Thinking about two 48" heat mats for 4x4 tent

Reverend Budbreath

Reverend Budbreath

Red-eyed and mystified
I'm struggling with low temps and high humidity in my tent, which is out in a garage. With regard to the temps, I have a space heater out there on a thermostat setup, but it makes me really nervous having it out there unattended. And I'm afraid to take it to the higher of two heat settings in the close quarters of the tent, if only because I'm afraid it will really dry out the side of the fabric pot it's pointed at and maybe harm the plant in it.

The vicious circle is that if I crank up my exhaust fan a lot higher and/or have it on constantly to deal with the RH which is ranging from 50% to 64+% with the lights on (78% and higher for hours after watering!), then it will suck every bit of warmth out of there, resulting in decent RH levels but unacceptably cold temps. On the other hand, if I set my exhaust fan to be off most of the time (e.g. 30 minutes off/5 minutes on cycle), the temps get warmer but the RH flies up to 70%.

Then I had the idea of using big seedling heat mats, 48" x 20", two of which would almost cover the floor. Of course it's already been suggested that I use foam insulation sheet to get the plants (in 7-gal fabric pots) off the floor, and I was about to go get a sheet but then I wondered why not go one better and actually provide a bit of heat to the bottom root zone? Surely it would come up and warm nearly the whole of the soil, no? Unless I'm mistaken, when air temps are only moderately cold as they are in this case (60-66F), it's the roots and not the plant that really need to be warmer. If I were to do this, what would I set as the temp of a heat mat that fabric pots are sitting directly on top of? 72F?

I have a small dehumidifier (https://www.amazon.com/gp/product/B07WWFRG5R/ref=ppx_yo_dt_b_asin_title_o00_s01?ie=UTF8&psc=1) on the way that seems perfectly sized for a grow tent, and I hope it will solve both problems in one shot, but I don't know if it will be effective. Plus, I doubt the warmth from a dehumidifier would warm the bottom surfaces of pots sitting directly on the floor of a tent that is sitting on a cold concrete garage floor. So I think the mat idea might still be good.

Your thoughts...?
 
Will work fine and set @ 75 - 78 and heating the roots is the way to go but only reason to keep the heat higher for the plants is to keep Powdery Mildew away from lower temps and high humidity but you won't have to worry about this till much later as they grow so carry on soldier you got this D.O.W.N. :thumbsup:
 
UniGrower said:
Will work fine and set @ 75 - 78 and heating the roots is the way to go but only reason to keep the heat higher for the plants is to keep Powdery Mildew away from lower temps and high humidity but you won't have to worry about this till much later as they grow so carry on soldier you got this D.O.W.N. :thumbsup:
Click to expand...

Then in your opinion, air temps between 60F-66F are not cold enough to significantly slow down growth? I've always been led to believe that they are and that every effort should be made to get things up into at least the low 70s, even when lights are off. But—in a way, that just doesn't wash logically, because obviously plenty of outdoor plants grow in nature with ambient temps in the low 60s and do okay, and often do great. I mean the big ol' outdoor plants in B.C. for example must have overnight temps in the 60s, even in summer.

So maybe I'm worrying unnecessarily about growth rate based on temps and should just relax. As far as PM, I'm so ready it's hilarious. I have about six weapons for that in the arsenal. But haven't seen hide nor hair of it, or even a single insect so far. This grow might be a little slow and cold for my taste, but it's the most pest-free one I've ever had so far for sure.

Speaking of cold spaces, how is the bombmaking going? Things good out there in the shed? ;)
 
My $.02 - if you are in fabric pots (I am too) I cut about 1 foot sections of PVC and lay them flat in the tray and put the pot on top. Keeps them from having their feet stay wet and also away from the heat (cold) sink of my concrete basement floor. I haven’t had to do much for temps but when I was looking at it I was leaning toward a radiant heater (think of the old fashioned oil filled ones) and I had concerns about not enough humidity so I could put a shallow tray of water on top. May not be what you’re after but there are a few ways to go about it.
 
Olday said:
My $.02 - if you are in fabric pots (I am too) I cut about 1 foot sections of PVC and lay them flat in the tray and put the pot on top. Keeps them from having their feet stay wet and also away from the heat (cold) sink of my concrete basement floor. I haven’t had to do much for temps but when I was looking at it I was leaning toward a radiant heater (think of the old fashioned oil filled ones) and I had concerns about not enough humidity so I could put a shallow tray of water on top. May not be what you’re after but there are a few ways to go about it.
Click to expand...

