@damon thanks for the quick reply, and that’s quite the glowing reference for Viessmann. Thanks for explaining.

@editor thanks Mars.  This is for our home.  I have already had a visit from an installer that you recommended and we got on well.  I am expecting a quote from him. 

One of my potential installers is proposing a combined solution of complete roof replacement, loft insulation, in-roof solar, battery and heat pump.  I am not confident in their approach to heat pumps.  They have refused to work with another heat pump installer and are saying it is an all or nothing deal.  I am talking to an alternative roof and solar provider.

Posted by: @grahamf

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They have refused to work with another heat pump installer and are saying it is an all or nothing deal.

Definite alarm bells there!

Posted by: @damon

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it uses an Esbe 4 way valve in a three way configuration, hot water, buffer, heating.

when it senses it requires a defrost it loads the buffer to around 55/60°c and uses this to defrost. So on a direct system it doesn’t impact the house load or temp.

Nice idea. Clever system, sensing the frost quite early in order to raise the temperature quick enough. As you say it’s a high temp solution. But sounds like it maintain emitter temps.

Daft idea:

I was kite-flying an idea following a recent discussion about having a volume buffer with a clever valve which diverted all the defrost water into a buffer store and trickle-fed the chilled volume over the (assumed) 40 minutes between defrosts. It would need a self contained temperature sensing valve but I don’t know if such a valve exists.

This would ensure an even distribution of the 50 ltrs or so of chilled water over the maximum period between defrosts, allowing the HP to raise its flow temp in its normal operation.

Imagined benefit. A self actuating valve might be able to be fitted to any HP with no wired in controls; No forced FT boost; thorough mixing over a longer period with reduced upset to the heat transfer.

if such a valve existed 🤔. Would it even work?

A lot of things can work if you are SCoP chaser, do they they add value, are they really cost effective to do? Small buffer £50 to 100, some valves and pipe £100, so soon got to £200, would they really save that over 10 years, doubt it.

I did look at a home brew version, but complexity, cost and benefits - just not worth it.

@johnmo did you add a buffer to your install?

Posted by: @editor

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@johnmo did you add a buffer to your install?

On our original boiler install, I did have a 140L buffer first in 4 port then 2 port configuration (boiler min output 8kW, against a max demand of 3.5kW, plus 5 zones). It ran fine, but wasn't easy to set up at first. Soon learned zones and low energy houses are the work of the devil. You need so much rubbish added to make it work. Over 2 years, mostly removing things, dropped heating costs by over a half.

At one point I had a 50L volumiser on the ASHP installed, but later removed it, at the same time took away the UFH pump and mixer. Let the ASHP do everything. On WC get a max of two cycles per hour and on batch charging floor it will run several hours non stop.

But when I did our hybrid design (added our original boiler into the system), I eliminate the buffer on the boiler side, as I was running single zone. Instead set a flow temperature that didn't cause cycling (run UFH as single zone). So just upped the flow temp a degree at a time until it didn't cycle. Boiler is connected to ASHP circuit via a plate exchange, with boiler flow set at 37 degs and UFH receiving about 34 degs. Boiler would run for as long as needed without stopping - used a -0 +0.1 hysterisis thermostat to control boiler.

So suspect most open loop heating systems should run without buffer, if you don't have have suitable volume add a volumiser or better still bigger radiators so you can run cooler.

But also be aware that flow temp and emitter size need to match to get good running. It's no use trying to run 25 degs if your emitters cannot give off the heat. You maybe better off running 30-35 degs and have better cycle frequency and uses thermostat to manage house temp (not room temperature). This should lead to lower running costs.

Low temp heating is a real balancing act, but run everything open loop makes things way easier. Add a couple of thermostats, you are also adding a buffer, (volumiser will not be ok, as suitable flow rate are no longer supported). More material cost, installer costs and running costs for ever more due to system distortion as zones switch on and off.

@editor yes, alarm bells are ringing for me to.  Still, to be fair to them, they have been burned trying to work with other companies in the past.  They also explained the benefits of only one company dealing with the DNO and all the certification, which makes sense.

There are a few interdependencies between the heat pump and the other projects and they would need to work in some of the same spaces.  I need to find a way to separate them, so that we can do one after the other.

Posted by: @grahamf

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I need to find a way to separate them, so that we can do one after the other.

Good luck with this… I’d also suggest you start a new topic dedicated to your project because it sounds like it’s going to be an interesting one, and it’ll be useful for other people to see how it all progresses.

@damon Whilst we are waxing lyrical about Viessmann as a pinnacle of heat pump system design, attached is an extract from their manual. They state that a buffer tank can "significantly" improve the operation of their heat pump. It should be connected "in parallel" to the heat pump i.e. 2 port connection. Furthermore, it should be used on radiator only systems with TOU tariffs? Somebody please explain. So, buffer tanks are good, are they?

@heatgeek this is for an additional buffer vessel, used for separation.

it can prolong run times but imo you would be better with a volumiser rather than a buffer, in certain situations buffers can work.

low load, hot water being sized for rather than heat demand,

large property and low occupancy for example, it all comes down to design and what the system requirements are for that client and property. 

no panacea 

Posted by: @heatgeek

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They state that a buffer tank can "significantly" improve the operation of their heat pump.

Yes it would prolong run times as they say to use a minimum of 200L.  Great if you have space, but then you have lots of radiators so should have lots of water capacity anyway. They also say if running on radiators only.