiler controller. I thought that the heat exchangers I put in place would be plenty to get the hot tub up to temperature, however it seems this is not the case. The temperature differential isn't enough to get anywhere near my rated 55Kbtu/hr rating of my smallest exchanger; I believe this rating is at 140F differential, which I overlooked.

I am going to try modifying my boiler control to get me closer to 200F boiler output temps, for the sake of a better delta T as well as increased thermal storage in the propane tanks.

The tub will heat itself up even at 0F outdoors, but it takes a long time at my current temperature levels. I have not used the tub yet but I can only assume that the temperature loss while using the tub without a cover will be exponential during use and that the heat exchangers will not be able to keep up, causing short bathings! I am glad I added all the insulation to the tub because at this point, even without heat on all night, the temperature drop appears to be minimal.

Just some thoughts I thought I would share for the next guy. Feel free to chime in! Merry Christmas!


Feeling the Heat
Mar 5, 2013
South Central Pennsylvania
Is your boiler only getting to 163, because the top sensor is down into the sensor well to deep? I would try moving the sensor and see if you can get more heat.
I played with it a bit last night. I took the side off the boiler and experimented with moving the sensor from the top down the sides a bit. Using a temp gun I discovered that it really doesn't matter where I put it on the boiler as it doesn't stratify like the storage tanks do. I ended up tucking it in the insulation a little bit on the top of the boiler. It helped. I am using a Taco Viridian Delta T pump for the boiler with a seperate sensor on the top of the boiler for that so I can adjust my output temps, but the problem was the boiler would throttle back the fan once it got to 71C. I'm about to throw my second load through it this morning; my logs showed top storage temps last night around 170, so that is an improvement.


Minister of Fire
Sep 15, 2011
Nova Scotia
Is that max boiler temp a built in thing to the Eko? Not sure I could live with that - aside from limiting max heat output, it really hampers your storage capacity.

On the hot tub, a 10 plate heat exchanger is really pretty far on the small side, and tube & shells are not as capable as flat plates. I have a 20 plate for my DHW. It works very well for that, but I suspect your hot tub load is a lot more. You might have to upgrade exchangers? Or you could try upgrading one at a time & measure results, starting with upsizing the 10 plate. I would think big there. Delivering the hottest temp possible to the tub HX would be the goal - might even think about going to a 40.

BTW have you done measuring of all your HX ins & outs? That should help reveal weak spots. Does the hot tub side of the shell & tube only circulate via the hot tub pump? Or do you rely on some convective flow for when the tub pump isn't running?
The hot tub side has a grundfos alpha stainless model running at low speed ( 4 watts) 24/7. I'm super happy about the load the hot tub requires. Even over night with no pumping at all, it loses almost 0 degrees, which doesn't seem possible, but I'm happy either way!

I'm changing some rules around in the vesta so only 150+ water hits the exchangers, will see if that works. I do think that ultimately though in order to maximize efficiency, you are right, I should have larger exchangers because the delta T just isn't there and won't be able to keep up with the cover open. I haven't measured the temps of the exchanger sides yet.
I love the Alphas, they show wattage and everything! I put a titanium exchanger on the tub loop because I read something about chemicals eating anything except titanium. It is only 55k/btuhr - probably my biggest bottleneck.