ASHP - Defrost cycle - MVHR exhaust

[MarkA]

I have been reading posts about ASHP issues and the thread below has provoked some thought:

 

 

Since we at still at the stage where we still have lot of options over where to locate an ASHP plus MVHR system I am wondering if there was any value in positioning the MVHR exhaust duct so that it could blow over the ASHP?  From what I have read there may only be a few degrees C above the external temperature in the exhaust stream during the cooler months, but would that be of any value in helping reduce the frequency of defrost cycles that the ASHP goes through?  I am assuming that it would be relatively simple to direct the MVHR exhaust away from the ASHP once the ambient temperatures rise, should the ASHP be used for cooling in the summer months (this could be manually done).

 

The energy (combined with the flow rate) from the MVHR exhaust may be insufficient to make any real difference but it seemed worth asking the question since it will be running continuously anyway.

 

[Jeremy Harris]

My own view is that it's not worth the hassle for the miniscule return.  The ASHP air flow rate is massive - ours has a fan that's around 450mm in diameter and blows a small gale when it's running.  By contrast the MVHR exhaust is 150mm in diameter and only flows at a pretty low rate most of the time, barely enough to feel.

 

I suspect that the ASHP flow rate on the suction side may well serious unbalance the MVHR, too.

[MarkA]

Yes, that all makes sense.  I have not been near to either of these pieces of kit while they are in action, so thank you @JSHarris for the feedback.

With your point about the ASHP blowing a small gale - does that actually mean it is relatively noisy due to the airflow (versus the motor noise)?

[jack]

I don't think I'd want to either have the ASHP high enough to be near the MVHR outlet, nor the MVHR outlet low enough to be near a ground-based ASHP.

 

Depending on the indoor and outdoor temperature (and your MVHR's efficiency), the MVHR outlet might only be a degree or two higher than ambient. As Jeremy says, the volume of air involved is low, too, relative to what the ASHP pushes when it's outputting a lot of heat.

 

Re noise, our ASHP is almost silent when in heating mode. I don't know about DHW mode, as I'm never awake when it's operating like that!

[Jeremy Harris]

Ours is very quiet, too, but it does shift a heck of a lot of air!

[MarkA]

That's all reassuring as this is most likely to be on the neighbours boundary

[jack]

The spec for all ASHPs will include noise parameters. Worth checking before buying, especially if your neighbor's the one who has to hear it!

[PeterW]

8 minutes ago, jack said:

The spec for all ASHPs will include noise parameters. Worth comparing before buying, especially if your neighbor's the one who has to hear it!

 

A lot of councils require you to list the ASHP on the planning or apply for planning if installing one. 

[Stones]

Ours is very quiet in heating mode, noisier for DHW when running full tilt, but in both cases no more than the noise from a gas boiler flue.

[MarkA]

@PeterW - very good point - I shall add it to the list of variations we are talking to the planners about at the moment.