MVHR running properly???

[joe90]

As some will know we have only recently finished (well nearly) our build and moved in, heating been working for a couple of months and gradually getting it trimmed. I have been a little dissapointed that with UFH downstairs only and electric towel rads and electric UFH in our en suite the upstairs room temp is a little low. Downstairs stays between 20’ and 22 but upstairs is between 16 and 18. I have just measured the air temp coming out of one of the bedroom terminals and it’s 16’. I would have thought that with 20’ downstairs I would be getting warmer air delivered upstairs. It is currently 6.6’ outside so is the temp uplift from the lossnay MVHR about right?

[ProDave]

That is similar to what we are finding, so perhaps the Lossnay units are not as good as they claim?

 

It is colder here. When it's sub zero outside, 20 downstairs, the bedrooms have got down to 16 and I have been told that is too cold.  The fix so far has been fire up the stove, get downstairs up to 25 and then all that excess heat finds it's way upstairs (mostly by convection up the stairwell) to warm up the bedrooms a bit.

 

Currently about 5 degrees outside (and blowing a hoolie) Supply air to bedrooms is measuring 17 degrees so about 3 degrees cooler than downstairs exhaust air.  17 degrees supply air is a lot better than 5 degrees it would be today without the mvhr.

 

Remember, just because downstairs exhaust air  is at 20 degrees does not mean it will heat all supply air to 20 degrees.  Some (half) of the exhaust air will be from the upstairs rooms that are also a bit cooler.

[joe90]

Thanks @ProDave, yes fresh air at 16’ is better than trickle vents or open windows at 6’ ?. I did wonder if the summer bypass was stuck on, but no. My fall back for cold weather was always small electric plug In Radiators, an hour @ 500w would make a big difference. I had thought  of an in-line electric heater within the manifold but no sense in heating all rooms when only one bedroom is occupied at a lower temp. 

[jack]

I knocked up a spreadsheet in summer when I was trying to understand how cooling downstairs affected the upstairs temps. You basically fill in the orange boxes with starting state, and see what the impact is on other temperatures.

 

Assuming 80% efficiency, 6 degrees outside, 18 degrees upstairs and 22 degrees downstairs, you can see that the temp of the supply air is actually only 17.2 degrees, so :

 

 

Bumping efficiency helps - at 90%, the supply temp is 18.6 degrees.

 

Lots of assumptions in this, but as a coarse model I've found it interesting to play with.

 

The other thing is that towel rails and other forms of bathroom heating only heat air that's already passed through the bedrooms. While they'll have an impact on the temperature of the supply air due to the MVHR heat exchanger, half of the heat they supply will end up in the supply air to downstairs.

 

Our bedrooms are at about 20 degrees at the moment. Great for sleeping, very slightly cooler than would be ideal for getting out of bed in the mornings. 

 

 

[joe90]

Thanks guys, In fairness I have not balanced the MVHR yet so don’t know what airflow I have, I did however restrict the air into the bedrooms a little as they were the shortest runs.

[Oz07]

Is 20c really cold to get out of bed for you @jack?!

 

I have my temp set around 16 and everyone who visits comments that it's toasty. What's going on?!

[joe90]

6 minutes ago, Oz07 said:

Is 20c really cold to get out of bed for you @jack?!

 

I have my temp set around 16 and everyone who visits comments that it's toasty. What's going on?!

 

I must admit I don’t like the house or bedroom too warm, the downstairs here is a constant 20’ to 21’ but does not feel overly warm, in our last house (very poorly insulated but gas central heating) if the stat was set to 21’ it “felt” warm!!!

[AliG]

That is similar to what we see with out MVHR, the incoming temperature is increased from a low outside temperate, but not to the same temperature as extract air.

 

I don't think 90% efficiency means that you get 90% of the temperature of the extract air, you would have to calculate the energy capacity of air which would probably be based on its temperature in Kelvin.

 

Also I assume that some of the extract air is coming from colder upstairs rooms such as bathrooms.

 

Basically when there is a 15C+ temperature difference between inside and outside air MVHR will help but you will probably still need some heating.

