Overheating plant room (inc MVHR)

[Besidethewye]

Hi there

 

steadily (if slowly) moving forward and getting close to plasterboarding.

 

We've got a small (3m2) plant room on the middle floor (3 storey) build - the new part including the plant room is designed to be passive house standard.  But the old part isn't - so will need heating much more of the time - including a good part of the year when the new part does not. 

 

The plant room has the hot tank, heat pump indoor unit, buffer tank, a lot of piping (three heating circuits and associated pumps, one manifold etc etc) and the MVHR unit.  I am about to go around and insulate all the pipes I can get to - but that won't be all of them!

 

Because it opens on to the stairs we have to have a firedoor - so no vents. 

 

Not switched anything on as yet but have a strong sense it's going to be hot in there.  With the tank standing loss, losses from the buffer when that's warm and losses from the pipes even a poorly insulated airing cupboard would get pretty toasty.

 

I am wondering about putting an MVHR extract in there (and rebalancing the rest of the system to cope) - at least that way the air would move and hot air could at least warm the incoming air when it's cold out. 

 

Anyone been there and done this - ie small plant room in an airtight, passive house?

 

Thanks

[ragg987]

4 minutes ago, Besidethewye said:

Anyone been there and done this - ie small plant room in an airtight, passive house?

A number here have. Some have found overheating, I have not though the room tends to be a touch warmer (maybe 1-2 C) than the surrounding rooms.

 

Our plant room also doubles as utility and is about 6m2. 300l DHW, 90l buffer, indoor ASHP unit, 3 circulation pumps, MVHR, electrical meters, UFH manifold, PV inverter, PV diverter, internet and router, washing machine. I have a MVHR extract in there as we also dry clothes on an indoor line.

 

I have taken care with insulation - DHW tank was pre-insulated at above normal level, I took care of all pipes including junctions to the tank.

[Jeremy Harris]

I doubt that doing this would make any significant difference, TBH, as MVHR only moves a small volume of air, and air is a pretty lousy way to move heat, as it has a low specific heat capacity. 

 

MVHR will typically change the air about once every two hours or so, so not really much.  The specific heat capacity of air is around 1210 J/m³.K, so if the plant room volume was, say, 10m³, and it was 10°C warmer than you wanted it to be, then the MVHR would typically move about 60 kJ of heat out of the room each hour, or around 0.017 kWh. 

[ProDave]

Is it too late to relocate or "distribute" the plant room?

 

For instance, out hot water tank ended up in a bedroom (will be boxed in as an airing cupboard) as that put it more central to points of use than the plant room.  Any heat that leaks for most of the year will be a bit of beneficial heat to that bedroom that otherwise has no heating.

 

And lag ALL the pipes. If you can get at them to install them, you can get at them to lag them?

 

The whole plant room might have been better in the old part of the house.

[Besidethewye]

24 minutes ago, ragg987 said:

A number here have. Some have found overheating, I have not though the room tends to be a touch warmer (maybe 1-2 C) than the surrounding rooms.

 

Our plant room also doubles as utility and is about 6m2. 300l DHW, 90l buffer, indoor ASHP unit, 3 circulation pumps, MVHR, electrical meters, UFH manifold, PV inverter, PV diverter, internet and router, washing machine. I have a MVHR extract in there as we also dry clothes on an indoor line.

 

I have taken care with insulation - DHW tank was pre-insulated at above normal level, I took care of all pipes including junctions to the tank.

Thanks for that - I suspect you are in a better position with a bigger and open room - otherwise similar amount of kit!

[Besidethewye]

16 minutes ago, JSHarris said:

I doubt that doing this would make any significant difference, TBH, as MVHR only moves a small volume of air, and air is a pretty lousy way to move heat, as it has a low specific heat capacity. 

 

MVHR will typically change the air about once every two hours or so, so not really much.  The specific heat capacity of air is around 1210 J/m³.K, so if the plant room volume was, say, 10m³, and it was 10°C warmer than you wanted it to be, then the MVHR would typically move about 60 kJ of heat out of the room each hour, or around 0.017 kWh. 

JS - thanks - a thoughtful and number-filled reply as ever. 

 

We have a rather oversized extract in the room below and I was thinking I could steal some of that - say 10m3/h and live with air being pulled up the stairs on the basis it all sorts itself out via the stair well etc.  It's a tiny volume probably no more than 5m3 after allowing for all the kit in there so the extract rate would bear no relation to room size or any ventilation based calculation. 

