I've started and commented on a number of discussions relating to the issue of overheating in our passivhaus new build and received some excellent advice - however I still need to do something before next summer and would appreciate your thought on another option.

Firstly, we will replace the MVHR unit next year - an old Titon unit - with one that allows us to use summer by-pass when we need it rather than the machine deciding for us. This will allow us to move some of the daytime heat out during the cooler evenings. However, one of the big heat sources in our house is the solar-panel fed 400l hot water tank that gets pretty warm in the summer, just when we don't want a big "storage radiator" in the house. I was thinking yesterday of maybe installing an extension to the extract ventilation system into the airing cupboard to extract some of the heat - usable in the winter months and with the summer by-pass in operation, a chance to remove some of the heat in the summer.

Any thoughts guys?

Thanks, regards, Jonathan

If you reduce the air temperature in the airing cupboard by say 10degC, the driving force for heat losses from the tank will increase accordingly, so it will give out more heat losses which you don't want anytime.

If you suck more air out of the house it will have to be replaced by air drawn from outside, which can itself be uncomfortably hot during summer.

As someone said you need an awful lot of airflow to move a small amount of heat, maybe more airflow than a mhrv branch could do.

Have you tried
- adding more insulation to the tank walls - I built a PIR box round our cylinder - lining the airing cupboard walls with PIR or duvets would help too?
- reducing the temperature of water in the tank during the summer and/or only heating the top part of the tank - you presumably don't get through 400l of steaming hot water in a day? (But consider legionella).

Is the tank and surrounding pipework fully insulated? Our tank has 75mm of sprayed on foam and it sits in a ground floor boiler room in the centre of the house with all the pipework insulated. With the 250 ltr tank contents sat at 70+ in the summer the boiler room is one of the coolest rooms in the house.

Maybe worth double chdcking all the hot pipework is fully insulated, particularly the solar feed as that gets pretty hot. Also check the immersion is insulated.

Thanks for the feedback guys.

Insulating the tank - it only has the insulation it came with so packing more round it will help, but the insulation will slow the heating of the house, not stop it. The feed from the panels is through the roof space and directly into the top of the tank on the first floor, so although the pipes are too much of a problem, the tank heats up all the areas around it - including the floor I guess!

Simple fan into the roof space to use in the summer is a neat idea tony, especially as it could shift a lot more air than the MVHR I guess! We do open windows on the tilt and turn, although living in the country and surrounded by dairy farms we do have issues with flies!! I'll think on about getting power across!!

All the first floor windows have solar powered external shutters which help and the south facing ground floor windows are covered by a deep porch, so not a lot more to do there.

GreenPaddy - we bought the Titon unit in 2012 and it was cheap (probably out of date then!!). The summer by-pass works off the temp of the incoming air and was coming on at such random times (at full speed which sounded like being next to Heathrow) we had to fit a control switch to switch it off. As it is a Titon unit (and they have offered a 10% discount as we are replacing an older version!) I wanted to replace it with similar so all the fittings and fixings would be the same, but the latest model has a decent control panel option that we can utilise to set the bu-pass when we need it. I'm more than happy to work the controls if it eases the stress of summer overheating, especially as the next option would be AC units working off PV panels.

Thanks djh - something to think about during the weird Christmas we are going to experience!!

Posted By: WillInAberdeenWould be surprising if an extra 50W was sufficient heat to cause the house to overheat?

It won't be enough by itself to overheat the house, but it's enough to notice.

Tony, your house is well renowned, but 60W is the body heat from four cats. If the house only loses 60W of heat in October, and a fifth cat came to visit, the house would overheat.... they couldn't allow any people into the house (100W each, way too hot!).

Only kidding. Are you sure that figure was really the total heat loss, and it wasn't perhaps the external heating requirement after accounting for solar and incidental gains?

How much heat does your hot water cylinder lose into the house, have you put any special insulation on it?

http://www.c60design.co.uk/many-cats-take-heat-passive-house/

I agree / understand that the tank isn't the primary source of the excess heat, but it's part of the problem and I'm just trying to deal with each of the sources (hence the expensive external shutters on the first floor windows).

We also have a 3m wide french door / window arrangement that faces east and gathers all the early morning sun. Again, not a huge source, but we will fit an awning once the patio outside is finished, to reduce that problem. At our advanced ages we are struggling with the higher internal temperatures.

Posted By: bxmanDo you actually need all this hot water within the envelope in the summer ?

When you collect heat from the sun, it's very useful to store it when it's available, since there can easily be periods of several days even in summer when there is very little collected.

Posted By: bxmanin winter for sure but in the height of summer it becomes an embarrassment .

No, I specifically said "there can easily be periods of several days even IN SUMMER".

And heat in a tank isn't embarrassing if the tank is well insulated and ventilated. The size of the tank won't matter all that much. I expect there is some temperature control since allowing it to boil is dangerous.

Heat on demand is a fine policy but doesn't work with solar heat.