I hear ya Olday... that's my concern, the heat sink floor. What do you think of my heat mat idea though? Wouldn't that be great to just supply mild heat to the whole floor (most of it anyway) instead of raising up the pots to prevent contact with the floor? I found one online made by AC Infinity that I feel confident about because it's a step of evolution past the simple alloy wire heat mats. It's some kind of little infrared cells instead. Anyway they're UL-certified and MET-certified and I think I feel confident they wouldn't do something weird. I even called AC Infinity and talked to a sales guy to see if they'd tell me any reason not to use them the way I have in mind, but he seemed pretty clueless. I can't some up with any reason not to do this, as it would solve the problem pretty inexpensively and doesn't carry any risks that I can see. I would control them simultaneously with a heat mat thermostat that I already have. The product info says the heating range is 80F-87F, so the lowest it goes is 80 I guess... I would just set the thermostat to 80 or 81 with the probe laid right on the mat with a pot on top of it, and it would never get any hotter than that at the interface, and hopefully some nice 78, 77, 76, 75 degree warmth would wick up the soil and generally warm up the whole lower zone of 6-8" from the floor upward.
 
AcInfinity makes fantastic gear, I have their fan and a friend of mine got their new tent, I love my gorilla gear tent but at the price point and quality of that one, I’d buy different today. I’d say go for it
 
We keep our bloom lights on overnight.
The heat from the HPS heats the whole workshop.
During the day I run an oil filled radiant heater at 75 degrees.
Keeps the workshop at about 70.
We also use a dehumidifier set at 40%. This unit produces a bit of heat also.
After several years I feel safe with the setup. The radiant heater is awesome.
The only caveat to the heater is that it can’t be turned on by a timer. It can be turned off but not on. That has not been a problem though.
Good luck!!!
 
Rev, I waited to see what others might say before I chimed in. I'm a worrier ;) so here are my concerns:

1) Pots drying out too soon due to bottom heat. 2) Roots suffering because soil temp is too high.

I think you'd do better placing a sheet of foam insulation under your tent. The heat from the light, circulated by a fan, should be sufficient to warm the roots. You don't want the root zone above ~70F, IMHO. :2c:
 
I would go with the sheet of blue board as well. I have very similar situation in the fall and thats what i do.
 
180 said:
Rev, I waited to see what others might say before I chimed in. I'm a worrier ;) so here are my concerns:

1) Pots drying out too soon due to bottom heat. 2) Roots suffering because soil temp is too high.

I think you'd do better placing a sheet of foam insulation under your tent. The heat from the light, circulated by a fan, should be sufficient to warm the roots. You don't want the root zone above ~70F, IMHO. :2c:
Click to expand...

Just for the sake of an interesting discussion, let me counter with a couple of things, even though I'm a fellow worrier: One, I'm really talking about the cold temps of the light off period, not so much about when the light is on. I do expect that the warmth of the light is warming the floor. Yet, even with a heater in there, yesterday the temps never got above 70. Had I not had a heater in there, it would have been in the low or mid 60s. When the light goes off, of course it gets colder (even though that happens in the daytime). And naturally, when I go into 12/12, the light off period is going to be 12 hours instead of 8, so there will be four more hours of cold floor.

Regarding the risk of the soil in the pots drying out too quickly, I doubt that could happen here because I'm not doing a kind of wet/dry cycle this time as I've done with soil in the past. My aim is to keep the pots nicely hydrated the whole time, more like a coco approach but with no runoff. With the fabric pots I believe this is possible (I'd never try it in plastic pots, of course). One big reason is to maintain optimal conditions for the microherd in the soil. I know the plants will grow slower this way (and they are!), and you all may hear me complain occasionally, but I think ultimately it's going to be better for this organic pesca-veganic living soil approach.

Hmm, maybe I should set it up on a timer so that the floor mats are warm only during lights out. The mats have been ordered and are on their way, so I'm compelled to try this experiment in some form.
 
Joe Dirt said:
We keep our bloom lights on overnight.
The heat from the HPS heats the whole workshop.
During the day I run an oil filled radiant heater at 75 degrees.
Keeps the workshop at about 70.
We also use a dehumidifier set at 40%. This unit produces a bit of heat also.
After several years I feel safe with the setup. The radiant heater is awesome.
The only caveat to the heater is that it can’t be turned on by a timer. It can be turned off but not on. That has not been a problem though.
Good luck!!!
Click to expand...

