UFH to help cooling?

[Mike]

2 hours ago, JohnMo said:

6 hours ago, Mike said:

No more than 6 circuits per manifold

That's a strange one - any obvious reason for this rule?

It isn't obvious, but I've tracked down the 1998 version of the regs that includes an explanation. That says to limit the circuits to 5 or 6 and to optimise the routing of loops to avoid concentrations of pipes, particularly in hallways, but to insulate them if necessary. Which I would take to mean to avoid condensation on parts of the floor that would, if pipes are too bunched up and not insulated, be a few °C cooler than the general floor temperature.

 

UFCH with cooling is pretty common France - apparently 1/3 of all new houses have it - and it has a long history, so their current regs (last updated in 2023) ought to be a good guide; I can spot a few things that have changed since 1998.

 

Edited May 22 by Mike

[richo106]

Hi all

 

I set my UFH temperature to 15 deg for cooling last heat wave and I monitored dew on the manifold and there was nothing. However when I turned it on Sunday for this hot period, I saw dew on my manifolds and the tiled floor felt very very slightly damp ish yesterday so I have upped the temp to 17 deg. This seemed ok until this evening when the manifolds had dew on them again albeit only very slightly.

 

how/what is the best way to monitor my dew point within the house?

 

is there anything I can integrate into my system retrospectively?

 

any advice/information greatly appreciated!

[Gus Potter]

2 hours ago, richo106 said:

any advice/information greatly appreciated!

Houses are not an exact science. If we design them for every eventuality then the build cost would be to high.  

 

With my SE hat on we design to tolerate occasional short term elevated moisture content as you are experiencing. Any condensation will quickly dry out. 

 

If you are worried about dew points.. open some windows and doors, strip down to your underpants, fill the paddling pool and enjoy the warm weather.. it won't last that long. 

 

 

[SteamyTea]

8 hours ago, richo106 said:

how/what is the best way to monitor my dew point within the house?

 

is there anything I can integrate into my system retrospectively?

Yes, you need to monitor temperature and relative humidity, then apply Td = (b × α(T, RH)) / (a − α(T, RH))

Where Td = Dew point temperature

• Td = dew point temperature
• T = actual air temperature (in °C)
• RH = relative humidity (in percent)
• a = 17.27
• b = 237.7°C
• α(T, RH) = [(a × T) / (b + T)] + ln(RH / 100)

Then use the Td result to adjust your UFH flow temperature up or down.

 

 

[dpmiller]

does your system control on flow or return temp tho?

[richo106]

33 minutes ago, dpmiller said:

does your system control on flow or return temp tho?

No, i just set the water temp

 

[richo106]

7 hours ago, Gus Potter said:

Houses are not an exact science. If we design them for every eventuality then the build cost would be to high.  

 

With my SE hat on we design to tolerate occasional short term elevated moisture content as you are experiencing. Any condensation will quickly dry out. 

 

If you are worried about dew points.. open some windows and doors, strip down to your underpants, fill the paddling pool and enjoy the warm weather.. it won't last that long. 

 

 

Yes is quickly disappeared when I opened some windows, I didn't get home till late and the mrs hadn't opened any windows so this helped when the temp was cooler outside and purged my MVHR

 

There was hardly any dew just a very small amount on the manifold, nothing on the tiled floor. Can the moisture do any damage apart from to woodwork/make the floor slippery?

 

I have now set the temp to 17 deg but i may try and monitor the temp/humudity. Where is the best place to have the monitor?

Edited 13 hours ago by richo106

[richo106]

I have been looking into but has anyone successfully cooled the supply air for a MVHR system? I am not after AC like performance but just something to take the edge off on the really hot days

 

Many Thanks 

[saveasteading]

Sitting in our old draughty house, thinking how controllable it is, using Mediterranean techniques and 2 hours of aircon unit in one room. And how mvhr would be making it worse.

 

[SteamyTea]

57 minutes ago, richo106 said:

Where is the best place to have the monitor?

If you have MVHR then in the ductwork just before the unit.

If you don't, then it gets tricky. A hallway used to be the place that thermostats were placed, but that was pointless as in the 'olden days' people keeps interconnecting doors closed.

Really a case of monitoring where you think there will be a problem i.e. floor in living room or kitchen.

Place the monitor close to the problem area as it is common to have a temperature and RH height gradient in a room.

[jack]

1 hour ago, saveasteading said:

And how mvhr would be making it worse.

 

How so? All MVHR does is try to maintain any temperature difference between inside and outside. If it's cooler inside than out on a hot day, then MVHR will help.

Or are you thinking about how MVHR would (or wouldn't!) work in your draughty house? 

[JohnMo]

3 hours ago, richo106 said:

I have been looking into but has anyone successfully cooled the supply air for a MVHR system? I am not after AC like performance but just something to take the edge off on the really hot days

 

Many Thanks 

I can work BUT

Flow rates are pretty much multiple times higher than normal MVHR flow rates as set for BR or Passivhaus.

 

Download datasheets read water flow temp requirements, air flow required and kW of cooling output

[saveasteading]

3 hours ago, jack said:

If it's cooler inside than out on a hot day, then MVHR will help

Please help me understand?

A fan takes the internal (24°C)  air out, through a heat exchanger. It partially cools the incoming air currently at 33°C.

What temp is the incoming air likely to be?

I'm not understanding how that helps.

 

I have to admit to scepticism in general, from a decade ago when mvhr systems were mostly inefficient bodges which the reps couldn't, or wouldn't, explain apart from ticking a sustainability box.

 

We've gone Spanish. A bit of work outdoors early. 12 til 5 indoors. Evening outside.   Dinner later than usually. (Football agony at midnight.)

