Steve.N wrote:My experience with the exchangers is that they don't seem to give the performance that the catalog implys. We typically install a 50 plate for an average house and sub zero temps really seem to strain them. Of course this is heating the existing radiation which might work fine with 220 deg F off of the oil boiler but might be a little light at 180deg off of an out door boiler. Direct is always better if you can get away with it.
In my professional engineering opinion once you start using heat exchangers in a hydronic heating system you have reached the limit of "rule of thumb" design. You need to do an "engineering" design not just use the nominal suggested typical, temperature drops, water flow rates, etc. I'm not surprised your experience with plate heat exchangers do not live up to data sheet expectations, particularly during high demand conditions.
The best text on "engineering" hydronic heating systems is "Modern Hydronic Heating for Residential and Light Commercial Buildings, 2E (Hardcover)" by John Siegenthaler, P.E. List price is over $150! I'm waiting for delivery of a discounted book from Amazon.com.