Wet or Dry heating?

[smart51]

My new build will have MHRV feeding all the house.  It will also have a ground source (or air source, to be decided) heat pump.  Most of what I've found online is a discussion of whether a heat pump should drive oversize radiators or under floor heating. 

 

Can the heatpump be coupled to the MHRV so that the warmed incoming air is heated a bit more by the heat pump?  It would save a lot of piping and radiators.  It seems a shame not to if the hot air ducting is already there.  What are the reasons this is not more popular?

[SteamyTea]

14 minutes ago, smart51 said:

Can the heatpump be coupled to the MHRV so that the warmed incoming air is heated a bit more by the heat pump?  It would save a lot of piping and radiators.  It seems a shame not to if the hot air ducting is already there.  What are the reasons this is not more popular?

Main reason it isn't done is that the energy transfer is low.

Or in English, you have to shift a lot of air.

MVHR is not designed to do this, it moves only a few litres a second.

This can be overcome with larger pipework, doubling the diameter, quadruples the flow rate for the same airspeed, this helps to keep noise down.

Also worth remembering that sound can easily travel through pipework. So a quiet chat in the kitchen may well be heard in the bedroom.

 

Have you had your place properly thermally modelled?

[Mr Punter]

MVHR is just to do with providing ventilation without heat losses.  It is not suitable for heat distribution.

[IanR]

26 minutes ago, smart51 said:

Can the heatpump be coupled to the MHRV so that the warmed incoming air is heated a bit more by the heat pump?  

 

Yes it can...but, MVHR does not move sufficient air to deliver very much energy in to the house. Depending on MVHR size, perhaps 0.5kW - 2.5kW.

 

Add to this, the lower temp you can run your HP the more efficient it is, so for space heating if you are running your buffer at 35°C say, that's not going to add much energy to the MVHR supply, so you'll be in the lower range of the scale. An Electric Heater on the supply duct, which could be at 80°C, could be more effective, but you are not getting the benefit of the HP efficiency.

 

If you plan to be at Passivhais levels of energy loss, then it may be possible to heat 100% via MVHR, but using a direct electric in-duct heater.

 

Heat Pumps are best matched with UFH, and if not large or fan convector radiators.

Edited October 1, 2021 by IanR

[SteamyTea]

58 minutes ago, IanR said:

Electric Heater on the supply duct, which could be at 80°C, could be more effective

And smelly.

[smart51]

I wondered if it was to do with not being able to set each room to a different temperature, but flow rate sounds about right.

 

I'm planning passiv haus levels of insulation, and airtightness.  My simple spreadsheet calculation suggests 130W heat loss for every °C difference in outside temperature, so maybe 4.5kW on an extremely cold night. 

 

On MHRV, I was up on the 2nd floor of our current house yesterday and could hear the missus running the shredder in the kitchen, but through the MHRV not up the stairs.  I guess running separate pipes to each room and joining them by the heat exchanger would solve that, rather than running a single pipe and tapping off for each room.

[Temp]

To get enough heat transfer warm air systems need much higher flow rates than MVHR. A friend if mine had ducted warm air heating and even with the higher temperatures of a gas boiler compared to a heat pump the flow rates were high enough to be noisy in bedrooms at night. Our MVHR is inaudible.

[IanR]

18 minutes ago, smart51 said:

On MHRV, I was up on the 2nd floor of our current house yesterday and could hear the missus running the shredder in the kitchen, but through the MHRV not up the stairs.  I guess running separate pipes to each room and joining them by the heat exchanger would solve that, rather than running a single pipe and tapping off for each room.

 

Mine's piped separately via distribution boxes that are lined with a bit of sound deadening foam. I was concerned this wouldn't be sufficient, but it is effective - I have no discernible noise transfer between rooms.

 

18 minutes ago, smart51 said:

I'm planning passiv haus levels of insulation, and airtightness

 

The PH principle is to avoid the need for traditional boilers/HPs and heat emitters, so that the space heating energy requirements can be supplied via the MVHR, with a direct electric duct heater, avoiding the capital investment for boilers/HPs and emitters.

 

But, I personally didn't feel a house would be saleable in the UK without "central heating" and you still need to heat your DHW, plus I knew I would have some cooling requirements.

 

Edited October 1, 2021 by IanR