Right, those heaters are good but they're power hogs. For a small tent grow like this, I'm wishing they made a mini one that pulled only 300W or so instead of 750/1500. My hope is that when I start running the mini dehumidifier, the warmth it adds will be helpful (and I hope it actually works well as a dehumidifier, as I'm going to be counting on it in flowering).
 
Bush Dr said:
I’m in coco and some of my best products have been grown with below freezing temperatures, the feed is warmed and they get 6 feeds a cycle instead of 4

As long as the media doesn’t get below 15*C growth will continue

This was grown with the air temperature on the inlet fan @-12C
Click to expand...

Thanks Doc, that's definitely food for thought. I might be just worrying too much, but I feel like the plants are kind of slow and not very vigorous and I'm blaming the temperatures. Maybe I need to reduce my expectations a bit.

What kind of overall air temps did you have in the room you were growing in when outside was below freezing?
 
Last edited:
Well well, it turns out that the little $60 dehumidifier off Amazon was completely useless... no big surprise there I guess. Back it goes. It's getting serious I think because the temps and humidity are like 65F-70F and 68-70%, which seems like a dangerously high RH/low temp combo.

Anyone know of a dehumidifier that is smaller than what would be used in a 10x10 room, but still works? I hope I don't have to buy the same sized $250 type that I used to use in a 12'x12' room! That would be total overkill, plus there wouldn't even really be room for one of those in there.
 
My suggestions are to heat and dehuey the whole space - spend the extra few bucks on power to grow your plants in the optimum conditions.
We heat the whole shop to keep the grow at 70. the Dehuey works the whole space and the grow gets the benefits.
good luck!
Reverend Budbreath said:
Well well, it turns out that the little $60 dehumidifier off Amazon was completely useless... no big surprise there I guess. Back it goes. It's getting serious I think because the temps and humidity are like 65F-70F and 68-70%, which seems like a dangerously high RH/low temp combo.

Anyone know of a dehumidifier that is smaller than what would be used in a 10x10 room, but still works? I hope I don't have to buy the same sized $250 type that I used to use in a 12'x12' room! That would be total overkill, plus there wouldn't even really be room for one of those in there.
Click to expand...
 
Thanks for the replies guys.

By the time the heat mats arrived, I had realized a surprising thing—the floor doesn't even really get very cold! I had assumed that hey, concrete floor in winter, of course it's going to get really cold. That's why the mats seemed like a good idea. But then I started going in and actually feeling the floor with my hand at various times including the coldest times of morning, and it just wasn't cold! I guess it warms up enough with the light being on in there, in combination with whatever insulation two layers of tent material provide.

Witchking, I've been running a space heater in there on a thermostatic controller, but it makes me nervous (visions of fire). However, I did find a dehumidifier that I think is going to be great in there. It was $170 as opposed to $60 so I think it's a serious one. It has really good reviews. And it's compact enough (barely) to make sense in the tent: 11.3 x 6.7 x 19.7 inches. In case anyone's interested, it's a Vacplus 30 pint dehumidifier off Amazon. Once that's in there, it should solve both problems of cold and humidity. Hopefully it will get me through flowering this grow and for many more. I'll report on how well it works.

Joe, I know you're talking about putting the dehumidifier in the garage rather than in the tent itself. I have a buddy in town... Pistilwhipt if you remember him... who is advising me to do the same thing. I believe it's going to come down to that once the plants really fill the tent. For now though, I'm going to run it in the tent itself so it won't be running too much.

Another nice thing is that now, finally, the plants are getting to the kind of size where I'm not worrying about their resilience like I was even a week ago. They're really starting to look good, in fact. Finally in full gear. I'll post an update pic later today in my soil forum thread. That's soil for ya... it took a month of veg for them to get to 13" and pumping. :pointlaugh:
 
Update on the dehumidifier, which arrived yesterday after I posted the above. I put it in there and wow, it was clear immediately (like within an hour) that it was going to work well. I set its control to maintain 50% RH, and it pulled it down to that range very quickly and has kept it there all night last night and is continuing to. I feel pretty confident that it's going to handle flowering just fine.

So, I'm posting this in case anyone now or in the future might be looking around for a small but effective tent-scale dehumidifier. This one branded "Vacplus" seems to be a good one.

My little hero:

Vacplus-30-pint-dehumidifier.jpg
 
You must log in or register to reply here.
Back
Top