 

@Oz07 that's freezing!

 

 

[Gone West]

8 minutes ago, Oz07 said:

Is 20c really cold to get out of bed for you @jack?!

 

I have my temp set around 16 and everyone who visits comments that it's toasty. What's going on?!

Our upstairs temperature is around 22C and downstairs around 23C, if it wasn't there would be complaints. 16C maybe toasty to an Innuit.

[joe90]

3 minutes ago, AliG said:

Also I assume that some of the extract air is coming from colder upstairs rooms such as bathrooms.

 

Ah, @AliG not thought of that, go to the top of the class.

[AliG]

If you think about it, MVHR is always cooling down your house (assuming outside is colder than inside). The more it runs, the more cold air it brings in, it just isn't as cold as it might be due to the heat exchange capturing energy from the extracts.

 

If you switch off the MVHR upstairs might get warmer, but it would be stuffy.

Edited December 13, 2018 by AliG

[ProDave]

8 minutes ago, joe90 said:

 

Ah, @AliG not thought of that, go to the top of the class.

I made that point in post #2, and Jack explained it with a spreadsheet in post #4

[ProDave]

4 minutes ago, AliG said:

If you think about it, MVHR is always cooling down your house (assuming outside is colder than inside). The more it runs, the more cold air it brings in, it just isn't as cold as it might be due to the heat exchange capturing energy from the extracts.

 

If you switch off the MVHR upstairs might get warmer, but it would be stuffy.

Is there merit in switching off the mvhr in the daytime if we are all out?

[AliG]

1 minute ago, ProDave said:

I made that point in post #2, and Jack explained it with a spreadsheet in post #4

 

@ProDaveSo you did, I always tell people nothing to put something important in the last line of an email as I usually give up before then.

 

2 minutes ago, ProDave said:

Is there merit in switching off the mvhr in the daytime if we are all out?

 

That's a good question. You probably could and you would save on running the MVHR and heating, but then you might forget to switch it back on. 

 

Does your MVHR have any programmes? Ours has various programmes that can be set to run more slowly when it is less likely that people are in or when you want it to be quieter. It is Dantherm, so nothing fancy.

 

I think running it constantly on a low setting is maybe better, but I am guessing.

[Oz07]

My bad it is a gas ch house with thermostat. The stat is set to 16 but the heating does click this off so must be somewhere between 16-18? It's not as if visitors are upstairs where I dare say it's a touch warmer. 

 

Regardless it can't be above 16 when I jump out of bed in AM. Heating goes off at ten pm. 

[Oz07]

Just googled it ideal sleep temp 16-18

[Cpd]

3 minutes ago, Oz07 said:

Just googled it ideal sleep temp 16-18

I am happiest sleeping in about 14-15 I am uncomfortable at 17+ I am away from home at the moment working and staying with a friend, the room I sleep in very rarely gets above 10..... I find this to cold and need an extra blanket and when it’s -6 outside I wear a hat ! 

[Ed Davies]

2 hours ago, AliG said:

I don't think 90% efficiency means that you get 90% of the temperature of the extract air, you would have to calculate the energy capacity of air which would probably be based on its temperature in Kelvin.

 

It should be 90% of the temperature of the extract air relative to the outside air (not absolute zero) if the flows are balanced, there's no condensation going on and so on.

 

e = (Ts - To) / (Te - To)

 

where:

 

e = efficiency (as a proportion; multiply by 100 to get percent, of course).

Ts = supply air temperature (air going into the rooms from the exchanger).

To = outside temperature (air going into the inlet from outside to the exchanger).

Te = extract air temperature (air going from the rooms to the exchanger).

 

The exhaust air temperature (air going from the exchanger to the outside) doesn't matter, that's lost and gone.

 

So:

 

Ts = e × (Te - To) + To

 

or, if all temperatures are measured relative to outside, it's simply:

 

Ts = e × Te

[Ed Davies]

2 hours ago, ProDave said:

Is there merit in switching off the mvhr in the daytime if we are all out?

