ASHP and LPG boiler connected together?

[mattnj]

Hi,
We have new LPG boiler here ( installed just before we moved in)
I have a Mitsubishi Ecodan 8.5kw ASHP from the last place ready to connect up, ideally I would like to install so that the ASHP runs most of the time but I retain the option fire up the LPG if it is super cold, the ASHP fails etc.
What options are there for plumbing it it to have 2 heat sources to the rads, I don't have space to add a tank.
Thanks in advance.

[JonG]

Hi Martin, it depends on which variant of controller you have. Only the latest (FTC4) have the functionality built in to control an external source like a boiler.

You also need to think through whether you will apply for the RHI, in which case the install will need to be carried by an MCS accredited installer, and in order to be compliant the system must be metered, the coverage of each source assessed and confirmed and the controls would have to be interlocked in such a way that the boiler is recruited when the bivalent point is reached (point when the heat pump can no longer cover the heat load).

This may be possible using outside stats to switch the boiler at certain target o/s ambients, but you would also need to blend the flow of the boiler down to avoid tripping the HP stat on the heat pump, or set it low, if you intend to use them in parallel or set up a relay to switch the heat pump off if you are going to switch to the boiler alone.

if you aren't going down the MCS route you have much more flexibility, but it depends on how much automation you want in terms of the boiler's recruitment.

Jon

[Worcester]

Definitely doable, we've got both an GSHP / LPG and an ASHP / Oil hybrid to do over the next few weeks.

For ASHP, we'd tend to run it down to 3°C switch over 100% to the fossil burner and then switch back when ot gets up to 6°C - that keeps the ASHP in the sweet spot.

Ideally you need a buffer tank to separate all the flows, you may get away with a low loss header. This way you can keep the existing rads and depending upon outside temp run then at 35-40-45-50 as required if 50 isn't coping then swap over to the LPG. (With a buffer tank you'd run the rads at Δt10 and the heat pump at Δt5)

You also need to think about how you're going to heat the DHW, if you're not going to change the tank (yet) and put in one with a heat pump coil, then run that permanently off the LPG. 
 
Not checked out the Mitsubishi controls manual yet, thought it's really easy to configure the Samsung units to work this way.

Have a look at this from Samsung: http://www.samsungehs.co.uk/index.php/downloads/finish/6-installation-manuals/159-schematics-showing-what-you-can-do-with-samsung-ehs-monobloc-heat-pumps-bi-valent-hybrid-joining-them-together

[mattnj]

Thanks for the replies!

Ok, so not interested in RHI/MCS and I have DHW sorted for the moment (have solar thermal with e7 immersion)
I only have the standard FTC1 controller for the Ecodan

So...from a plumbing point of view what are the options to connect the 2 heat sources, I was thinking a single circuit, with a 3 way valve diverting the return to either the boiler or ASHP and a simple outside stat to control the valve (ie <3deg run on boiler >3 run on ASHP) but I am sure there is issues there I haven't thought of yet. I cant get my head around the options plumbed directly (split) or with a low loss header or with buffer tank.

Any chance of someone sending me a hydraulic diagram of the best option ? :-)

[baker]

google
systemlink.ie
may help
baker

[dullnote]

I have a danfoff ASHP and LPG boiler connected, should come on a -5degrees or when the  counter reaches 600, however last year which was cold but not extreme , the LPG boiler never on at the coldest time the ASHP would be on for about 15 to 20  hours costing 3kw at 13p. 39p an hour cost £7.80. A day in worst conditions. The LPG boiler costing approx £1 an hour to run, the year I had no ASHP was pay more than £13 a day, but the conditions that year were extreme.

This year I have still the LPG boiler connected but have removed the automatic changeover will do this myself if needed.

Please note the two work through a balance tank, feeding underfloor heating.

This is the second winter with my ASHP and I love it, but be aware there are still people out there that will sell anything to you, research, do some more, ask lots of questions, take everything with a pinch of salt( don't no where that saying comes from) .

Mattnj, don't be afraid to ask the supplier how everything work, go back to them with how you think it is to work and get them to confirm you toughs are correct, develop situations in you mind and put these through the potential system, What if questions

Dullnote