Controls for slab radiant zone, tenant wants setback

vilord

Good day!

We run hydronic baseboard in the main house, but have an in-law apt with radiant in the slab.
The apt is only about 400sq ft.

In winter, the tenant likes the temperature to be 62 from 11pm-8am, 72 during the day. She can't sleep if it is too warm and will open a window in the middle of the winter.
(Yes I know, slab radiant with a big setback is just asking for pain, well here we are)

The space also has a minisplit with winter heat capability.

I'm thinking of installing a Tekmar 521 with floor sensor, and setting it in floor-only operation with a floor temp of 65 from about 8pm-6am, 75 from 6am-8pm so that the bulk of heating is being done by the radiant and the slab has time to cool and heat, and then letting her run the mini-split to boost as needed.

Please shoot holes in this plan? Is there a better way to get the desired result?

Jamie Hall

Let her run her mini-split. Use the slab to maintain the base temperature, and set the mini-split to pick the space temperature up to the tropics in the daytime.

Much as one might like it to, there's no way the slab is going to respond that fast.

I presume she pays the utility bill for the mini-split?

vilord

She doesn't pay any utilities...

So should I just lock in the floor at 65?

vilord

I wonder if she'd be able to sleep with a slightly higher night time temperature (65ish, with slab at 70ish) if I installed a ceiling fan over her bed, heh.

nibs

Depending on the mass of the slab, she may have to shut down the boiler at 4 pm, and start it up at midnight.
New tenant maybe?

HVACNUT

Put in the 521 and lock it to 62°.

STEVEusaPA

Get a new tenant, seriously. Sounds like a PIA. I wouldn’t have a tenant and pay for their heat and running a mini split simultaneously.

Zman

vilord said:

She doesn't pay any utilities...

So should I just lock in the floor at 65?

Yes! Your tenant will not have toasty warm floors but that is the trade off.

Jamie Hall

What @Zman said.

Jean-David Beyer

I have a fancy thermostat that allows 4 different temperatures each day, so I could have 28 different temperatures each week. But it controls my radiant slab at grade. I have a mod-con with outdoor reset and the reset curve is set very tight so I can supply just a tiny bit more heat than the thermostat is set for. On really cold days, the heat is on steady for 18 hours..

So I thought to put some night-time setback in there, so I started the setback at 10 PM and the house did not drop more than two or three degrees all night even though the thermostat was supposed to learn and apply the new temperature sooner. No go. I then made the setback start around 4PM, and the temperature did drop enough to notice by bed-time, but not nerely the setback amount. And in the morning, even though by then I set the temp to come back up at 4AM, it was nowhere near back by 8AM.

Basically, I have to figure about 24 hours for a temperature change at the thermostat to be approached by the temperature change to be fully in effect and 48 hours for the temperature to stop hunting. So now I do no setbacks. For my radiant slab at grade, as a practical matter, it just does not work. Maybe if I left the windows open ... .

vilord

Even if you had the windows open, that slab isn't heating or cooling any time quick.
Maybe I'll set it to 70 from midnight to 4pm, and 65 from 4pm to midnight...
It won't really have any meaningful effect, but even a degree or two of heat from the slab is a degree or two i don't have coming out of my electric.

To others: Tenant is a friend of the family who just got out of surgery for cancer, so we're not going to ask her to leave any time soon.

Glad to hear the plan of controlling the slab with a floor sensor is a good one, even if the setback won't do anything.

Ironman

What's controlling the supply water temperature to the slab? It needs to be on some form of outdoor reset. That will more than a floor sensor for controlling it.

Jamie Hall

Quite seriously, it is perfectly possible to have a heating system with setbacks which uses radiant -- but only if the radiant is only one component of the system. Doing it with the combination you have -- radiant plus a mini-split -- is not out of line at all. Use the radiant to provide the base heat load, and the mini-split to raise the temperature as desired and when needed. If you were designing from scratch, one could do the same sort of thing with all hot water -- radiant as the base, and panel or conventional radiators as boost when wanted.

One thing you might consider -- but only if the radiant itself is zoned -- is setting a programmable thermostat to boost the floor temperature in one space -- say the bathroom -- at specific times. No, it won't change the space temperature significantly. But the floor might be just a bit warmer which might be a nice comfort feature.

vilord

Ironman:
supply comes from a thermostatic valve that is fed from one zone off a new G115 boiler.
for ODR to be effective on the radiant I'd need a mixing valve and mixing valve control that supports ODR (or a logamatic control for the boiler) which would add significantly to my install... and I'm already at $14k or so for the boiler and the added baseboard in the main house where we ripped out the air handler that was pumping heat into the basement and walls

maybe next year i can consider that, though I would need to find a company that actually knows what they're doing with radiant to do it right, so far I've been impressed by how many companies have no f'ing idea how radiant works (we had 4 different companies come through to quote installing the boiler)

HVACNUT

The Hydrostat 3250 Plus does accept ODR.

vilord

True, but it is also feeding a bunch of baseboard zones, including some 30 year old baseboard.
The 3250 also uses algorithms to guess what temperature it should heat to based on how long a thermostat called for heat.
Since the radiant might circulate for 4 hours without actually needing to call for heat due to the thermostatic valve, the radiant is just going to screw up those algorithms.
Also, the baseboards need a higher temperature than the radiant, so even with ODR would it really help?

Ironman

This is what you want:

https://www.supplyhouse.com/Taco-I075T3R-1-3-4-3-Way-Outdoor-Reset-I-Series-Mixing-Valve-Thread

It would replace the thermostatic valve and provide ODR only for the slab.