 

I've not tried to do a heat balance and we will bleed heat out through the internal walls possibly rendering the whole exercise futile but if we stagnated 10 degrees warmer then that sort of extract would take out something approaching a kWh a day which ought to be somewhere near what the hot tank is going to lose.

 

Clearly spending time and attention on lagging everything I can makes good sense in there.  Maybe it's time for that thermopile I've always wanted to play with dumping heat directly into the MVHR outlet when it's all a bit hot.....

 

 

[Besidethewye]

15 minutes ago, ProDave said:

Is it too late to relocate or "distribute" the plant room?

 

For instance, out hot water tank ended up in a bedroom (will be boxed in as an airing cupboard) as that put it more central to points of use than the plant room.  Any heat that leaks for most of the year will be a bit of beneficial heat to that bedroom that otherwise has no heating.

 

And lag ALL the pipes. If you can get at them to install them, you can get at them to lag them?

 

The whole plant room might have been better in the old part of the house.

Dave - thanks.  Yes too late I am afraid.  We are hemmed around by all sorts of restrictions in terms of layout and space and the old part is listed and a church so no plant room in there - though the logic is obvious. 

 

I will get on it and lag everything in sight (though quite a lot of it is no longer in sight as we keep putting more stuff in there!

 

 

[lizzie]

@Besidethewye my plant room is approx 2m x 1.5m.  I have large hot water tank, ufh manifolds, mains gas boiler, mvhr, a wall of drivers for lighting, a data cab and god only knows what else....they place is a mystery to me!   Not one single pipe is lagged in anyway at all. I have an MVHR extract in there too.

 

It was as hot as hades in there and that transferred to the utility room next door, couldnt get it below 26 on a cool day.  My MVHr and UFH were not set up properly and this was exacerbating it.  The boiler was firing almost continuously (large gas bills). Now I have those things functioning better it has improved but it is still very warm in there. 

 

 

[Jeremy Harris]

10 minutes ago, Besidethewye said:

JS - thanks - a thoughtful and number-filled reply as ever. 

 

We have a rather oversized extract in the room below and I was thinking I could steal some of that - say 10m3/h and live with air being pulled up the stairs on the basis it all sorts itself out via the stair well etc.  It's a tiny volume probably no more than 5m3 after allowing for all the kit in there so the extract rate would bear no relation to room size or any ventilation based calculation. 

 

I've not tried to do a heat balance and we will bleed heat out through the internal walls possibly rendering the whole exercise futile but if we stagnated 10 degrees warmer then that sort of extract would take out something approaching a kWh a day which ought to be somewhere near what the hot tank is going to lose.

 

Clearly spending time and attention on lagging everything I can makes good sense in there.  Maybe it's time for that thermopile I've always wanted to play with dumping heat directly into the MVHR outlet when it's all a bit hot.....

 

 

 

My experience is that the heat loss from a tank will be a great deal higher than the specification, if the tank is kept hot most of the time.  The standing loss test method uses a cyclic measurement where the tank is assumed to be cold some of the time, I believe.

 

I measured the heat loss on our double spray foam insulated 210 litre thermal store and it was specified to be around 1.5 kWh per day, but in reality it was over 3 kWh/day at 65°C.  I added an additional layer of 50mm thick PIR foam all around it, as a foamed in place octagonal jacket, with all the joints foamed and taped, and managed to get the heat loss down to a bit over 2 kWh/day.

 

Our Sunamp is a lot better, it loses around 0.7 kWh/day.

[ProDave]

I assume as you are heating with a heat pump you will be running the hot water temperature lower?  Mine is an unvented cylinder normally heated to just 48 degrees and it does not seem to heat the room much.  I am pretty sure all the connecting pipework gave off more heat than the tank until I carefully lagged all that with good thick pipe insulation.

[Ed Davies]

Pity the heat pump can't take heat from the room, somehow. Can it?

[TerryE]

5 hours ago, Besidethewye said:

I will get on it and lag everything in sight (though quite a lot of it is no longer in sight as we keep putting more stuff in there!

 

A bit of a mistake for the very reason that you mention.  So you should really make an effort to do this lagging properly and the sooner the easier.   As Jeremy says, the SunAmps are amazing for their low heat losses (maybe just over 1kWh /day for two).  I have all of my plant for the DHW, CW, UFH including a Harvey water filter and 2×SunAmp PVs in a 700×1400 full height services cupboard off our ground-floor toilet.  All of our hot runs are lagged and our DHW manifolds are in a lagged box.  The cupboard temp is typically about 2°C warmer than the ground floor rooms.  (The MVHR unit is in the store room on the 3rd floor).  So it can be done.

