MVHR + ASHP Post Heater control

[john0wingnut]

Hi all

 

Just wanted to clarify if my plan below is ok:

 

I have a NIBE F2040 12Kw heat pump, doing my UFH and DHW currently. I plan to fit MVHR, when the first floor extension is done.

 

So, my plan is, to fit a water based post-heater in the house supply duct, after the MVHR unit, then add a feed to / from it, from my buffer tank supply loop, which goes to the manifolds.

 

If, any room in the house, calls for heat, it will activate the UFH pump and this would in turn provide the heated water to the post heater. If no heat was being requested by a room, no water would flow to it.

 

Is that ok, or, should I have another stat, or temperature feed from the MVHR unit controlling the post heater supply?

 

My thought process is, that if no room is asking for heat, then you dont need to have any heating in the supply air, but if heat is being called for, then having it help warm the supply is a good thing, and would not require any further controls, so to speak

 

Thanks

[A_L]

52 minutes ago, john0wingnut said:

If, any room in the house, calls for heat, it will activate the UFH pump and this would in turn provide the heated water to the post heater

 

1) Every other room, not calling for heat, will receive some and overheat.

2) The post heater will only receive water at the temperature of the buffer tank and the heat transferred will be very small

3) Why does the heat pump not keep the cold room(s) up to temperature?

[ProDave]

Would it not make more sense to just fit UFH upstairs?

 

Do you actually need heating upstairs?  I am one of several on here that doesn't.  currently about 2 degrees outside (sub zero overnight) 20 degrees downstairs. 19.5 upstairs with no heating upstairs.

[john0wingnut]

6 minutes ago, A_L said:

 

1) Every other room, not calling for heat, will receive some and overheat.

2) The post heater will only receive water at the temperature of the buffer tank and the heat transferred will be very small

3) Why does the heat pump not keep the cold room(s) up to temperature?

 

1) Should in theory receive similar level to that being extracted, and MVHR is not all that good at moving heat around, from what I have read.

2) True, but the temperature supplied from the buffer is the same that goes to all UFH zones,  and small transfer is not an issue, given the HE will get the air to within a few degrees of teh extract air, and thus this would take the final edge off and get it up to supply temp - thats my thought process anyway.

3) It does. All rooms have their own zone / stat, so only call for UFH when needed.

[john0wingnut]

2 minutes ago, ProDave said:

Would it not make more sense to just fit UFH upstairs?

 

Do you actually need heating upstairs?  I am one of several on here that doesn't.  currently about 2 degrees outside (sub zero overnight) 20 degrees downstairs. 19.5 upstairs with no heating upstairs.

Im not fitting it upstairs - just rads in bathrooms.

It was more around, whether having a post heater is a good thing to help bring the air backup to supply temp, as from what I have read, the supply is usually a few degrees cooler than extract, so in the depth of winter, having a post heater to give a bit of heat from the UFH loop might help the air comfort side of things

[ProDave]

1 hour ago, john0wingnut said:

Im not fitting it upstairs - just rads in bathrooms.

It was more around, whether having a post heater is a good thing to help bring the air backup to supply temp, as from what I have read, the supply is usually a few degrees cooler than extract, so in the depth of winter, having a post heater to give a bit of heat from the UFH loop might help the air comfort side of things

Yes with mvhr the supply air into the rooms will be a little colder than the room temperature, but that compared to uncontrolled trickle ventilation where the fresh air in would be a LOT colder than the room air.

 

But just heating ALL the incoming air might overheat rooms that are already up to temperature.

[john0wingnut]

30 minutes ago, ProDave said:

Yes with mvhr the supply air into the rooms will be a little colder than the room temperature, but that compared to uncontrolled trickle ventilation where the fresh air in would be a LOT colder than the room air.

 

But just heating ALL the incoming air might overheat rooms that are already up to temperature.

Ok, so I shouldn't bother with a post heater then?

[SteamyTea]

8 minutes ago, john0wingnut said:

Ok, so I shouldn't bother with a post heater then?

I would not say that.

When you only need a small increase, or decrease, in temperature, you may find it works well.

Really just a case of doing the numbers and seeing how much can be transferred on the highest flow settings.