 

I think there's a lot of merit in controlling the MVHR speed depending on the CO₂ and humidity levels in the house. Turn it down automatically when it's not needed, boost it when it is. Also drying the house out when it's mild and dry outside by running the MVHR a bit more to get the RH inside down then throttling back when it's wet and cold out to reduce wasting energy.

[Jeremy Harris]

@SteamyTea did some number crunching on data that both I and Damon Hart-Davis had collected, that included temperature, CO₂ and RH, and concluded that there was a very close correlation between CO₂ and RH when the house is occupied.  The usefulness of this is that accurate RH sensors are pretty cheap, whereas decent CO₂ sensors aren't (I have a few NDIR sensors bought surplus, but they retail for around £80 each IIRC).

 

I suspect that just using RH for MVHR control would work OK, with one proviso.  Our's boosts using an RH sensor in the extract, and I find I need to adjust the threshold between winter and summer to make it function effectively.  I keep meaning to make a more sophisticated controller, that is able to use rate of change of RH to trigger boost.  Making the same controller vary the MVHR speed, or turning it off when there's no one home, should be fairly straightforward.

[Ed Davies]

Suppose @joe90 's problem is imbalanced flows, how imbalanced would they have to be to see the temperatures he reports with decent efficiency in the actual heat exchanger?

 

Suppose x is the eXtract airflow (kg/s or whatever) and y is the supplY airflow. Then the efficiency would become:

 

e = [y × (Ts - To)] / [x × (Te - To)]

 

The (im)balance ratio would then be:

 

y/x = e × (Te - To) / (Ts - To).

 

He says downstairs stays between 20 and 22 and upstairs between 16 and 18. Let's assume 21 downstairs and 17 upstairs with equal extracts from each so an average of 19 °C. His supply is at 16 °C and the outside air at 6.6 °C. Assuming 90% or 95% efficiency for the heat exchanger gives:

 

y/x = 0.90 × (19 - 6.6) / (16 - 6.6) = 1.187…

y/x = 0.95 × (19 - 6.6) / (16 - 6.6) = 1.253…

 

In other words the supply airflow only needs to be 18 or 25% more than the extract airflow for 90 or 95% exchanger efficiencies respectively to get this somewhat disappointing supply air temperature. I'll admit to being a bit surprised, my hunch before doing this calculation was that it would be more like 50% difference.

 

If only @SteamyTea had put some temperature sensors in @joe90's ducts ?

[Ed Davies]

33 minutes ago, JSHarris said:

The usefulness of this is that accurate RH sensors are pretty cheap, whereas decent CO₂ sensors aren't (I have a few NDIR sensors bought surplus, but they retail for around £80 each IIRC).

 

Yes. I have a CO₂ monitor but it was about £80 though for the whole monitor including an ESP8266, temperature, pressure, RH and so on sensors. How decent it is another question but it seems to give reasonable readings. What's important is knowing when CO₂ is high and when it's low for your house, an absolute value in ppmv doesn't matter really.

[jack]

4 hours ago, Oz07 said:

Is 20c really cold to get out of bed for you @jack?!

 

Not for me, no, but I hear about it a fair bit from everyone else in the house!  I think it's just dark and miserable, and people are looking for excuses not to get out of bed.

 

3 hours ago, AliG said:

I think running it constantly on a low setting is maybe better, but I am guessing.

 

I suggested turning the MVHR rate down in hot weather, and the same applies when it's very cold. A slower fan speed will reduce the rate at which the energy is transferred, which naturally becomes more critical as the temperature outside diverges further from the desired temperature inside. 

[joe90]

49 minutes ago, Ed Davies said:

If only @SteamyTea had put some temperature sensors in @joe90's ducts ?

 

Ed, he did, completely forgot about it. It’s a shame he’s not here to answer, perhaps I will email him and see if he can give us sine answers (I have no idea what your maths are above?)

[Ed Davies]

Yes, he mentioned the sensors in an email about something else (refrigerators). Are they logging to anything, yet?

 

Bottom line, an imbalance of 20% or so between your supply and extract airflows could knock the supply temperature down as much as you seem to be finding.