Edited April 7, 2019 by TerryE

[joe90]

12 hours ago, ProDave said:

I assume as you are heating with a heat pump you will be running the hot water temperature lower?  Mine is an unvented cylinder normally heated to just 48 degrees and it does not seem to heat the room much.  I am pretty sure all the connecting pipework gave off more heat than the tank until I carefully lagged all that with good thick pipe insulation.

 

I am the same as @ProDave, not a plant room but an airing cupboard with tank as above and er indoors is complaining it’s not hot enough as an airing cupboard ?.

[Gone West]

12 hours ago, ProDave said:

I assume as you are heating with a heat pump you will be running the hot water temperature lower?  Mine is an unvented cylinder normally heated to just 48 degrees and it does not seem to heat the room much.  I am pretty sure all the connecting pipework gave off more heat than the tank until I carefully lagged all that with good thick pipe insulation.

I've found the same. I keep the unvented hot water tank at 45C and don't notice any increase in the temperature of the utility room. I think I calculated the heat loss of the tank at just over 0.5kWh/day.

[Besidethewye]

18 hours ago, Ed Davies said:

Pity the heat pump can't take heat from the room, somehow. Can it?

Ed - that would be nice - but it's an air source unit with the heat drawn from the outside unit.  

[Besidethewye]

15 hours ago, TerryE said:

 

A bit of a mistake for the very reason that you mention.  So you should really make an effort to do this lagging properly and the sooner the easier.   As Jeremy says, the SunAmps are amazing for their low heat losses (maybe just over 1kWh /day for two).  I have all of my plant for the DHW, CW, UFH including a Harvey water filter and 2×SunAmp PVs in a 700×1400 full height services cupboard off our ground-floor toilet.  All of our hot runs are lagged and our DHW manifolds are in a lagged box.  The cupboard temp is typically about 2°C warmer than the ground floor rooms.  (The MVHR unit is in the store room on the 3rd floor).  So it can be done.

Indeed so - thought in mitigation it's a bit of a trade off between being sure that everything is in there with no leaks and covering it up in insulation - and the tank had to go in obstructing pipes behind before we could put water in.  It's also very crowded but I did get the plumber to space pipes so they could be lagged so not completely lost.

 

I did wonder about sunamp units but decided against.  

[Besidethewye]

6 hours ago, PeterStarck said:

I've found the same. I keep the unvented hot water tank at 45C and don't notice any increase in the temperature of the utility room. I think I calculated the heat loss of the tank at just over 0.5kWh/day.

Peter - that's a good point - yes lower temperature which will reduce the standing loss significantly.  That takes us in the right direction at least.  

[Besidethewye]

20 hours ago, JSHarris said:

 

My experience is that the heat loss from a tank will be a great deal higher than the specification, if the tank is kept hot most of the time.  The standing loss test method uses a cyclic measurement where the tank is assumed to be cold some of the time, I believe.

 

I measured the heat loss on our double spray foam insulated 210 litre thermal store and it was specified to be around 1.5 kWh per day, but in reality it was over 3 kWh/day at 65°C.  I added an additional layer of 50mm thick PIR foam all around it, as a foamed in place octagonal jacket, with all the joints foamed and taped, and managed to get the heat loss down to a bit over 2 kWh/day.

 

Our Sunamp is a lot better, it loses around 0.7 kWh/day.

interesting point on the calculation of standing loss - I don't know the protocol but experience of another hot water tank containing cupboard (even with the lower temperature of a heat pump) suggests a small, sort of sealed room with a modern tank in it gets very warm.  

[Ed Davies]

5 hours ago, Besidethewye said:

Ed - that would be nice - but it's an air source unit with the heat drawn from the outside unit.  

 

Yes. What I was wondering was if there was any way to warm the gasses returning to the outside unit but, on reflection, I think they'll still be quite warm as the expansion nozzle will be in the outside unit.

[Besidethewye]

31 minutes ago, Ed Davies said:

 

Yes. What I was wondering was if there was any way to warm the gasses returning to the outside unit but, on reflection, I think they'll still be quite warm as the expansion nozzle will be in the outside unit.

I was thinking exhaust air unit on the MVHR output - but sadly it's not one of those and I can't see a way to do it - better than sending a bit of hot air into the MVHR and giving that a small boost.