[Gone West]

4 hours ago, john0wingnut said:

My thought process is, that if no room is asking for heat, then you dont need to have any heating in the supply air, but if heat is being called for, then having it help warm the supply is a good thing, and would not require any further controls, so to speak

I think your idea is good and a simple solution to overcoming the problem of cool supply air in the winter. It's unlikely that at normal UFH water temperature you will get too much heat. Our MVHR has a built in EASHP which keeps the air supply at least as warm as the room. It will also provide warm air if required up to a maximum of 50C, any hotter and apparently the dust in the system starts to smell.

[PeterW]

1 hour ago, john0wingnut said:

Ok, so I shouldn't bother with a post heater then?

I fitted one of these with a controller to reduce the power to it, controlled by a Shelly that checks the external air temperature, MVHR supply temperature and the post heater temperature. 
 

With it on about 475W it takes the air from 14-27°C which is ideal for the flow rates. It also only pipes it into the upstairs rooms. 

[john0wingnut]

5 minutes ago, PeterW said:

I fitted one of these with a controller to reduce the power to it, controlled by a Shelly that checks the external air temperature, MVHR supply temperature and the post heater temperature. 
 

With it on about 475W it takes the air from 14-27°C which is ideal for the flow rates. It also only pipes it into the upstairs rooms. 

Smart idea!!

One question that might be a tad daft - why only to upstairs?

 

I have seen some MVHR units which have the ability to control a post heater using the MVHR sensors, so very similar to what you’ve done, but likely a more expensive option.

Edited December 22, 2020 by john0wingnut

[joe90]

11 minutes ago, PeterW said:

I fitted one of these with a controller to reduce the power to it, controlled by a Shelly that checks the external air temperature, MVHR supply temperature and the post heater temperature. 
 

With it on about 475W it takes the air from 14-27°C which is ideal for the flow rates. It also only pipes it into the upstairs rooms. 

That’s a good idea, we have no heating in the bedrooms (towel rads in bathrooms only) and they can be a tad chilly in very cold weather. I could create another manifold for just the bedrooms only with one of these and time it fir early and late daytime (in and out of bed) ?

 

I was pondering a wet post heater using pipework that heats the DHW cylinder as it’s just underneath the MVHR unit in the loft but not worked out how to control it yet ?.

[SteamyTea]

29 minutes ago, joe90 said:

I was pondering a wet post heater using pipework that heats the DHW cylinder as it’s just underneath the MVHR unit in the loft but not worked out how to control it yet

And at 125 quid each, they are pretty cheap, and being electrical resistance heater, about as reliable as you can get.

Just getting a control and safety system for them may be a bit tricky, unless there is an off the shelf one.

Maybe incorporate a smoke sensor.

[joe90]

I fancied using ASHP water as it’s COP means cheaper to run than direct electricity. 
 

1 hour ago, PeterW said:

I fitted one of these with a controller to reduce the power to it, controlled by a Shelly that checks the external air temperature, MVHR supply temperature and the post heater temperature. 
 

With it on about 475W it takes the air from 14-27°C which is ideal for the flow rates.

can you post the controls stuff Peter as Googling “Shelly” didn’t work (for me?).

[john0wingnut]

33 minutes ago, joe90 said:

I fancied using ASHP water as it’s COP means cheaper to run than direct electricity. 
 

can you post the controls stuff Peter as Googling “Shelly” didn’t work (for me?).

Lol

They are a wireless switch brand I believe:

https://shelly.cloud

[PeterW]

 

2 minutes ago, john0wingnut said:

Lol

They are a wireless switch brand I believe:

https://shelly.cloud

 

Yep that’s exactly what it is ..!

 

A 1PM running with a thermo probe header, and 3 DS18S20 Sensors. 
 

https://shellystore.co.uk/product/shelly-1pm

 

Plug it all together and connect to the WiFi and it then allows you to set the scenario when you want it to come on and off. Nice thing is you can also override a Shelly with a standard switch, I’ve used one for outside lights which is on a dusk-ish timer and goes off around 10:45pm but a quick push of a button and they are on - no hunting around for a phone or iPad (which you can use) to turn the lights on. I call that the Luddite option..! 
 

 

 

 

[joe90]

14 minutes ago, PeterW said:

I call that the Luddite option..! 

? that’s me alright, thanks