Heating required

[stubiff]

Assuming an MVHR and fairly airtight, and appreciate that everybody's setup/situation is different.

Just wanted to get a picture for heat input, say on an average winters day (no sun!).

 

Do you have the heating on in the morning and it lasts for a few hours (with MVHR on), or right through to early evening, or right through the night.

 

If the house requires an MVHR does it makes sense to incorporate extra heating supply via that, thus using less of the 'main' heat source.

 

Thanks.

[Conor]

An MVHR can only move about 1.5kw of heat through it, unless you have massive flow rates. I'm incorporating a water heater matrix in my MVHR for the first floor as we've no heating up there. It's more to help cope with those rare very cold periods. We're building a passive House and the peak heat load is only 2.5kw, so this air heating should help in our case. Same for summer cooling. It'll take the edge off.

 

The short answer is no, don't rely on an MVHR for significant heating, but look at your overall heat losses and peak heating loads. A well insualted air tight house (near passive) should need minimal heating and ground floor UFH once a day should keep you happy.

[jack]

We have an ASHP heating a single downstairs zone and no heating upstairs.

 

I don't actually know exactly what ours does, but at the moment I just leave it on continuously and let it do its thing. As far as I can tell, in cold weather it just comes on periodically for a while at quite a low output.

 

3 minutes ago, stubiff said:

If the house requires an MVHR does it makes sense to incorporate extra heating supply via that, thus using less of the 'main' heat source.

 

Of all the things I'd consider changing or tweaking, adding any form of MVHR heating is at or near the end of the list. You can't easily add enough heat via MVHR to make much difference to the overall temperature of the house. To accomplish that, you'd need a huge amount of airflow. In a typical MVHR system with much narrower ducts than a typical ducted heating system (like Americans use), the required airflow rate will be high, making it noisy. High airflow also mean lower humidity inside, especially on cold dry winter days. You'll also use a lot more energy running an MVHR at high rate for long periods, and the heat exchanger efficiency drops at high flow rates.

And finally, I prefer the bedrooms to be cooler, and that's hard to accomplish if you have heated air coming through the MVHR system.

 

9 minutes ago, stubiff said:

... thus using less of the 'main' heat source.

 

Curious about why you think that might be something to aim for? Your main heat source is likely to be the most efficient and effective way of heating your house. Why would you need/want more?

[pdf27]

11 minutes ago, stubiff said:

Assuming an MVHR and fairly airtight, and appreciate that everybody's setup/situation is different.

Just wanted to get a picture for heat input, say on an average winters day (no sun!).

 

Do you have the heating on in the morning and it lasts for a few hours (with MVHR on), or right through to early evening, or right through the night.

 

If the house requires an MVHR does it makes sense to incorporate extra heating supply via that, thus using less of the 'main' heat source.

It's part of the definition of the Passivhaus standard - the 10W/m² comes directly from the amount of heating that can be provided by the MVHR system without the supply temperature exceeding 50°C, which is associated with burning dust smells. It makes some sense if you're only using resistance heat, but if you are then it absolutely needs to be a full-fat Passivhaus to ensure the heating actually works rather than an approximation using rules of thumb. https://www.paulheatrecovery.co.uk/heat-recovery-explained/feasibility-heating-via-air/ has a good explanation.

On a maximum cold day with no solar gain it'll be on continuously at max power (i.e. 10W/m²: not very much), and will probably modulate by switching on and off as required when the heating load drops. Because you're heating the air directly you're unlikely to be able to make much use of something like Economy 7 however.

[ProDave]

Our house is pretty much built to passive standards and has a maximum heat input of just under 2.5kW, most of the time it will be way less than that.

 

Heating is an ASHP and UFH downstairs and upstairs only in bathrooms.  The heating is timed to come on at 6AM and off at 9PM and is then under the control of room thermostats.

 

The UFH runs at low temperatures such that you barely notice the floor as being "warm" even id you walk on it with bare feet, instead it is "not cold".  So the UFH will not heat the house quickly.  But that is an important point. Wth a well insulated air tight house with mvhr it will retain it's heat for a long time and only cool down very slowly. so you only need a low level of heat input and there is no harm in leaving it on all the time.  Forget the old fashioned notion of having the heating on in the morning and the evening and letting the house cool down in the middle of the day.  It won't.

[stubiff]

54 minutes ago, jack said:

Curious about why you think that might be something to aim for? Your main heat source is likely to be the most efficient and effective way of heating your house. Why would you need/want more?

 

Not that you need/want more, but to provide it in a different way (even if it's not as efficient, it may be simpler or preferred).

 

The link from @pdf27 is very useful, thanks.

"As only limited heat can be transferred by air, the heat load of the house has to be very low, if air heating is the only space heating system."

 

Am not necessarily wanting to have all the heating provided by the MVHR, but to get a picture of what 'main' heating is required to make things work. As, if the MVHR is 'required' in an airtight house then it makes sense to at least look at what can be done with it, on top of ventilation. 

One may choose (just) to have a woodburner, say, to be the source of a 'lot of heat', and let the MVHR + heater do the rest.

 

I know a lot of people have ASHP + UFH (as the main source of heat), but there are other options (preferred or otherwise, which I am looking at).

 

 

[jack]

30 minutes ago, stubiff said:

As, if the MVHR is 'required' in an airtight house then it makes sense to at least look at what can be done with it, on top of ventilation. 

 

As mentioned above, an assumption that air heating via MVHR would be the primary source of heat for a PassivHaus is a major factor in the peak heat load set by the PH standard. However, the fact that so few PassivHaus projects use MVHR heating should tell you something.

[SteamyTea]

1 hour ago, stubiff said:

but to get a picture of what 'main' heating is required to make things work.

Just take the modelled contribution from the MVHR away from the modelled contribution of the main heating system.

If ASHP, then up the capacity to the next available size that is 30% of so oversized.

[Iceverge]

You'll achieve the same cheaper with straightforward electric heating. A couple of towel rads in the bathrooms maybe. 

 

In general asking anything to do something that's not its primary function is always more expensive and less satisfactory. 

[Bitpipe]

We don't even have a timer on our heating, it's 'on' 24/7 year round but driven by a stat which is set to 20^o.

 

Same for the bathroom under tile mats. We just turn off the towel rads at the controller when summer starts and back on when it gets colder.