Curtains and blinds closed according to sun movement.

[-rick-]

3 minutes ago, saveasteading said:

Please help me understand?

A fan takes the internal (24°C)  air out, through a heat exchanger. It partially cools the incoming air currently at 33°C.

What temp is the incoming air likely to be?

 

Depends on your unit. They can be between 60 and 90% efficient.

 

33-24 = 9

9 x 0.1 = 0.9

0.9 + 24 = 24.9 incoming temp in the 90% efficient case? (not 100% sure I've done this right but you get the idea).

 

The air from outside will inevitably be warmer than the inside air in this situation, but would be much less warm than just opening the windows and blowing the outside air in.

[JohnMo]

31 minutes ago, saveasteading said:

4 hours ago, jack said:

If it's cooler inside than out on a hot day, then MVHR will help

Please help me understand?

A fan takes the internal (24°C)  air out, through a heat exchanger. It partially cools the incoming air currently at 33°C

If house is cooler than outside

The heat exchanger is a counter current one, so the air entering the heat exchanger from the house and leaving the exchanger to enter house are at the same end.  This gives an approach temp that is very close. So if air in house is say 22 the air being pushed into house will be sat 23/24. So will warm house up over many hours.

 

Warmer inside than outside

the air is warmed to very close to the inside air temp, this will cool the house but very slowly - many hours. In summer a bypass around heat exchanger operates to allow cooler outside air to by go around the heat exchanger.

[JohnMo]

But remember MVHR only changes house air every 2 to 3 hours, so in cooling/heating terms pretty rubbish.

[saveasteading]

Thanks. Warms the house rather than cools it but not a lot.

We are simply leaving doors open after 6pm ish and the house cools ready to start again tomorrow. 

[jack]

14 minutes ago, saveasteading said:

Thanks. Warms the house rather than cools it but not a lot.

We are simply leaving doors open after 6pm ish and the house cools ready to start again tomorrow. 

 

You need some ventilation even when everything is closed up. Draughts will do it if have a draughty house. MVHR is a way of providing that ventilation without bringing in full temp air from outside to replace your precious cool air.

 

As noted, MVHR isn't 100% efficient. I tend to keep it turned on as low as possible during weather like this, as that reduces the losses in absolute terms (i.e., you lose double the amount of energy with the same 90% efficiency if you double to airflow).

[saveasteading]

I've only ever lived in draughty houses tho the steading is pretty well airtight but doesn't get stuffy.

I'm of the opinion that the essential vents at fans and through opening of doors etc we get enough oxygen. 

But maybe when we relax and doze of watching Repair Shop it is lack of oxygen.

[JohnMo]

38 minutes ago, saveasteading said:

through opening of doors etc we get enough oxygen. 

Not if oxygen is really the issue as you get more airtight, it's dilution and getting of excess CO2 and removing moisture.

 

Other thing you will notice as you get airtight is opening an outside door doesn't blow through the house, unless you have a window open as well.  As it has nowhere to blow out off.

Edited 5 hours ago by JohnMo

[NSS]

 

This was at 13.35 today. We have hard (Karndean) flooring throughout. Had a few areas where the surface has got a little damp this afternoon, but I can live with that.

[jack]

52 minutes ago, NSS said:

Had a few areas where the surface has got a little damp this afternoon, but I can live with that.

 

I lowered the flow temp on our underfloor cooling from 16 deg to 14 deg a few days ago, in anticipation of the several day heat wave.

 

The surface of the door (polished concrete) slowly fell to a minimum of 18.7 deg a couple of days ago. It's slowly drifted up to 18.9 deg since. 

 

In the middle of the afternoon today, I noticed some darker spots on the surface of the concrete. Within three hours of noticing that, entire sections of the floor in the kitchen were slightly damp.

 

We've never seen so much as a whiff of moisture in 7 or 8 years of using underfloor cooling. It's amazing what a two degree flow temperature difference coupled with high humidity (~80 inside at the moment!) can do.

 

I've turned the flow temp back up and will monitor. 

[NSS]

20 minutes ago, jack said:

 

I lowered the flow temp on our underfloor cooling from 16 deg to 14 deg a few days ago, in anticipation of the several day heat wave.

 

The surface of the door (polished concrete) slowly fell to a minimum of 18.7 deg a couple of days ago. It's slowly drifted up to 18.9 deg since. 

 

In the middle of the afternoon today, I noticed some darker spots on the surface of the concrete. Within three hours of noticing that, entire sections of the floor in the kitchen were slightly damp.

 

We've never seen so much as a whiff of moisture in 7 or 8 years of using underfloor cooling. It's amazing what a two degree flow temperature difference coupled with high humidity (~80 inside at the moment!) can do.

 

I've turned the flow temp back up and will monitor. 

Ditto Jack, we've never had this before, but we've also had the MVHR running slower than normal (speed 1 rather than lifting to speed 3 overnight).

[JohnMo]

1 minute ago, NSS said:

Ditto Jack, we've never had this before, but we've also had the MVHR running slower than normal (speed 1 rather than lifting to speed 3 overnight).

Aren't you better to run the MVHR at the normal speed. Then the MVHR actively pulls the humidity out the house, moving dew point upwards.

[jack]

29 minutes ago, JohnMo said:

Aren't you better to run the MVHR at the normal speed. Then the MVHR actively pulls the humidity out the house, moving dew point upwards.

 

Generally yes, but only if humidity is lower outside than in.

Where I am, humidity is still high, and is forecast to rise through the night into the high 80s. Thankfully it should drop a fair bit through tomorrow.